Characterisation of Residual Char From Biomass Gasification: Effect of the Gasifier Operating Conditions

Accepted Manuscript
Characterisation of Residual Char From Biomass Gasification: Effect of the Gasifier
Operating Conditions
Juan J. Hernández, Magín Lapuerta, Esperanza Monedero
PII:

S0959-6526(16)30592-3

DOI:

10.1016/j.jclepro.2016.05.120

Reference:

JCLP 7302

To appear in:

Journal of Cleaner Production

Received Date: 13 April 2015
Revised Date:

21 April 2016

Accepted Date: 23 May 2016

Please cite this article as: Hernández JJ, Lapuerta M, Monedero E, Characterisation of Residual Char
From Biomass Gasification: Effect of the Gasifier Operating Conditions, Journal of Cleaner Production
(2016), doi: 10.1016/j.jclepro.2016.05.120.
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CHARACTERISATION OF RESIDUAL CHAR FROM BIOMASS

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GASIFICATION: EFFECT OF THE GASIFIER OPERATING CONDITIONS

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Juan J. Hernándeza(*), Magín Lapuertaa, Esperanza Monederob

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Escuela Técnica Superior de Ingenieros Industriales

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Avda. Camilo José Cela, S/N

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13071 Ciudad Real (Spain)

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Universidad de Castilla-La Mancha

Universidad de Castilla-La Mancha

Instituto de Investigación en Energías Renovables

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Calle de la invetigación, S/N

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02006 Albacete (Spain)

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(*)

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Author for correspondence:

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ABSTRACT

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Char, together with tars, are the main wastes derived from biomass gasification. The

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removal of tars and the valorisation of char are necessary to avoid technical problems

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and to increase the overall efficiency of the gasification plant, respectively (both aspects

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encouraging the commercial implementation of biomass gasification systems).

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However, char properties may not be suitable for an easy valorisation. This work

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analyses the effect of the main gasifier operating conditions (relative biomass/air ratio,

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temperature and steam content of the gasifying agent) on the properties of char

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produced from gasification of dealcoholised marc of grape. Those properties allow both,

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to analyse the phenomena taking place during the conversion process and to assess

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potential applications (valorisation) of this waste. Gasification was carried out in a

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small-scale drop-tube pilot plant. Char characterisation includes structural, thermo-

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chemical and compositional analysis. Results show that an increase of the relative

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biomass/air ratio leads to a higher char production as well as to an increase in the extent

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of the carbonisation, which makes the char more aromatic and stable. Intermediate

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relative biomass/air ratios (~3.5), indicate a trade-off between fuel conversion and char

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specific surface area. On the other hand, higher operating temperatures lead to a less

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reactive char as a consequence of a higher fuel conversion. All obtained chars have low

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specific surface area (<70 g/m2), which discourages their application as activated carbon

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without further activation. Inorganic elements have in general a high retention level in

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the carbonaceous matrix (~70-80% for most compounds), which decreases at higher

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temperatures and/or lower steam content in the gasifying agent. The char could also be

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recirculated to the gasifier or to a combustor. However, the alkali index values of both

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the initial biomass and the gasification chars are far above the limit considered as

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troublesome, thus being likely to cause slagging and fouling problems.

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Keywords: Biomass gasification, operating conditions, char characterisation, char

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valorisation.

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Nomenclature:

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Symbols

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AI

Alkali Index (kg/GJ)

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F

Biomass/gasifying agent mass ratio

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Frg

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LHV

Lower heating value (MJ/kg)

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m

Mass (kg)

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Pchar

Char production (kg char/kg fuel daf)

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ρap

Char bulk density (kg/m3)

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R

Reactivity (min-1)

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Relative biomass/air mass ratio

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SBET

BET specific surface (m2/g)

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t

Time (s)

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T

External reactor temperature (ºC)

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X

Conversion degree

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Y

Mass fraction

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Subindexes

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a

Air

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daf

Dry, ash-free basis

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ga

Gasifying agent

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s

Steam

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1. INTRODUCTION

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Gasification is a complex thermochemical process in which a carbonaceous solid fuel

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(coal, biomass, wastes) is transformed at high temperatures (700-1500ºC) and in the

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presence of a gasifying agent (air, oxygen, steam, carbon dioxide, hydrogen, or mixtures

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thereof, with the total oxygen content remaining under sub-stoichiometric conditions)

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into a gas with a useful heating value, called producer gas or synthesis gas (depending

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on its composition). The key advantage of gasification is the possibility of converting a

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solid fuel into a gas (easier to clean, transport and burn efficiently) which keeps 70-80%

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of the chemical energy of the original fuel (Higman and van der Burgt, 2008).

