Volume 5 biomass and biofuel production 5 05 – biomass co firing

5.05

Biomass Co-Firing

A Nuamah, The University of Nottingham, Nottingham, UK; RWE npower, Swindon, UK
A Malmgren, BioC Ltd, Cirencester, UK
G Riley, RWE npower, Swindon, UK
E Lester, The University of Nottingham, Nottingham, UK
© 2012 Elsevier Ltd. All rights reserved.

5.05.1
5.05.1.1
5.05.1.2
5.05.2
5.05.2.1
5.05.2.2
5.05.2.2.1
5.05.2.2.2
5.05.2.3
5.05.2.3.1
5.05.2.3.2
5.05.2.3.3
5.05.2.3.4
5.05.2.3.5
5.05.2.3.6
5.05.2.3.7
5.05.2.4
5.05.2.5
5.05.2.5.1
5.05.2.5.2
5.05.2.5.3

5.05.2.5.4
5.05.2.6
5.05.3
5.05.3.1
5.05.3.2
5.05.3.3
5.05.3.4
5.05.3.5
5.05.3.5.1
5.05.3.5.2
5.05.3.6
5.05.3.6.1
5.05.3.6.2
5.05.3.6.3
5.05.4
5.05.4.1
5.05.4.2
5.05.4.3
5.05.5
5.05.5.1
5.05.5.1.1
5.05.5.2
5.05.5.2.1
5.05.5.2.2
5.05.5.2.3
5.05.5.3
5.05.5.3.1
5.05.5.3.2
5.05.5.3.3
5.05.5.3.4

Introduction
Global Trend
Challenges Facing the Power Industry
Available Biomass Materials
Wood-Based Fuels
Energy Crops
Short-rotation coppice
Miscanthus
Agricultural Residues
Olive residues
Oil palm residues
Shea residues
Rice husks
Straw
Grass
Bagasse
Processed Wood (Wood Pellets and Torrefied Wood)
Liquid Biomass
Tall oil
Tallow
Jatropha oil
Sewage sludge
Gaseous Biomass
Combustion Technology
Pulverized Coal Combustion
Fluidized Bed Combustion
Stoker Combustion
Cyclone Boilers
Gasification

Direct gasification
Indirect gasification
Gasification Techniques
Fixed bed gasifiers
Fluidized bed gasifiers
Entrained flow gasifiers
Co-firing Methods
Direct Co-firing
Parallel Co-firing
Indirect Co-firing
Global Overview of Biomass Co-firing Plant
United States
McNeil generating plant
Netherlands
Amer 9
Borssele
Maasvlakte 1 and 2
United Kingdom
Aberthaw power plant
Didcot power plant
E.ON (Kingsnorth)
Drax

Comprehensive Renewable Energy, Volume 5

doi:10.1016/B978-0-08-087872-0.00506-0

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Case Studies

5.05.6
Health and Safety Issues Associated with Co-firing
5.05.6.1
Spontaneous Fires
5.05.6.2
Exposure to Biomass and Coal Dust
5.05.7
Technical Issues regarding Biomass Co-firing
5.05.7.1
Fuel Delivery, Storage, and Preparation
5.05.7.2
Supply of Biomass
5.05.7.3

Properties of Biomass and Their Effects on Plant Operations
5.05.8
Conclusions
References
Further Reading

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5.05.1 Introduction
The global demand for energy has increased drastically over the last decade. Coal is regarded as a major source of pollution,
but continues to be one of the most reliable and widely available sources of energy in most countries. This, coupled with the
changing phase of the BRIC countries (Brazil, Russia, India, and China) toward industrialization, contributes enormously
toward global carbon emissions, thereby causing global warming and its associated problems. There is a growing acceptance
that energy from renewable resources must replace the use of fossil fuels, in order to reduce the rate of global climate
change.
There are, however, technologies that have the potential to mitigate these emissions, whether from coal, biomass, or other
resources. Carbon capture and storage (CCS) and renewable energy technologies have been identified as carbon abatement
technologies which could drastically reduce the carbon emissions from power plants. CCS, as innovative as it may be, still has a
lot of technical challenges to overcome before it can be properly commercialized. However, carbon capture, while novel in the
power generation sector, has already been employed in other sectors such as the oil and gas industry, where carbon is captured,
transported, and stored in depleted oil and gas fields to increase the pressure and the flow of oil beneath the ground – this gives

grounds for optimism for coal CCS [1].
This chapter limits itself to biomass co-firing as a renewable energy option and will not discuss other technologies in detail.
Biomass co-firing, as the name suggests, is the burning of biomass along with other fuels. The principal objective of adding
biomass as a partial substitute fuel in high-efficiency coal boilers is that the combustion of biomass is carbon neutral if the biomass
is grown in a regenerative manner. Moreover, there should be minimal changes in total boiler efficiency as a result of co-firing.
Currently, co-firing is the most effective use of biomass for power generation, with efficiency ranging between 35% and 45% [2]. It
has also been shown that the introduction of 5–10% biomass in co-firing requires only minor alteration of the handling equipment.
If larger amounts of biomass are used (exceeding 10%), modifications in the mills, burners, and dryers may be needed [2]. Other
advantages are that it promotes the use of renewable and/or other waste organic materials, thus augmenting the global effort to limit
the use of land for landfilling activities. Legislation around landfilling may be an effective means of promoting the use of biomass in
the fuels market.
While there are many forms of renewable energy, from tidal power to solar power, biomass is an important source of energy that
can be transported and used as a solid, liquid, or gaseous fuel. Bioenergy, or energy from biomass, has huge potential, especially in
countries with renewable forest resources, in wealthier countries with an excess of agricultural land, and in countries where
specialized high-yielding biomass species can be grown.

5.05.1.1

Global Trend

Globally, about 10% of the total primary energy demand is met by biomass. Biomass dominates predominantly in developing
countries where it is used for cooking and heating [2]. In industrialized countries, the use of biomass is below average although
Finland, which is part of the International Energy Agency (IEA) member countries, meets 11% of its energy demand with solid
biomass. The United States, which is the largest producer of biomass, meets only 1% of its energy demand with this renewable
feedstock. Other European countries such as Sweden, Austria, and Portugal generate more than 2% of their total energy from
biomass [3].
In recent years, many governments have encouraged the use of biomass for power generation by initiating incentives to entice
power generators. The relatively high percentage of biomass usage enjoyed by Finland is as a result of government policy which saw
the use of solid biomass being exempt from carbon tax on fossil fuels during the period 1990–97. In the United Kingdom, as part of
the government effort to meet its stringent emission target, the government has set a target to increase the proportion of electricity

generated from biomass from 3% in 2003 to 10.4% in 2011 [3]. The final target is 15% by 2015 [2, 4]. This is backed by a generous
incentive package termed the Renewables Obligation, where power generators are issued with a Renewables Obligation Certificate
(ROC) after generating 0.5–4 MWhe from renewable sources, depending on the ROC band, which can be sold to generate further
revenue for the industry.


Biomass Co-Firing

5.05.1.2

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Challenges Facing the Power Industry

The major technical challenges in biomass co-firing lie in the storage, preparation, and handling of biomass. Many countries lack the
necessary infrastructure to transport biomass to the power plants since such crops are generally grown over a large geographical area.
The seasonal nature of biomass means that abundant supplies exist only around harvesting, with significantly reduced quantities
being available during cultivation and growth. The high moisture content of biomass (up to 70%), coupled with its low bulk
density and heating value [5], adversely affects the behavior of the fuel during combustion, handling, and transport. This also means
that higher volumes of fuel will need to be collected, transported, stored, milled, and burned to achieve the same thermal heat
output as coal, which has a very low moisture content.
Many of these drawbacks are reduced if biomass is co-fired with coal. Co-firing can avoid the need for the high capital cost of
building a new plant. Retrofitted boilers can be altered to fire varying amounts of biomass with coal while maintaining its originally
designed capacity. The energy conversion efficiency of biomass is significantly increased when co-fired in larger plants. This
efficiency ranges between 35% and 45%, which is far higher than the efficiency in biomass-dedicated plants [2]. Apart from reduced
carbon dioxide (CO2) emissions, the sulfur and nitrogen content in biomass is very low, which subsequently reduces NOx and SOx
emissions by diluting the contributions from coal [6]. In other scenarios, the operating cost associated with co-firing is likely to be
higher due to the higher cost of some biomass fuels compared with coal; nevertheless, co-firing is usually the cheapest form of
‘renewable’ energy, which is another important factor that favors the use of biomass when seeking to meet the European Union
(EU)-level regulations on emissions.

5.05.2 Available Biomass Materials
Traditionally, solid biomass fuels have been the main form of biomass used to generate energy. In most of the developing countries,
solid biomass fuels have been utilized to provide heating and also for cooking. In co-firing, other forms of biomass materials have
also been used, such as liquid and gaseous biomass fuels. While solid biomass still dominates the market, the liquid and gaseous
biomass fuels are gaining some impetus as highly efficient energy sources.

5.05.2.1

Wood-Based Fuels

The characteristics, both physical and chemical, of wood-based fuels vary significantly in the form of sawdust, shavings, bark,
and chips. Residues from forestry, sawmills, and the furniture industry have been identified as viable options for co-firing. The
heating value of oven-dry sawdust is around 20.5 MJ kg−1 (high heating value (HHV)) depending on the type and the
percentage presence of bark [7]. Wood is one of the most widely spread sources of fuel for residential, commercial, or
industrial utility boilers or furnaces for producing thermal and/or electrical energy. Locally, the United Kingdom produces
large amounts of wood residue – it is estimated that it produces 10 Mt of waste wood each year [41]. While this amount is not
significantly higher than in many countries, the United Kingdom does not have a large-scale producer of wood pellets, which
is needed to maximize the potential of this fuel source. Wood pellets are mostly imported from Russia, North America,
Scandinavia, and other northeastern European countries. Large-scale production facilities are under development, however, in
Scotland [41].
The type of wood used varies from country to country, but the United Kingdom has indigenous supplies of pine and spruce,
wood from fruit trees like apple and pear, and eucalyptus wood among others. Eucalyptus is a nonnative species and was first
introduced into the United Kingdom in the late eighteenth century.

5.05.2.2

Energy Crops

Energy crops are specifically grown for use as a fuel and are therefore designed to maximize energy yields per hectare at the lowest

cost possible. The commercial ones are usually densely planted monocultures with high yields. Short-rotation coppice (SRC) and
miscanthus are popular choices for co-firing applications.

5.05.2.2.1

Short-rotation coppice

SRC cultures are high-density plantations with high-yielding varieties of willow and poplar. The shoots are harvested every 2–5
years, but the roots are left intact in order to avoid replanting. Approximately 3000 ha are planted with SRC across the United
Kingdom. The shoots from SRC are produced in the form of rods, chips, or billets. The inherent moisture content, depending on the
form, can be between 45% and 60%. A typical yield from a UK-based SRC can be between 5 and 18 oven-dry tonnes per hectare per
year (or odt ha−1 yr−1 for short).

5.05.2.2.2

Miscanthus

Miscanthus is native to Asia and is a perennial fast-growing grass. It uses the C4 photosynthesis pathway and hence is more efficient
in fixing carbon and in water use than the majority of native species in Europe. It grows rapidly during the summer months to
produce canes that can be harvested annually, rather than on a 2- to 5-year cycle as with SRC. The calorific value (CV) of oven-dry


58

Case Studies

Miscanthus is 19.0 MJ kg−1 (HHV), with a yield similar to the average SRC at 7–12 odt ha−1 yr−1. It can continue to grow from the
same rhizomes for at least 15–20 years. One advantage over SRC is that if the landowner decides to stop growing energy crops,
Miscanthus can be removed easily by spraying a herbicide like glyphosphate [8].

5.05.2.3

Agricultural Residues

Usually, agricultural processes produce waste residues, which can end up as compost or animal bedding. Reuse is a cheap source of
biomass and is more benign since landfill sites will inevitably generate methane and other gaseous emissions that increase the
environmental burden. Residues that have been used in co-firing applications include olive residues, oil palm residues, and shea
residues. Other agricultural residues of interest include wheat straw, corn stalks, nutshells, sugarcane bagasse, orchard prunings, and
vineyard stakes.

5.05.2.3.1

Olive residues

Olive residues are produced globally, but the main sources are from around the Mediterranean, with Spain, Italy, and Greece
accounting for 97% of total production [7]. Olive oil production is the main source of olive residues, where only about 21% of
the weight of an olive is actually oil, the rest being residues that are normally treated as waste [6]. The residues include crushed
olive kernel, shell, pulp, skin, water, and any remaining oil. Olive plantations can produce between 500 and 10 000 kg olives per
hectare [7].
The advent of a three- and two-phase production system of olive oil has generated new types of residues. In Spain, about 90% of
olive production systems utilize the two-phase process, which produces residues called ‘alpeorujo’. Alpeorujo is a solid residue from
olive oil extraction and is made up of stones, skins, flesh, water (50–60%), oil (2–4%), and ashes (2%). High moisture content and
high alkali metal content of alpeorujo create problems for boilers due to the low melting temperatures of the ash, although co-firing
can reduce the problem.

5.05.2.3.2

Oil palm residues

Oil palms are mainly grown in Southeast Asia, South America, and Africa. Malaysia and Indonesia currently dominate the

world market in the production of palm oil. Palm oil is extensively used in the food and chemical industries. Forty-five percent
of the palm fruit remains after oil is extracted, which can be used as a fuel. This residual material consists of the empty fruit
bunches, kernel, shell, and fibrous material. Palm residues can also be burned in oil palm processing mills to generate heat and
power. The production of palm oil creates a vast amount of waste biomass after the milling and the crushing of palm kernel.
Over the last decade, palm kernel expeller (PKE) was investigated by power generators as a potential biomass fuel for co-firing.
It has now been burned commercially for at least 5 years by many generators as one of the most popular co-firing fuels in the
United Kingdom. However, price fluctuations make its popularity more ‘volatile’ than other choices of biomass feedstock.
PKE’s lower moisture content and higher CV (compared with other biomass types) make it an excellent choice for co-firing with
coal (Table 1). The kernel shell could also be used as a fuel during co-firing, but it is hard and therefore difficult to mill, thus
making it less popular than PKE.

5.05.2.3.3

Shea residues

The shea tree is native to Africa, where its butter is extracted from the kernel for use in cosmetics and foods. After butter
extraction, significant quantities of waste are produced in the form of shell, husk, and the fleshy mesocarp. These residues can
be processed and used as fertilizers, domestic fuels, or as a waterproofing agent. Similar to palm wastes, the characteristics of
the shea residue can vary according to the processing methods used to extract the butter. This material has been used for
co-firing at a number of power stations in the United Kingdom, but there have been reports of issues with self-heating and dust
handling problems.

5.05.2.3.4

Rice husks

Rice husks are the waste materials after the rice grains have been removed and are predominantly composed of silica. They can
be used as an energy source, but the high ash content, relative to other biomass materials, makes their use problematic during

Table 1

Properties of palm biomass

Fiber
Shell
Empty fruit bunches
PKE cake

Moisture content
(wt.%)

Calorific value
(kJ kg−1)

37
12
67
3

19 068
20 108
18 838
18 900


Biomass Co-Firing

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co-firing. Ash contents of 15–20% (on a dry basis) are not uncommon with >60–70% SiO2 content. Pellets made from a

mixture of rice husks and olive residues have also been marketed, but it is unclear as to whether this product has been
commercially successful.

5.05.2.3.5

Straw

Straw is a product that is available in abundance in most farming-intensive countries. Denmark has been one of the leaders in the
use of straw as a power station fuel. Denmark introduced legislation banning the practice of burning straw in the fields in 1990,
which made straw a more practical fuel source. There are disadvantages with using straw in combustion processes since its ash has an
extremely low melting point, which can result in slagging problems, and its fibrous nature makes it very difficult to handle during
milling and transportation. Either specialist equipment is required for handling and grinding or the boiler needs to be reconfigured
to burn straw bales.

5.05.2.3.6

Grass

Grass can be used as a biomass fuel since it is abundant in many countries. Switchgrass and reed canary grass are examples of
popular varieties of rapidly growing grasses. As with straw, blending grass with coal can be problematic in terms of handling
and milling, particularly without retrofitting existing equipment, thus making it less popular than other biomass types. Grasses
do have the advantage that they can be grown outside the general harvest season and can also be harvested more than once
a year.

5.05.2.3.7

Bagasse

Bagasse is the residue after sugarcane or sorghum stalks are crushed to extract their juice. It contains high amounts of fixed
carbon due to the high bioconversion by the sugarcane plant during photosynthesis. Sugarcane is a major commercially grown

agricultural crop in the vast majority of countries in Africa and in the southern part of America, in particular Mauritius and
Brazil.
Table 2 shows details for different types of solid fuels in terms of elemental and proximate composition.