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Moreover, the producer gas can be used in a wide range of applications: production of

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heat and power, as well as a feedstock for the synthesis of fuels and chemicals (Brown,

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2006)(Milne et al., 2002)(Sims et al., 2008). In the case of biomass gasification, the

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advantages associated to the use of biomass (an abundant, widespread renewable energy

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source) are added to those of the gasification technology.

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However, during the gasification process, part of the fuel (around 20%, although the

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amount depends on the operating conditions and the type of gasifier) is not transformed

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into producer gas, but remains as a condensable stream (tars) and as carbonaceous solid

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residue (particles and char). The amount of these residues influences the gasifier

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performance, the design and efficiency of the gas cleaning system and the final

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application of the producer gas (Gil, 2005). Tars are the most troublesome pollutants of

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producer gas, and are the main technical hurdle for the commercial implementation of

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biomass gasification (Higman and van der Burgt, 2008)(Gil, 2005)(Milne et al.,1998).

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Tars are a complex mixture of organic compounds (including aromatic and

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heteroaromatic species as well as polycyclic aromatic compounds (PAHs)) which

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condense at room temperature, thus causing several operating problems (Gil,

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2005)(Milne et al, 1998)(Li and Suzuki, 2009)(Reed and Das, 1988)(Devi et al., 2003).

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Carbonaceous particles in the reaction gas stream (which are mainly collected in filters

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and cyclones) has a high abrasive potential and they are the responsible of engines wear

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if an efficient cleaning is not performed (Hindsgaul et al., 2000). Its concentration in the

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gas stream of a conventional biomass gasification plant typically represents between 3

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and 10% wt, of the biomass feedstock. These particles have different origins and may be

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generated through different routes (Fitzpatrick et al., 2009), which determine their

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chemical composition and physical properties. Some may be formed as a result of direct

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attrition of the carbonized biomass inside the reactor due to mechanical interactions

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with the surrounding elements (biomass feeder and reactor elements). In this case, the

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carbonized biomass stays in the reaction chamber until it is small enough to be carried

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over by the purge gas. Solid particles may also derive from the inorganic components of

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the biomass feedstock once the carbonaceous fractions has been completely gasified. A

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third alternative relates to the formation of soot via condensation of gaseous organic

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molecules (recombination of volatile fraction). The mechanisms governing the latter are

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highly complex, involving preliminary nucleation of organic molecules in the gas phase,

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followed by coagulation, agglomeration and aggregation of the primary particles into

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larger solid structures. High reaction temperatures and reduced oxygen concentrations

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are known to promote these processes (Kozinnski et al., 1988) (San Miguel et al., 2012).

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On the other hand, char is the unreacted carbonaceous solid residue obtained after

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gasification. Frequently, char and ashes are collected together in the ash-tray. This

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residue is formed during the final breakdown of the char structure after reacting with the

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pyrolysis and combustion gases. Char is an amorphous, disordered, isotropic,

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heterogeneous structure which contains elemental carbon (50-80% wt.), heavy

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compounds such as polycyclic aromatic hydrocarbons, and almost all the ash content of

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the original solid fuel (Reed and Das, 1988)(Turn et al, 1998). The ash content could be

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up to 10 times higher than that of the original feedstock (Mülen et al., 1995).

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Among the different and above-mentioned wastes formed during gasification, this work

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is focused on the char (which derives from direct carbonization of the solid fuel), since

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it contains the most of the ash in the original fuel (thus conditioning the alternative uses

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as well as the gasifier operating problems (fouling)) and it is the most abundant waste

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after gasification. In fact, char can have several applications, such as fuel in gasifiers or

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combustors (Galhetas et al., 2012), domestic charcoal, activated carbon (García-García

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et al., 2003), fertilizer or soil conditioner, manure treatment, feed-additives (Boateng et

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al., 2007) (Alburquerque et al., 2016) (European Biochar Foundation, 2012), or even as

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a tar reforming catalyst (Abu El-Rub et al., 2008)(Byrne and Marsh, 19995). However,

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the composition of ash contained in the char can cause several operating problems. In

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particular, elements such as Si, K, Na, S, Cl, P, Ca, Mg or Fe are involved in reactions

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leading to ash deposition and fusion, thus causing fouling, corrosion, emissions,

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erosion, efficiency losses and eventually plant shutdown. There exist several parameters

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related to the tendency of carbon ash to fouling and slagging, such as the alkali index or

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the base/acid ratio, although they are not fully reliable for biomass ash (Fernández-

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Llorente, 2004).

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In this work, a study of the effect of the main gasification operating conditions (relative

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biomass/air ratio, temperature, air/steam content of the gasifying agent) on the

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properties of the resulting char has been carried out. In contrast with most of the

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published works about this topic, which are not focused on the char obtained after the

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whole gasification process but after the pyrolysis step and are based on devices with

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conditions far from real entrained-flow gasifiers (mainly heating rate, particle residence

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time and reaction atmosphere), the results and conclusions obtained in this work can be

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more reliably extrapolated to commercial plants. Moreover, these results, together with

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those previously published about tar formation and properties (Hernández et al., 2013),

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can provide valuable information about the physical and chemical phenomena occurring

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during the conversion process, as well as about the alternatives for the valorisation of

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the carbonaceous solid residue.

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2. METHODOLOGY

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All experimental gasification tests have been carried out in a small-scale drop-tube

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gasification pilot plant described elsewhere (Hernández et al., 2010)(Hernández et al.,

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2012)(Hernández et al., 2013)). The reactor, an alumina tube (1.2 m long, 60 mm inner

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diameter) is surrounded by an electrical furnace with three independent heating zones in

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order to compensate for heat losses. Below the reactor, a stainless steel hopper with

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inner refractory lining allows the collection of the solid residue after each test. A cold

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trap (consisting of a sealed stainless steel container surrounded by a water jacket at

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room temperature) installed at the gas outlet enables to collect the condensates (water,

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tars and entrained particles) contained in the producer gas. Moreover, a gas meter makes

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possible to obtain the flow rate of the non-condensable fraction of the product gas flow.

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The dry-basis producer gas composition is determined on-line by a gas micro-

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chromatograph Varian CP-4900 PRO located at the end of a sampling line.

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All experimental tests have been carried out using dealcoholised marc of grape, an

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abundant solid residue produced in the distillery industry (with an estimated production

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in Spain of 750000 tons/year), composed of grape stalks, skins and seeds. This biomass

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has been selected because of its wide availability, its relatively easier grindability

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(crucial issue in entrained flow gasification), and its interesting thermochemical

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behaviour caused by its ash composition and content (Lapuerta et al., 2008). The

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characterisation of this biomass fuel is shown in Table 1.

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Table 1. Characterisation of dealcoholised marc of grape.

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The experimental schedule for the study of the effect of the relative biomass/air ratio

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(Frg, defined with respect to the stoichiometric one) is shown in Table 2, whereas the

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experimental plan for analysing the effect of temperature (considered in this work as the

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external reactor temperature) and air/steam content of the gasifying agent is displayed in

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Table 3. For the study of the relative biomass/air ratio, the external reactor temperature

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has been held constant at 1200ºC, and Frg has been varied from 1.6 to 4.6 by changing

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the fuel flow rate. For the second experimental schedule, the external temperature of the

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reactor was modified from 750ºC to 1200ºC under three different compositions of the

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gasifying agent: air, a mixture of 54.6% air and 45.4% wt. steam (composition within

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the optimal range found in a previous work (Hernández et al., 2012)), and pure steam.

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In all cases, the biomass/gasifying agent (air + steam) mass ratio (F), was kept

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approximately constant. In order to check the reproducibility of the gasification tests

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leading to the production of char and tars, duplicate experiments were carried out for

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some conditions (Frg=3.5 and T=1200ºC). The two results obtained for those conditions

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are shown in the corresponding figures. Since no significant differences in the char

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properties were observed, the rest of the gasification tests were not repeated.

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Table 2. Experimental schedule for tar and char sampling: effect of the relative

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biomass/air ratio.

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Table 3. Experimental schedule for tar and char sampling: effect of temperature and air/

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steam content in the gasifying agent.

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Prior to the experimental tests, the biomass feedstock was milled, ground and sieved

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below 0.5 mm. Then, the lock hopper was filled with a weighed amount of biomass.