5.05.2.4

Processed Wood (Wood Pellets and Torrefied Wood)

Processed wood fuels, such as wood pellets and torrefied wood, can be generated from a variety of wood residues. Wood pellets
have a higher and more uniform CV than raw wood. The production of uniform shape and size means that handleability problems
are predictable. They also have a moisture content of 5–10% [7], which is considerably lower than the 60–70% moisture that is
present in the fresh ‘parent’ material.
Torrefaction is a process of improving, or upgrading, the properties of lignocellulosic materials like wood (Table 3). The process
involves slow pyrolysis of the feed material with a hold temperature between 200 and 300 °C. The process lowers the moisture
content and increases the CV (around 21 MJ kg−1, which is similar to subbituminous coal); removes the volatiles that cause smoke
during combustion, resulting in a product that is still approximately 70% of its initial weight but with 80–90% of the original CV;
and also increases the hydrophobicity, making it more durable while improving grinding properties. The additional investment and

Table 2

Properties of different solid fuels [6]

Property

Coal

Peat

Wood
without

bark

Ash content
(db)
Moisture
content
Net CV
C (% db)
H (% db)
N (% db)
O (% db)
S (% db)
Cl (% db)
K (% db)
Ca (% db)

8.5–10.9

4–7

0.4–0.5

2–3

1–3

1.1–4

5

6.2–7.5

2–7

6–10

40–55

5–60

45–65

50–60

50–60

17–25

15–20

60–70

26–28.3
76–87
3.5–5
0.8–1.5
2.8–11.3
0.5–3.1
< 0.1
0.003

4–12

20.9–21.3
52–56
5–6.5
1–3
30–40
< 0.05–0.3
0.02–0.06
0.8–5.8
0.05–0.1

18.5–20
48–52
6.2–6.4
0.1–0.5
38–42
< 0.05
0.001–0.03
0.02–0.05
0.1–0.5

18.5–23
48–52
5.7–6.8
0.3–0.8
24.3–40.2
< 0.05
0.01–0.03
0.1–0.4

0.02–0.08

18.5–20
48–52
6–6.2
0.3–0.5
40–44
< 0.05
0.01–0.04
0.1–0.4
0.2–0.9

18.4–19.2
47–51
5.8–6.7
0.2–0.8
40–46
0.02–0.1
0.01–0.05
0.2–0.5
0.2–0.7

17.4
45–47
5.8–6
0.4–0.6
40–46
0.05–0.2
0.14–0.97
0.69–1.3

0.1–0.6

17.1–17.5
45.5–46.1
5.7–5.8
0.65–1.04
44
0.08–0.13
0.09
0.3–0.5
9

17.5–19
48–50
5.5–6.5
0.5–1.5
34
0.07–0.17
0.1 (in ash)
30 (in ash)
No data

db, dry basis; % on a weight basis.

Bark

Forest
residues

Willow

Straw

Reed
canary
grass

Olive
residues


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Case Studies

Table 3

Properties of torrefied wood compared with others [9]

Moisture content (%)
NCV (MJ kg−1)
Bulk density (kg m−3)
Energy bulk density (GJ m−3)
Hygroscopic nature
Behavior in storage

Wood chips

Wood pellets

Torrefied wood

35
10.5
550
5.8
Wet
Gets moldy, dry matter loss

5−10
17
600
9
Wet
Deteriorates, get moldy

3
21
800
16.7
Hydrophobic
Stable

loss of product during torrefaction are outweighed by the lower transportation costs and higher CV. Torrefaction also allows for a
wider use of source material (including grasses and roots).

5.05.2.5

Liquid Biomass

Liquid biomass fuels are generally grouped into biodiesels and ethanol. Some of the liquid biomass fuels specifically used for
co-firing operations include palm oil, raw vegetable oil, tall oil, waste vegetable oil, rapeseed oil, and jatropha oil. These fuels are
converted into biodiesel by the process of transesterification, which is the chemical process of converting animal fat and vegetable
oil into biodiesel. Ethanol is produced by fermentation of sugar-bearing and starch crops such as wheat, maize, potato, and sugar
beet. In 2005, it was estimated that about 17% of biomass used in co-firing was liquid [10]. Disadvantages include lower CV and
potentially higher NOx emissions. However, liquid biomass has advantages over conventional liquid fuels like petrol and diesel,
particularly in that liquid biomass is renewable, biodegradable, and a superior lubricant (in the case of biodiesel) and has better
solvent properties. Another vital advantage of bioethanol and biodiesel is that they can be mixed with conventional petrol and
diesel, respectively, which allows the use of the same handling and distribution infrastructure [11]. Liquid biomass fuels have been
tried, tested, and proven to be a very useful substitute for petrol and diesel in the transport industry; however, their application in
the power generating sector is still in its infancy.

5.05.2.5.1

Tall oil

Tall oils are essentially by-products from the kraft pulping process in the papermaking industry. During this process, pulp is created
by the digestion of wood, which is influenced by a combination of factors including the high cooking temperature of the chemicals
and its elevated pH level. Black liquor is produced as a by-product of the pulping process, which contains cooking chemicals,
residual pulp, and resin or pitch from the trees. Tall oil is therefore produced from the refined form of the resin or pitch and is used
in the manufacture of soaps, lubricants, and emulsions. Moreover, it can easily replace, or be blended with, current energy fuels to
supply energy. Its physical properties vary based on the type of tree it is obtained from and the processing method employed. Tall oil
can be very corrosive and aggressive to low-grade steel. Care must therefore be taken before it is introduced to ensure that the
integrity of the combustion system is not compromised.

5.05.2.5.2

Tallow

Tallow is a product from rendered animal by-products. The rendering process drives off water at high temperatures to separate the

fat (or tallow) from the protein. About 30–35% of an animal’s mass can be rendered, and of this, 24% is tallow. Tallow is generally
used in the food and chemical industries, with around 250 000 t of tallow being produced annually in the United Kingdom with an
average CV of 40.0 MJ kg−1 [41]. All grades of tallow can be used as liquid fuel in place of fossil fuel since the CV of tallow is just over
90% that of fuel oil and very little modification of combustion equipment is needed to burn it [12]. However, the use of tallow in
co-firing applications has been prevented as a result of its classification as a nonwaste material according to the UK interpretation of
the European Waste Incineration Directive (WID). It only becomes classed as a waste material if it is burned.

5.05.2.5.3

Jatropha oil

The jatropha plant is pest and drought resilient with a high tolerance to poor soil conditions. Enhanced growth rates have been shown
to be achievable by applying fertilizers containing minerals such as magnesium, sulfur, and calcium. It is estimated that a hectare of
land can grow 2200 jatropha plants producing 7 t of jatropha seed yielding 2.2–2.7 t of jatropha oil. The oil content of jatropha kernel
is 63% [13], which is higher than palm oil (up to 45%). The fact that the plant cannot be used as a food source without detoxification
makes it very attractive as an energy fuel source. Drax, a power generation company in the United Kingdom, has recently announced
plans to develop and use jatropha oil in its plans for developing biomass-only power stations and units [14].

5.05.2.5.4

Sewage sludge

Sewage sludge is the final solid component produced during wastewater treatment. Approximately 1.5 Mt of sewage sludge is
produced in the United Kingdom each year, which when processed is suitable for co-firing. After the sludge component has been
separated from the water fraction, it is dried and pelletized. The drying is energy intensive since producing 1 t of sewage sludge


Biomass Co-Firing

61

pellets requires 20 t of sewage sludge. The CV of dry sewage sludge is highly variable, with an average of 12 MJ kg−1. As a waste
product, this falls under the control of the WID of the EU and can only be burned in a WID-compliant plant, which prevents its use
in most co-firing plants as they tend to be not WID compliant.

5.05.2.6

Gaseous Biomass

Solid biomass can further be processed into gaseous forms by the process of gasification. The gasification of solid biomass implies
incomplete combustion of the material resulting in production of combustible gases, such as carbon monoxide (CO), hydrogen
(H2), and traces of methane (CH4). The technique of converting solid biomass into gaseous forms has been described in detail in
subsequent sections.

5.05.3 Combustion Technology
Biomass co-firing relies on existing coal technologies to function, as it is not a stand-alone technology. This is achieved with slight
or, in some cases, no modification at all to the parent plant. Common technologies employed in biomass co-firing are pulverized
coal combustion (PCC), fluidized bed combustion (FBC), cyclone boiler, stoker combustion, and gasification.

5.05.3.1

Pulverized Coal Combustion

PCC technology is a widely utilized technology to generate energy from fossil fuel, especially coal [15]. In this technology,
pulverized coal is injected to combust in a furnace in the presence of a controlled level of air. The heat generated is used to produce
high-pressure steam driving a steam turbine to generate electrical power. The average efficiency for such plants is about 36% in the
OECD (Organisation for Economic Co-operation and Development) countries and 30% in China [16]. The concept of PCC has
been enhanced to operate at higher temperatures and pressures to produce supercritical (SC) steam and also ultra-supercritical
(USC) steam (>374 °C and 218 atm). These two advanced technologies have efficiencies far greater than PCC, with efficiency
ranging between 40% and 55%. However, the full commercialization of SC technology has been limited by the need for materials

that can withstand high temperatures and pressures. These technologies are seen as a major carbon mitigation route, as it is
estimated that a percentage point increase in plant thermal efficiency can lead to a double reduction in CO2 emissions [17].
Therefore, replacing old pulverized fuel (PF) plants with SC pulverized coal plants has the potential of reducing emissions by
10–25% [15].

5.05.3.2

Fluidized Bed Combustion

FBC can be either a bubbling bed (BFBC) or a circulating bed (CFBC). BFBC is achieved by combusting the fuel with a bed material
that has a depth of around 1 m operating at gas velocities sufficient to fluidize the fuel and the bed material. CFBC operates at higher
gas velocities, high enough to entrain the fuel and bed particles in the gas flow leaving the combustion chamber, where the particles
are separated in a cyclone or beam separator and recirculated to the combustion chamber.
These technologies result in lower NOx and SOx emissions than PF technology, which is due to the fact that FBC operates at
temperatures (800–900 °C) below the temperatures required for thermal NOx formation [15] and also there exists an intimate
contact between the fuel and the bed material. Moreover, SO2 can be totally removed, negating the need for flue gas desulfurization
or recirculation, by addition of limestone to the bed material. FBC technologies are ideal for high-ash coals or coals with poorer
burnout properties. Their thermal efficiencies are normally about 3–4% below that of PF combustion. However, with the advent of
pressurized fluidized bed combustion (PFBC) technology, which employs the same processes but with higher pressures, thermal
efficiencies can exceed 40%. There is also the possibility of improving upon the PFBC by the application of combined cycle
technology [15].

5.05.3.3

Stoker Combustion

In the stoker or grate-fired boiler system, the fuel is fed onto a moving grate while air is blown through the bed of fuel. Smaller
particles burn out suspended above the grate while larger particles burn on the grate, as the fuel moves from the back to the front of
the boiler. These boilers are capable of firing a wide range of fuels, including coal, peat, straw, waste, and wood residues in fairly
large pieces (not more than 3 cm).

This technology has a low maintenance and operational cost, but it is limited to a maximum capacity of about 100 MWe and has
a lower efficiency compared with PCC and FBC [18]. Modern stoker units are equipped with cyclones, electrostatic precipitators, or
baghouses, sometimes with gas scrubbers to remove particulate from the stack phases. There are often problems when firing
low-melting fuels in stokers, but these can be reduced by using mechanical or water-cooled grates and by avoiding the use of
preheated combustion air in the final burning region.


62

Case Studies

5.05.3.4

Cyclone Boilers

Cyclone boilers are another combustion technology suitable for biomass co-firing. Here the mineral matter in the fuel forms a
slag that holds and captures the large particles, allowing the volatile and fine particles to burn in suspension providing intense
radiant heat for slag layer combustion. The burners for cyclone boilers are generally large, water-cooled, and horizontal with the
combustion temperature in the external furnace ranging between 1650 and 2000 °C. For optimum cyclone performance, the
fuels are specified to meet certain requirements, such as ash content must be greater than 6% (too high for many pure biomass
types), volatiles must be greater than 15%, and the moisture content must be less than 20% unless the fuel is dried [41]. Cyclone
boilers only need fuels to be crushed and not pulverized making them suitable for co-firing in that they require minimal
modification for feeding and mixing the biomass and the coal [42].

5.05.3.5

Gasification

Gasification converts solid or liquid carbon-based fuels into a gas (syngas or biosyngas) in a high-temperature environment.
The process is initiated in the presence of oxygen, air, or steam and also heat. The gas produced is mainly made up of CO and

H2, and other components including CO2, H2O, and CH4. The percentage of these gases (CO, H2, CO2, H2O, and CH4)
depends on the composition of the raw materials and the gasification conditions such as pressure and temperature. To obtain
the highest efficiency for this technology, the gasification process is integrated with a combined gas turbine set, called the
integrated gasification combined cycle (IGCC), where efficiency is maximized by using the syngas to drive a gas turbine as well
as powering a steam turbine by utilizing the exhaust heat generated from the gas turbine. Work has been done to suggest that
efficiencies of up to 56% can be achieved under IGCC [16]; however, it is understood that the practical thermal efficiency is
about 40% [43].

5.05.3.5.1

Direct gasification

In direct gasification, reaction temperatures are produced by partial combustion of the feedstock in the presence of air or oxygen in
the reactor. Usually, the syngas produced from this process is very dilute when air is used due to the high amount of nitrogen in air.
The use of pure oxygen produces syngas with high CV, which is good for combustion purposes, but the process becomes
considerably more expensive.

5.05.3.5.2

Indirect gasification

Here, the heat required for gasification is supplied by an external source outside the main gasifier usually by the use of steam. Steam
contributes to increasing the CV of the product gas due to its high hydrogen content and, moreover, it is very attractive due to its low
cost and easy production. An example of an indirect gasifier using gas as heat source is fluidized bed gasifier equipped with heat
exchanger tubes. Here, part of the product gas is burned with air as oxidizing agent in a pulse combustor. The resulting heat is used
to gasify the fuel that is fed into the reactor [19]. Two separate reactors are required when char is used as the heat source: a circulating
fluidized bed steam gasifier converts fuel to produce gas and a circulating fluidized bed combustor burns residual char to provide
heat which is needed to gasify the fuel. Bed material, usually silica sand, is circulated between the two reactors to facilitate better heat
transfer.

5.05.3.6

Gasification Techniques

Based on the type of technique and equipment used, three basic types of gasifiers can be distinguished: fixed bed, fluidized bed, and
entrained flow gasifiers.

5.05.3.6.1

Fixed bed gasifiers

Fixed bed gasifiers require mechanically stable fuel of smaller particle size (1–3 cm), such as pellets or briquettes [20], to
ensure free and easy passage of gas through the bed. Depending on the direction of flow of the feedstock and the gas, these
gasifiers are classified as updraft and downdraft gasifiers. In updraft fixed bed gasifiers, the fuel is fed from the top while air
is blown into the bottom of the reactor. This arrangement can withstand biomass of higher moisture content (up to
40–50%) [19]. This is because the hot gas exiting the gasifier initiates the combustion process by drying and pyrolyzing
the fuel as it moves down the gasifier until finally undergoing gasification and combustion at the bottom. The product gas is
generally useful for heat and power generation through a steam turbine and not particularly applicable for synthetic fuel,
chemical, or gas turbine applications, due to the amount of higher hydrocarbons (e.g., aromatic hydrocarbons and tars)
contained in it.
Alternatively, in downdraft fixed bed gasifiers, the fuel is fed in from the top while air is introduced at the sides above
the grate and combustible gas blown through the grate. The setup is very simple and of low cost. The gas produced
(mainly CO, H2, CH4, CO2, and N2) is relatively clean compared with that produced in updraft fixed bed gasifiers and
contains no or low amounts of tars or oils, making it suitable for application in heat and power generation using gas
turbines.


Biomass Co-Firing

5.05.3.6.2

63

Fluidized bed gasifiers

For biomass gasification in a fluidized bed, the temperature for successful gasification is at least 750 °C [44] while maintaining the
bed temperature below the ash melting point of the fuel. Failure to adhere to this standard may produce sticky ash that might glue
together with bed particles causing agglomeration and breakdown of fluidization. Hence, fluidized bed gasifiers are appropriate for
woody biomaterials, which have a higher ash melting point (above 1000 °C) than herbaceous biomaterial (e.g., straw), whose ash
melting point can be as low as 700 °C [21].

5.05.3.6.3

Entrained flow gasifiers

Entrained flow gasifiers operate at a very high temperature (1200–2000 °C) and pressure (about 50 bar) and turn the mixture of fuel
and oxygen into a turbulent dust flame, producing liquid ash, which deposits on the walls of the gasifier. This is sometimes very
problematic, especially when analyzing the ash melting behavior of solid biomass feedstock; another drawback is the high cost
associated with oxygen production and the milling of the fuel to suitably fine sizes for easy entrainment [44]. Due to the operating
conditions of this type of gasifier, only specific types of biomass are suitable to be applied. The technology is relatively mature and has
been commercially utilized in the petroleum industry for the gasification of petroleum residues.

5.05.4 Co-firing Methods
There is no dedicated technology for co-firing, which implies that it utilizes the existing technology for generating power from fossil
fuels. Some of these technologies have been described in the preceding sections. However, based on the different routes by which
the coal and biomass blend can be introduced into the boiler, three other techniques have been identified: direct, indirect, and
parallel co-firing.

5.05.4.1

Direct Co-firing

This is the most popular, simplest, and cost-effective way of co-firing coal with biomass. Here, the combustion of coal and biomass
takes place in the same boiler producing blended coal and biomass ash. There are four possible ways to this technique [4]:
1. Co-milling of coal and biomass with existing coal mill equipment or with dedicated individual milling equipment and firing
them through the existing coal feeding system
2. Direct injection of premilled biomass and firing of the biomass material through existing coal injection systems and burners
3. Installation of new, dedicated biomass milling equipment and firing the coal and the biomass through separate injection systems
4. Utilizing the biomass as a reburn fuel
Option 1 can be achieved in different ways depending on the milling system:
• Milling and firing of blended biomass and coal fuel through existing coal milling and firing equipment. Here, coal and biomass are milled
and dried together in existing equipment to achieve the desired particle size. The blended fuel is then fired into the furnace for
operation of the plant. It is the cheapest and most straightforward option. The main disadvantage of this technique is that the
grinding performance of the coal mill degrades due to the presence of the biomass. It also carries the highest risk of malfunction of
the fuel feeding system [42]. This option is suitable for biomass fuels such as olive/palm kernels or cocoa shells as well as sawdust
[45] but not for herbaceous biomass.
• Modifying existing coal mills on each boiler to mill biomass materials separately and firing the milled material through existing pulverized coal
pipework and burners [46].
• Milling of the biomass in dedicated biomass mills and the introduction of the milled fuel into the existing coal-firing systems.
With option 2, for the introduction of the premilled fuel into the furnace, three direct co-firing options can be applied:
• Injection of the biomass directly into the furnace, with no flame stabilization and no additional combustion air. This is relatively inexpensive
and simple to install as it involves direct injection through the walls of the furnace, albeit it does involve the installation of new,
small-diameter pipework for better furnace penetration [4]. The drawback of this process is that its application is limited by
conventional wall- or corner-fired furnaces. This approach has been used in downshot boilers at RWE npower’s Aberthaw power
station in the United Kingdom.
• Installation of new, dedicated biomass burners, with a combustion air supply. Here, the premilled biomass is fed into the same boiler as
the coal but through separate feeding systems. This option requires the installation of a number of biomass transport pipes across
the boiler front, which may already be congested, creating difficulties in maintaining an adequate burner performance over the
normal load curve. This is more capital intensive than co-milling, since it requires greater modification to existing coal plants.
Again, there are a number of potential problems that need to be resolved such as the location of new burner, alterations to

accommodate secondary air supply, and the lack of experience in large-scale biomass burners [4].