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The furnace was set on the selected temperature. Once the reactor tube had reached that

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temperature, the gasifying agent (air, steam, or a mixture thereof) was introduced

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according to the selected flow and pressure. In all tests the steam inlet temperature was

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set at 400ºC, and the maximum steam flow rate was 1.6 kg/h. Then, biomass was fed

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into the reactor (its volumetric flow set by means of a calibrated screw feeder). In that

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moment, the gasification run started. Producer gas samples were taken and analysed by

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the micro-GC every 2 minutes.

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During the gasification tests, both the gas composition and the gas outlet temperature

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were periodically registered. Once stable operation was reached, the average producer

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gas flow rate was determined from measurements in the gas meter. After 20-30 minutes,

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when the operation was steady (that is, when gas composition remained constant after

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3-4 similar chromatograms), the test was finished. Time was then registered, all flow

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rates (biomass, air, and steam) were stopped, and the final content of the lock hopper

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was weighed in order to calculate the biomass mass flow rate. The furnace was set to

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ambient temperature (with a controlled temperature slope of 10ºC/min in order to

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prevent the alumina tube from suffering thermal shock). When the temperature was low

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enough to ensure a safe operation, char contained in the bottom hopper was collected,

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weighed and sampled and stored for subsequent analyses.

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Char characterisation includes the following techniques: thermogravimetric analysis

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(TA Instruments TGA Q500, used both for the determination of proximate analysis and

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char reactivity), ultimate analysis (Leco HCNS-932), heating value (Calorimeter Parr

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1351), BET specific surface area (Micromeritics Gemini V), qualitative morphologic

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analysis via SEM microscopy (JEOL SEM 6490 LV), FTIR spectroscopy (Thermo

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Scientific Nicolet 6700 FT-IR), and ICP-OES spectroscopy (Varian ICP-OES 715-ES).

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The latter has been used for the determination of major inorganic compounds.

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Instantaneous reactivity of char and biomass samples under oxidising atmosphere (air),

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denoted as R (in min-1), was determined as a function of conversion X according to (Eq.

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1) (Di Blasi, 2009).

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(Eq.1)

X being the degree of conversion, calculated from (Eq. 2).

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1 dX
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X (t ) =

m (t ) − m 0
m0 − m∞

(Eq.2)

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where m (t ) is the sample mass at time t , m0 is the initial sample mass, and m∞ is the

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final sample mass (mass in mg, data obtained in thermogravimetric analyses).

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3. RESULTS AND DISCUSSION
3.1 Effect of the relative biomass/air ratio, Frg
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Figure 1 shows the char production, calculated as the mass of produced char with

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respect to the mass of solid biomass fed in each test. It can be seen that char production

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increases from 0.15 kg/kgdaf to 0.18 kg/kgdaf when increasing Frg from 1.6 to 4.6. This

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indicates a decrease in the fuel conversion, since the process tends to approach

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pyrolysis.

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Figure 1. Effect of the relative biomass/air ratio on the char production.

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Figure 2. Effect of the relative biomass/air ratio on the bulk density and the lower

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heating value (daf basis) of the char.

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Figure 2 shows that higher Frg values lead to a decrease in the bulk density of the

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produced char from 230 kg/m3 to 150 kg/m3. As a comparison, the bulk density of the

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initial biomass is approximately 630 kg/m3. The decrease in the bulk density indicates

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the fuel conversion through the release of volatile matter and the conversion of a

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fraction of the remaining char via heterogeneous reactions (partial oxidation, steam

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reforming, Boudouard reaction). When operating at lower Frg values, the higher char

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density might be associated to a higher fraction of ash due to the higher fuel conversion.

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On the other hand, the heating value of char slightly increases with Frg, but is kept

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within a narrow band of 25-30 MJ/kgdaf. These values are significantly higher than that

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of the initial biomass, approximately 20 MJ/kgdaf, and indicate enrichment in the carbon

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content of the char, in consistency with the ultimate analysis results shown in Figure 3.

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As can be seen, the carbon content of the char increases from 40% to 55% wt. when

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increasing the relative biomass/air ratio, which confirms the progressive carbonisation

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of the char associated to a lower fuel conversion.

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Figure 3. Effect of the relative biomass/air ratio on the ultimate analysis (dry basis) of

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the char.

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Figure 4. Effect of the relative biomass/air ratio on the proximate analysis (dry basis) of

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the char.