64

Case Studies

However, option 2 presents a relatively simple and cost-effective way of increasing the proportion of biomass co-fired in a typical
coal plant. Moreover, unlike co-milling, where there is an undue pressure from the biomass on the mill and feed system, this option
ensures that there is no interference on the existing coal milling and feeding system.
• Pneumatic injection of premilled biomass into existing coal pipework downstream of the coal mills or at the burner and firing through the
existing burners. The introduction of additional fuel and air reduces the mill primary air and coal flow rate accordingly to maintain
both the coal mills and the burners within their normal operating envelope. Albeit relatively inexpensive and simple to install,
there are significant interfaces with the mill and combustion control system, which have to be carefully managed. The available
options for the biomass injection points are directly into the burner, just upstream of the burner into the pulverized coal
pipework, and into the mill outlet pipework.

5.05.4.2

Parallel Co-firing

Here, the biomass and the coal are separately combusted producing individual ashes, supplying steam to a common header. The
fuel preparation and feeding are physically independent. The only potential limiting factor to this technique is the capacity of
existing downstream infrastructure, such as the steam turbine. The amount of steam that could be co-fired could be limited by the
capacity of the steam generator.
For this technique to be successful, there should be sufficient overcapacity of the steam turbine to accommodate the extra power
from biomass or a reduction in the coal boiler capacity to make room for the biomass. Since the coal and the biomass are converted
independently, an optimal system for each fuel can be chosen, for example, CFB for biomass and PC for coal. The capital investment
of installing parallel co-firing is significantly higher than for direct co-firing; however, this technique attracts some interest due to its
potential to optimize the combustion process, the ability to use fuels with high alkali and chlorine content, and the possibility of

producing separate ashes.

5.05.4.3

Indirect Co-firing

The process of indirect co-firing involves gasifying biomass separately and injecting the produced gas into a coal boiler to be
burned. Like parallel co-firing, this technique produces separate ashes while allowing very high co-firing ratios. Its main
disadvantage is the high capital investment of installation. However, this approach may suit the future of co-firing, which will
see, almost certainly, an increase in the biomass/coal ratio as well as in the range of different biomass fuels considered. Therefore,
high capital investment could pay off in the future with more advanced co-firing configurations giving better operability and
flexibility.

5.05.5 Global Overview of Biomass Co-firing Plant
The huge benefits of biomass co-firing as the most efficient renewable route and the corresponding incentives enjoyed by companies
for generating electricity from renewables have globally stimulated the development of the technology at a considerable rate. This
rate has been intensified in the past 5–10 years, which has seen the modification of existing plant to accommodate the biomass and
the introduction of new plant designs with the capacity to co-utilize biomass with fossil fuels [22]. According to the IEA Bioenergy
database (), a significant number of these plants are in the United States, with about 100 in Europe and 12
in Asia.

5.05.5.1

United States

Biomass co-firing has been practiced in the United States for a long period of time. It, however, gained impetus from the 1990s,
where energy generators started enjoying incentives from the government for generating electricity from renewable sources [23]. A
large range of biomass fuels, including residues, energy crops, and herbaceous and woody biomass, have been fired using PCC,
stokers, and cyclone boilers. The introduction of the energy generated tax credit in 1992, which was exclusively targeted to support
electricity generated from wind and closed-loop biomass, attracted electricity generators to invest in these technologies. Initially, the

incentive package covered only closed-loop biomass – that is, energy or forest crops where what is harvested is regrown – but
changes in legislation in 1999 allowed for extension and expansion of the existing incentive for electricity generated from biomass.
The qualified biomass fuels for incentives included any solid, nonhazardous cellulosic waste material, pellets, crates, trimmings,
and agricultural by-products and residues. Other incentives were introduced to encourage renewable technologies in the United
States including exemptions from property tax, state sales tax, and income tax, with offers for loan and special grant programs,
industry recruitment incentives, accelerated depreciation allowance, and net-metering provisions [3].
Over 40 plants in the United States have co-fired biomass and coal for several years mostly for testing and demonstration
purposes. Five plants currently operate continuously for testing wood or switchgrass, and one plant has been operating


Biomass Co-Firing

65

commercially for the past 2 years burning wood-based fuels and coal [24]. Almost all the co-firing plants in the United States utilize
the direct co-firing process, apart from the McNeil generating plant, which employs the indirect options (Table 4).

5.05.5.1.1

McNeil generating plant

The construction of this plant was by popular acclamation where residents in the city of Burlington, Vermont, USA, approved the
construction of a 50 MW wood-fired power plant. Electricity was initially supplied by an aging coal-fired plant with poor emission
records, and hence the idea of using locally available and environmentally friendly feedstock was widely accepted.
The McNeil generating plant operates by the indirect co-firing techniques, with an installed capacity of 50 MWe. Wood chips are the
main biomass fuel and contribute about 15% by heat to the co-firing plant. It utilizes a low-pressure Battelle gasification process that
consists of gasification and combustion reactors. The gasification reactor is heated by an indirect source to generate medium-CV fuel of
about 17–18 MJ m−3 and residual char at a temperature of 700–850 °C [25]. The combustion reactor burns the residual char to provide
heat for gasification. Sand is used as a medium of heat transfer between the reactors by circulating the sand between the gasifier and the
combustor. The product gas is co-fired in a stoker grate boiler for steam generation, which is used in steam turbines for power.

5.05.5.2

Netherlands

The Dutch government has set ambitious targets with 14% of its total energy from renewable sources by 2020 [26]. For that reason,
the government has entered into an agreement with six utilities companies, which operate coal-fired power plants, to commit
themselves to reduce CO2 emission by an equivalent of 5.8 Mt yr−1 in the period 2008–12. It is envisaged that more than half of this
target (3.2 Mt yr−1) will be achieved by the substitution of coal by biomass [27].
In The Netherlands, 4 GWe out of the total power production of 14 GWe is generated by coal-fired power plants, which co-fire
biomass. Table 5 shows the total number of coal-fired power plants and the types of biomass used. Apart from Amer 9, all co-fired
power plants in The Netherlands utilize the direct injection technique.

5.05.5.2.1

Amer 9

The Amer 9 plant utilizes both direct and indirect co-firing configurations. The plant co-fires biomass pellets up to a maximum of
1200 kt yr−1, generating 27% by heat through two modified coal mills. Only wood-based fuel has been used since 2006, due to
reduced subsidies for agricultural by-products.
For the indirect co-firing option, low-quality demolition wood is gasified in a CFB gasifier at atmospheric pressure and a
temperature of approximately 850 °C. The raw fuel gas is cleaned extensively and combusted in a coal boiler via specially designed
low-CV gas burners. An advantage of this concept is that there is no contamination of the fuel gas as it enters the coal-fired boiler.
This allows a wide range of fuels to be co-fired within existing emission constraints while avoiding problems with ash quality. The
Table 4

Major US biomass co-firing demonstration plants [3, 22]

Utility

Plant

Boiler type

Boiler size
(MWe)

Biomass type

Biomass heat input
(%)

Alabama Power
Allegheny Energy
Allegheny Energy
Alliant Energy Corporation
GPU
GPU
GPU/RE
La Cygne
NRG Energy
NRG Energy
NIPSCO
Nisource
NYSEG/AES
Madison G&E
Otter Tail
Santee Cooper
Southern Company Services, Inc.
Southern Company Services, Inc.

Tampa Electric
TVA
TVA
TVA
FERCO

Gadsden
Albright
Willow
Ottumwa
Shawville
Shawville
Seward
KCP&L
B. L. England
Dunkirk
Michigan City
Bailly
Greenidge
Blount St.
Big Stone
Jefferies
Hammond
Kraft
Gannon
Allen
Colbert
Kingston
McNeil

Tangentially fired
Tangentially fired
Cyclone
Tangentially fired
Wall-fired
Tangentially fired
Wall-fired
Cyclone
Cyclone
Tangentially fired
Cyclone
Cyclone
Tangentially fired
Wall-fired
Cyclone
Wall-fired
Tangentially fired
Tangentially fired
Cyclone
Cyclone
Wall-fired
Tangentially fired
Stoker grate

70
150
188
704
130
160

32
840
120
100
425
160
105
50
450
165
120
55
165
270
190
160
50

Switchgrass
Sawdust
Sawdust
Switchgrass
Wood
Wood
Sawdust
Wood
Wood
Wood
Wood
Sawdust

Wood
Switchgrass
Seed corn
Wood
Wood
Wood
Wastepaper
Sawdust
Sawdust
Sawdust
Wood chips

7
7
5–10
3
1.5
1.5
10
5
12 (mass)
10–15
5.5
10
10
10
1–4
10–20 (mass)
5–14 (mass)
20–50 (mass)

5
10
1.5
2.5
15


66

Case Studies

Table 5

Biomass fuels power plants in The Netherlands [22]

Plant

Type of co-firing

Co-firing

Co-firing
(%, thermal)

Status

Burner configuration

Gelderland
Amer 8

Amer 9
Borssele 12
Maasvlakte 1
Maasvlakte 2
Hemweg 8

Direct
Direct
Indirect
Direct
Direct
Direct
Direct

Demolition wood
Wood pellets
Demolition wood
Kernels, shells, fibers, paper sludge
Biomass pellets
Poultry litter
Sewage sludge

3
10–12
27
10–15
5
5
3

Commercial
Commercial
Commercial
Commercial
Commercial
Commercial
Test phase

Wall-fired
Tangentially fired
Tangentially fired
Tangentially fired
Tangentially fired
Tangentially fired
Wall-fired

challenge, as always, is working within the relatively stringent fuel constraints while avoiding the inevitable high investment costs
[22]. Amer 8 also co-fires at high biomass feed levels but uses a standard hammer mill configuration.

5.05.5.2.2

Borssele

Borssele is a 420 MWe, tangentially fired PCC unit, equipped with low-NOx burners with overfire air. This plant co-fires coal and
phosphor oven gas, which is transported from a nearby phosphor production plant through a pipeline. About 80 000 t of gas are
fired annually, which is equivalent to a 3.5% coal replacement. Initially, there were concerns over the quality of the fly ash, due to
risk of contamination by the presence of phosphate; however, the concentration of phosphorus is sufficiently low, and as such, no
adverse effects have been observed [27].
Lately, paper sludge, olive pulp, cocoa shells, palm kernels, and wood pellets have been co-fired over the coal belt through
existing mills. It is also possible to co-fire these fuels through separate mills and burners. Sewage sludge has also been fired at

Borssele 12, but it is no longer used due to odor problems [28].

5.05.5.2.3

Maasvlakte 1 and 2

Liquid organic waste from the petrochemical industry is co-fired with coal in the 518 MWe PCC, a tangentially fired plant fitted with
flue gas desulfurization. The liquid waste is handled and fired separately from the coal and constitutes about 1% of the output of the
plant. The disposal of the fly ash and the bottom ash produced is problematic due to molybdenum contamination of the liquid
waste.
At Maasvlakte 2, the co-fired biomass pellets consist of a mixture of wood, composted sewage sludge, and paper sludge.
The pellets are mixed with the raw coal and milled in the existing milling equipment. The pellet production plant is capable of
manufacturing 150 000 t of pellets with a heating value of 16 MJ kg−1, and is situated adjacent to the power station. During the
demonstration trials, the operation of the mills was found to be normal and inspections revealed no damage or accumulation
of woody material in the mills or transport lines. The quality of the fly ash, bottom ash, and gypsum produced was tested
and they fulfilled their regular quality specifications. Also, no adverse effects on atmospheric emissions or wastewater effluent
were observed. Co-firing started commercially in 1998 and the amount co-fired is equivalent to 5% of the output of the
plant [3].

5.05.5.3

United Kingdom

The United Kingdom has, by law, set a stringent target to reduce its carbon emissions by 80% by 2050, with a preceding short­
to-medium target, where 20% reduction is expected by 2020 and 10.4% and 15.4% by 2011 and 2016, respectively [41]. To meet
this target, it is expected that about 30–40% of electricity would be generated from renewables, which is made attractive by the
introduction of the Renewables Obligation, as power generators are awarded ROCs that are tradable. Table 6 shows the number of
ROCs awarded for the production of 1 MWh of electricity by different technologies. The certificates were traded at a price of around
£45 in 2010 [31]. If a supplier cannot provide the required number of ROCs, they have to pay a buyout fee, which was £37 per ROC
in 2010 [32].

Currently, all the 15 coal-fired power plants have co-fired biomass, but several of them have stopped doing so. A wide variety of
biomass fuels have been utilized, including baled straw, woody materials, poultry litter, residues, tallow, and bonemeal (Table 7).

5.05.5.3.1

Aberthaw power plant

This plant is located on the coast of south Wales in the Vale of Glamorgan. It is one of the three RWE npower power stations co-firing
a range of biomass. Originally, the plant was designed to fire only coal and has been operating at full scale since 1971 with three
operation units. It has the capacity to generate 1500 MW of electricity. As part of the company’s effort to invest in lower carbon
technologies, it has invested over £9.5 million in biomass co-firing technology to allow the substitution of some of the coal burned


Biomass Co-Firing

Table 6

The ROC bands for different approaches that use biomass [29, 30]
ROCs
(MWh−1)

Approach

Fuel type

Co-firing

Biomass

0.5

Co-firing of energy crops

Energy crop which is one of the following:
(1) Miscanthus giganteus;
(2) Salix (also known as SRC willow); or
(3) Populus (also known as SRC poplar)

1

Co-firing with combined heat and power (CHP)

Biomass by a qualifying CHP generating station and where the fossil fuel and biomass
have been burned in separate boilers

1

Co-firing of energy crops with CHP

Energy crop which is one of the following:
(1) Miscanthus giganteus;
(2) Salix (also known as SRC willow); or
(3) Populus (also known as SRC poplar)
by a qualifying CHP generating station and where the fossil fuel and energy crops
have been burned in separate boilers

1.5

Dedicated biomass power generation

Electricity generated solely from biomass

1.5

Dedicated energy crops power generation

Energy crop which is one of the following:
(1) Miscanthus giganteus;
(2) Salix (also known as SRC willow); or
(3) Populus (also known as SRC poplar) generating electricity solely from energy
crops
Electricity generated from biomass by a qualifying CHP generating station in a
calendar month in which it is fueled wholly by biomass

2

As above but with energy crops

2

Dedicated biomass with CHP
Dedicated energy crops with CHP

Table 7

67

2

Information on UK co-firing power plants [33]

Company

Power
station

Installed capacity
(MW)

Primary
fuel

AES
Alcan
British Energy

Kilroot
Lynemouth
Eggborough

390
420
2000

Coal/oil
Coal
Coal

Drax Power Limited
EDF Energy

EDF Energy
E.On UK
E.On UK
E.On UK
International Power
RWE npower
RWE npower
RWE npower
Scottish and Southern
Energy
Scottish and Southern
Energy
ScottishPower
ScottishPower
Uskmouth1 Power Company

Drax
West Burton
Cottam
Kingsnorth
Ironbridge
Ratcliffe
Rugeley
Aberthaw B
Tilbury B
Didcot A
Ferrybridge C

4000
2000

2000
2034
964
2000
1000
1553
1029
1940
2034

Coal
Coal
Coal
Coal/oil
Coal
Coal
Coal
Coal
Coal/oil
Coal/gas
Coal

Olive pellets
Wood pellets, olive pellets
2.5% PKE, olive pellets and pulp, shea pellets and
meal
3% Energy crops/wood pellets
5% Biomass blend – wood pellets, shea, miscanthus
5% Biomass blend – wood pellets, olive cakess
Cereal residues

Wood chips, PKE
None
None
Tallow, sawdust, PKE, wood chips, small roundwood
3% PKE, sawdust, olive residues
PKE, sawdust, olive residues, shea, wood pellets
10% Biomass – wood, olives, shea residues, PKE

Fiddler’s
Ferry
Cockenzie
Logannet
Uskmouth
Littlebrook

1995

Coal

10% Biomass

1200
2400
393
685

Coal
Coal
Coal

Wood pellets
Sewage sludge/wood pellets
Shea meal
Palm oil

Co-firing fuel


68

Case Studies

with biomass fuels such as sawdust, PKE, and wood chips in a 55 MW existing generating plant [34]. It employs the direct injection
technique, with 5% (thermal basis) of biomass. The biomass is milled in a separate biomass milling plant using hammer mills and
then injected into the boilers through dedicated injectors and burns in the coal flames, rather than milling the biomass and coal
together. This has been achieved without significant modification of the existing boiler. Liquid biomass fuels such as tallow and tall
oil have also been co-fired at the plant for a number of years.

5.05.5.3.2

Didcot power plant

RWE npower invested over £3.5 million in the Didcot power plants to provide enough renewable electrical energy, through biomass
co-firing, to over 100 000 homes each year [34]. It is estimated that this investment could replace about 300 000 t of coal and avoid
700 000 t of CO2 emissions into the atmosphere. Didcot A has an installed capacity of about 2000 MW, which was originally
designed to fire coal or gas. It has been modified to fire biomass materials such as PKE, sawdust, olive residue, shea residue, and
wood pellets alongside coal in its co-firing operations. The biomass is co-milled with the coal at Didcot and burned through the
existing unmodified coal burners.

5.05.5.3.3

E.ON (Kingsnorth)

Pelletized straw materials have been co-fired at the plant’s four 500 MWe, tangentially fired, low-NOx units. The pelletized biomass
fuel is preblended with the coal in the coal handling system and is fired through the existing handling and firing system. Before the
commissioning of the plant, pretrials and health and safety checks had been completed, which led to the installation of temporary
blend facilities and additional explosion suppression systems. During the pretrials, it was observed that for every percentage
increment of biomass in the co-firing mixture, there was a corresponding 2% reduction in mill capacity, even though this depended
largely on the type of biomass. There were also noticeable reductions in carbon, NOx, and SOx emissions, and the ash produced met
salable standards [3].

5.05.5.3.4

Drax

Drax has invested in a multimillion-pound co-firing facility to allow the company to meet its set target of generating 12.5% of the
total electricity from biomass co-firing, reducing carbon emissions by about 15% and saving over 2.5 million tonnes of CO2
annually. This investment puts the company in the forefront of developments to establish alternative fuels technology for power
generation in the United Kingdom. The company has again announced plans to build three biomass co-firing plants, with each
having an installed capacity of 300 MW. This could result in the company being responsible for the supply of at least 15% of UK
renewable energy and up to 10% of total UK electricity [35]. Drax Power Limited has been burning biomass since 2003, with its
main fuel being sustainable wood-based products and residual agricultural products such as sunflower seed husks and peanut
husks, which are secured through a supply agreement for biomass with local producer groups and supplier groups for all their
required biomass fuels.

5.05.6 Health and Safety Issues Associated with Co-firing
5.05.6.1

Spontaneous Fires

Spontaneous combustion has always been one of the most serious hazards in the power generation industry, as recorded in the
United States, France, Great Britain, and Australia from the 1950s to the 1990s [36]. Spontaneous combustion is most likely when
deep deposits of coal have been heated and moistened by steam purges. For vertical-spindle mills, deposits of coal in the air inlet to
the mill are particularly dangerous because the air inlet temperature is higher than the temperature in all other parts of the mill. In
addition, smoldering could take place, without causing excessive increase in mill outlet temperatures, which could remain
undetected for a considerable period of time. Biomass, like all materials with high-volatile matter, is often prone to spontaneous
combustion, which makes storage, milling, and transport often problematic [37].
A combination of low humidity levels and dust particulate accumulation can expose the plant to a high fire risk due to the
occurrence of combustion and deflagration. It has, however, been found that for an air/gas mixture, deflagration may fail if the ratio
of gas to oxygen is too high (fuel-rich mixture) or too low (fuel-lean mixture). For a coal dust/air mixture, the lower explosive limit
for lignite and bituminous coals is 30 and 140 g m−3, respectively. Biomass fuels have a lower explosion limit similar to lignite at
around 30–40 g m−3 [37]. The concept of a fuel-rich upper explosive limit for any PF milling system is questionable if air is used as
the transport medium due to the sufficiently energetic source of ignition. It has been suggested that PF particles can use the available
air to continue to react, even in the fuel-rich region. Generally, for bituminous coals, the risk of deflagration occurring can be
reduced if the air/fuel ratio within the PF supply system is limited. It has also been found, based on empirical data and laboratory
analysis, that applying a similar operating restriction when co-milling biomass should not pose any additional threat during normal
operation, provided that blend concentrations are kept within well-defined limits. This is due to the fact that the explosion
characteristics of biomass/coal blends containing up to 15 wt.% biomass are dominated by the coal blend [37].
Work by Caini and Hules [38] found that suppression systems could be used to minimize or prevent the occurrence of fires and
deflagration, especially in coal bunkers, feeders, and pulverizers. This system injects inert gases or particles like sodium bicarbonate
to dilute the oxygen concentration to a level where fire and deflagration of the mixture are not possible [43]. It is very important that


Biomass Co-Firing

69

the oxygen level be reduced to a level below 15% and above 12%, as below 15% oxygen fire cannot thrive and below 12% visible
signs of oxygen depletion might set in [39].

5.05.6.2

Exposure to Biomass and Coal Dust

Workers subject themselves to high risk of exposure to dust when working with biomass and coal. Dry biomass particles are easily
suspended in air due to their low density and large drag coefficient. Exposure to dust at work contravenes the Control of Substances
Hazardous to Health (COSHH) regulations, which clearly stipulate that workers should not unnecessarily be exposed to chemical,
physical, or biological agents that may harm their health. Where it is impossible to prevent exposure, steps must be put in place to
control the situation to reduce as much as possible the level of contamination and exposure. Moreover, workers must be given the
necessary training and protective equipment to protect themselves from unnecessary exposure when it becomes inevitable.
Information on suitable protective equipment against exposure can, in the UK, be provided by the Health and Safety Executive
(HSE) where recommendation has been made for suitable protection against exposure to specific materials such as wood dust and
grains. This information has been found to be applicable to biomass materials such as cereal pellet and wood pellet [37].
The effects of coal or biomass dust on health are very diverse, depending on the type of dust. But dust mainly affects the lungs
and the respiratory systems through inhalation, creating the risk of nasal cancer. Other dust-associated health problems include
dermatitis and soreness of the eyes, abrasion, and conjunctivitis. It is strongly advised that people who are allergic to dust must
avoid exposure completely. Palm and shea residues originally contained nuts; hence, it is advisable for people who have a nut
allergy to avoid working in storage areas or areas where dust inhalation is possible.
In general, the use of a disposable filtering facepiece respirator would be enough to provide adequate protection, unless
personnel are exposed to high concentrations of dust that might be beyond the designed limit of the respirator. In such cases,
dust levels must be continuously monitored to determine the risk of exposure. Dust formation can be suppressed or prevented by
the use of suppression systems such as moisture or foaming agents. This system of dust prevention requires very low capital
investment compared with full dust extraction systems. However, misting systems can also generate conditions that are favorable for
mold growth on stored biomass. This problem clearly depends on biomass type, plant design, atmospheric conditions, and cleaning
regimes.
Currently, there is limited information regarding the effects of mold on health, as well as human susceptibility which varies
considerably. Even though the majority of the molds found on biomass are common species and pose no known harm to
health, persons with impaired immune systems may be at risk when exposed to high levels of airborne molds and fungal
spores.
Good housekeeping such as minimizing storage times, the immediate cleaning up of spillages, and minimization of dust and

moisture levels can reduce the ambient spore loading in areas where biomass is handled. Currently, there is no regulatory limit on
airborne levels for spores, even though the level of bacteria and microbiological organisms is covered by COSHH. This, however,
does not negate the potential health hazards of working with biomass; hence, good housekeeping procedures and personal
protective measures must be adhered to in order to mitigate these risks.
Under COSHH, hardwood dust is classified as a carcinogen which has the potential to cause lung and respiratory
problems. However, only softwoods are usually used as biomass fuels and they present a significantly lower risk than
hardwoods [37].
Mycotoxins and endotoxins are by-products from the growth of mold and breakdown of bacterial cells, respectively. They
contribute to naturally occurring aerosols which are generally noninfectious. Nevertheless, they may cause irritations mainly of the
respiratory tract such as mucous membrane irritation (MMI), immunotoxic diseases, and allergic diseases (e.g., asthma and allergic
rhinitis). Like dust, there is no exposure limit; the general approach for protection against endotoxins and mycotoxins has been to
limit exposure.
The release of volatiles from many volatile organic compounds (VOCs) in biomass materials gives them their characteristic
smell. Some of these biomass materials, especially olive, may release substances such as carbon monoxide, hydrogen sulfide,
methane, CO2, and volatile fatty acids (e.g., acetic acid, propanoic acid, and butyric acid). In general terms, this may not be
harmful, but there may be potential health and safety hazards when personnel are exposed in a confined space with poor
ventilation. An employee from a power station in The Netherlands suffered carbon monoxide poisoning after unloading a
consignment of olive cakes in a confined space. Risk could be reduced by conducting an appropriate laboratory screening of the
fuel prior to delivery.

5.05.7 Technical Issues regarding Biomass Co-firing
There are vast characteristic differences between coal and biomass largely influenced by the behavior of coal and biomass blends
under co-firing conditions. Apart from the heating value of coal being almost twice that of biomass and the corresponding bulk
density of biomass being significantly less than that of coal, the moisture content of biomass is usually much higher than that of
coal, ranging from 25% to over 50%. The ash content of biomass can also vary from less than 1% to over 20%. Moreover, the fuel
nitrogen of biomass can vary from 0.1% to over 1%, but its sulfur content is usually very low [3].


70

Case Studies

5.05.7.1

Fuel Delivery, Storage, and Preparation

Work on the delivery, storage, and preparation of coal for power generation has been well documented. However, when coal is
co-fired with biomass, new challenges arise, which are primarily due to the differences in their properties. The low bulk density of
biomass requires different types of handling, storage, and preparation compared with coal. The low energy and high shear strength
require a receiving pit as open as possible to allow sufficient unloading for the boiler capacity, and a screening device designed to
meet the irregular shapes of the biomass material.
Storing biomass with moisture content greater than 20% for a long period of time can cause problems, which could lead to the
growth of biological activity causing self-heating of the storage piles, loss of dry matter, and significant deterioration of the physical
quantity of the fuel [4]. This means that moist biomass cannot be stored on-site for a long period of time; hence, co-milled mixtures
of biomass and coal must be prepared shortly before the fuel is fired or else the quality of the feedstock will be deteriorated due to
the degradation of the biomass [47]. There is also the possibility that high dust and spore concentration in the stored fuel can create
health and safety issues during subsequent fuel handling operations [37].
Taking appropriate steps prior to delivery can minimize biological activity during long-term storage but significantly add to the
cost of the fuel. Some of these steps are as follows [5]:
•
•
•
•

Storage of biomass in billets or larger pieces, if possible, to reduce the surface area available for biological activity
Using fungicides and other chemical agents to suppress biological activity
Predrying of the fuel to a moisture level where biological activity cannot flourish
Cooling the stored fuel by forced ventilation to temperatures where biological activity can be minimized

On the other hand, some biomass materials (such as PKE and wood pellets) have moisture content below 20% and are not affected

by biological activity to the same extent as wet fuels.
Due to the nature of biomass and its high moisture content, there is a risk of spontaneous combustion occurring. This can be
prevented by adhering to the following guidelines [3]:
• Storage piles should consist of a homogeneous material.
• Biomass piles should not be compacted.
• Temperature and the gas composition in the pile should be monitored.
Co-milling of blended coal and biomass with already existing coal mill equipment may require significant modification, since the
equipment was designed to suit the brittle nature of coal. Since biomass is not brittle, the breakage mechanisms are different.
Biomass/coal ratios may be limited if the biomass is not milled to the required specification. Moreover, the use of wet biomass can
alter the heat balance in the mill and wet biomass also has the tendency to accumulate in the mill, which can be problematic during
normal operation and when emptying. During milling and processing, biomass releases combustible volatiles at lower tempera­
tures than coal, which can result in health and safety issues that need to be considered especially during start-ups, shutdowns, mill
trips, and restarts [3].

5.05.7.2

Supply of Biomass

To meet the quantities of biomass suitable for co-firing, power generators usually rely on imported biomass fuels. Dried or
pelletized wood is widely available in countries such as North America, Scandinavia, Russia, and specific European countries.
Other biomass materials such as olive and palm residues can be sourced from countries with large olive or palm oil production such
as Spain, Italy, Greece, Turkey, Tunisia, Portugal, Malaysia, and Thailand [40]. Oil, sugar, and starch energy crops can be used for the
production of liquid fuels with high energy content as in biodiesel and bioethanol. However, they are a primary food stock; hence,
their full-scale utilization may compete with and defeat their main purpose of serving as food for human consumption.

5.05.7.3

Properties of Biomass and Their Effects on Plant Operations

As mentioned earlier, biomass and coal have diverse characteristics. Generally, biomass has a higher moisture content (about

50–70% in fresh wood) than coal (about 3% in bituminous coal), resulting in low CV of the biomass fuel. The volatile matter
content of biomass is close to 80% and 20% fixed carbon (on a moisture-free and ash-free basis), whereas bituminous coal has
around 20–30% volatile matter and 70–80% fixed carbon [5]. Moreover, both particle size variations and the high fiber content of
biomass contribute to the poor flow properties of biofuels. This means that, with the exception of pelletized fuel made from dry raw
materials, high internal and external frictions will occur during movement of the material, making it more abrasive and somewhat
corrosive [6].
Apart from the physical properties, the chemical properties of wood biomass set demanding requirements for power plant
operation. These properties include total ash content, ash melting behavior, and the chemical composition of ash. Alkaline metals
present in the ash are generally responsible for fouling the heat transfer surfaces and are abundant in wood fuel ashes which will be
easily released in the gas phase during combustion [6]. It is known that a small concentration of chlorine in the fuel can result in the
development of harmful alkaline and chlorine compounds on boiler heat transfer surfaces [6].


Biomass Co-Firing

71

EUBIONET concluded that most of the problems with boiler performance when co-firing arise from the difference in properties
between the coal and the biomass fuel (Table 8), which can be summarized as follows (Maciejewska 2006):
• Biomass has a higher inherent moisture content.
• Pyrolysis starts at lower temperatures with biomass than with coal.
• The volatile matter content of biomass is higher than that of coals, even that of high-volatile coals.
• The proportion of heat that is generated from the volatile fraction of biomass is approximately 70% compared with 30–40%
for coal.
• The CV of the volatile matter from biomass is significantly lower than that from coal.
• Biomass char contains more oxygen than coal char and is also more porous and reactive.
• Biomass ash tends to be more alkaline, which increases the chances of fouling in the boiler.
• Biomass can have high chlorine content, but typically low sulfur and ash content.
These variations mean that if biomass is blended with coal, the following implications may be expected:
•

•
•
•
•
•

Increased rate of deposit formation
More frequent soot blowing
Higher risk of corrosion of heat transfer surfaces
Bed material agglomeration (in fluidized beds)
Higher in-house power consumption, particularly with the mills
Higher flue gas temperature

Table 8

Properties of biomass and their effects on power plants

Properties
Physical
Moisture content

Bulk density
Particle dimension and size distribution

Chemical
Carbon (C)
Hydrogen (H)
Oxygen (O)
Chlorine (Cl)
Nitrogen (N)

Sulfur (S)
Fluorine (F)
Potassium (K)

Sodium (Na)

Magnesium (Mg)
Calcium (Ca)
Phosphorus (P)
Heavy metals

Impacts

Storage durability
Dry matter losses
Low CV
Self-ignition
Fuel logistics (storage, transport, handling)
Determines fuel feeding system
Determines combustion technology
Drying properties
Dust formation
Operational safety during fuel conveying
Gross CV (GCV; positive)
GCV (positive)
GCV (negative)
Corrosion
NOx, N2O, HCN emissions
SOx emissions, corrosion
HF emissions, corrosion

Corrosion (heat exchangers, superheaters)
Lowering of ash melting temperature
Aerosol formation
Ash utilization (plant nutrient)
Corrosion (heat exchangers, superheaters)
Lowering of ash melting point
Aerosol formation
Increased ash melting temperature
Ash utilization (plant nutrient)
Increased ash melting temperature
Ash utilization (plant nutrient)
Increased ash melting temperature
Ash utilization (plant nutrient)
Emission of pollutants
Ash utilization and disposal issues
Aerosol formation


72

Case Studies

While the magnitude of these implications depends on the quality and the proportion of biomass in the fuel blend, the overall
result is that operating and maintenance costs may increase. However, this can be reduced or avoided with appropriate fuel blend
control, where optimum amounts of the biomass fuel in the fuel blend can be defined with appropriate combustion tests together
with bed material and deposit quality assessment [6].

5.05.8 Conclusions
Coal will continue to play a critical role in the global energy mix for the foreseeable future, despite all the known impacts that come
from its combustion. There are many technologies that are destined to mitigate emissions from coal. Co-firing of coal with biomass

has been identified as a low-cost option for efficiently and cleanly converting biomass to energy by adding biomass as a partial
substitute fuel in high-efficiency coal boilers. Moreover, it makes good use of materials that would otherwise end up in landfill,
thereby serving a dual purpose of mitigating emissions and contributing toward a cleaner environment.
However, biomass co-firing cannot thrive without adequate financial support to entice power generators and favorable
legislation from the government to allow the technology to prevail. This support is common to many new technologies; however,
the huge benefits associated with biomass co-firing, in terms of its ability to use existing PF technologies and achieve direct and
immediate results, make such support essential.
Biomass co-firing is not a stand-alone plant; it employs the already existing coal-fired technologies with slight modification, or
no alteration at all if the biomass percentage is low. This chapter has discussed in detail the common technologies employed in
biomass co-firing. It is found that, among the three main biomass co-firing options, the direct co-firing method is the simplest to
operate and common among many biomass co-firing power stations. The indirect and parallel co-firing options also have some
advantages; however, the complexity and the cost involved in their operations deter power generators from embarking on that
route. Notwithstanding, there are pockets of plants, especially in Europe, where the technology is used.
Biomass co-firing technology, despite presenting numerous benefits, also presents significant challenges. Health and safety
issues associated with the handling of the biomass and the coal have been discussed in this chapter, and it is generally accepted that
adequate measures are needed to protect personnel, such as the wearing of personal protective equipment (PPE), and to prevent
spontaneous fires. Coal bunkers, feeders, and pulverizers may be protected by injecting adequate amounts of inert gas by using
suppression systems. This would dilute the oxygen concentration to a level where deflagration of the mixture is not possible.
Another challenge faced by the technology is the effects of the properties of biomass on boiler equipment. Some of these effects
include increased rate of deposit formation and soot blowing, risk of corrosion of heat transfer surfaces, and bed material
agglomeration. These effects are found to be reduced in lower biomass percentage and very pronounced in higher biomass, and
can be reduced if appropriate fuel blend controls are adhered to. Progressive developments based on an increasing awareness of
biomass behavior will lead to increases in the optimum amount of the biomass that can be introduced into the blend, thus
achieving maximum environmental benefit with minimal impact on the plant.

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www.ieabcc.nl (accessed 15 February 2011).
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Sciences 5(3): 179–183.
[7] Scurlock JMO (1999) Miscanthus: A review of European experience with a novel crop. US Department of Energy’s Environmental Science Division, ORNL/TM-13732, Publication
No. 4845. Tennessee, USA.
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Technical Conference on Coal Utilisation and Fuel Systems. Clearwater, FL, USA, 4–7 March.