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The results of the proximate analysis (Figure 4) show that when solid dry biomass

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undergoes gasification, it loses volatile matter (decreasing from 64% wt. to 30% wt.),

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the ash content increases from 8% wt. to 30% wt. (thus indicating the conversion of part

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of the carbonaceous matter to gas and tars), and the fixed carbon increases from 28% to

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40-50%. It can be shown that operating at higher Frg values leads to a carbonisation of

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the resulting char derived from a lower conversion. It is worth observing the relatively

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high content of volatile matter in the char, which might be attributed to the condensation

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and adsorption of the tars contained in the producer gas when going through the bed of

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char in the bottom part of the gasifier. Despite the capacity of char for capturing tars, the

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presence of toxic compounds (PAHs) prevents its use in soil amendment applications.

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In particular, the European biochar guideline states that the PAH content must be under

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12 mg/kg (dry basis) for basic grade biochar and under 4 mg/kg for premium grade

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biochar (European Biochar Foundation, 2012).

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Figure 5. Effect of the relative biomass/air ratio on the BET specific surface area of the

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char.

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Figure 5 shows a maximum in the BET specific surface of the char of approximately

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60-70 m2/g when operating at intermediate relative biomass/air ratios (Frg~3.5, i.e.

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Equivalence ratio (ER)~0.29). This maximum corresponds to the range of Frg at which

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there is a trade-off between fuel conversion, cold gas efficiency and tar production

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(Hernández et al., 2013). The isotherms obtained (not shown), display a hysteresis loop

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between the adsorption/desorption, which indicates that the produced char is mainly a

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mesoporous material. When increasing Frg from 1.6 to 3.5, there is a development and

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widening of micro- and mesopores due to the increase in the extent of the heterogeneous

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gasification reactions (mainly Boudouard and char steam reforming). During this

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process, two phenomena take place, namely the opening of closed pores and the

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widening of the open pores. Since there is an increase of both the number of pores and

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the average radius, the specific surface also increases. However, when increasing Frg

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beyond values of ~3.5, the relative amount of air with respect to biomass is decreased.

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Therefore, the extent of the combustion reaction is reduced, and thus the concentration

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of CO2 and H2O in the reaction atmosphere decreases. This reduces in turn the extent of

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heterogeneous gasification reactions, and therefore, the degree of fuel conversion. The

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collapse of the char structure and the fusion of adjacent pores might also play a

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significant role in the decrease of the specific surface.

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Figure 6. Effect of the relative biomass/air ratio on the thermogravimetric analysis

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(under air atmosphere) of the char and the initial solid biomass.

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Figure 6 shows the TGA analysis under oxidising atmosphere of biomass and chars. In

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the case of biomass, two peaks of mass loss rate can be observed, which correspond to

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the combustion of volatiles and char, respectively. On the contrary, the char samples

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show an overlapping of both peaks due to the loss of volatiles during the gasification

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process. The intensity of this peak increases with the Frg value of the gasification

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process, which indicates a higher carbonisation degree of the resulting char, in

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consistency with the results shown in this section. All chars show a small peak of mass

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loss around 600-650ºC, which is associated to the transformations of the inorganic

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matter (vaporization, carbonate decomposition, etc.). This peak shows the retention of

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inorganic species in the char after the gasification process, as discussed below.

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Figure 7. Effect of the relative biomass/air ratio on the FTIR spectra of the char.

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The FTIR spectra displayed in Figure 7 show an increase of the intensity of the bands

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located around 1500 cm-1 (corresponding to vibrations of aromatic C=C rings) and 1200

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cm-1 (associated to CO of O-CH3 and C-OH groups) when operating at higher Frg

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values. This indicates a decrease of the fuel conversion degree as well as a progressive

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increase of the carbonisation of the resulting char.

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To sum up this section, it can be concluded that even though the produced char from

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gasification with air is in all cases less dense and less reactive than the initial biomass,

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an increase of the relative biomass/air ratio (i.e. process tending to pyrolysis conditions)

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leads to a higher char production as well as to an increase in the extent of the

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carbonisation, which makes the char more aromatic and stable. Intermediate Frg values

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(Frg ~ 3.5) indicate a trade-off between fuel conversion and char specific surface area.

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3.2 Effect of the temperature and the air/steam content of the gasifying

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agent

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Figures 8 to 14 show the results when varying the temperature and the composition of

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the gasifying agent. As stated by other authors, temperature (and thus heating rate) has a

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significant effect on the reactivity, composition and morphology of the resulting char

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(Guerrero et al., 2005). Figure 8 shows a very significant increase of the BET specific

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surface of char when rising the temperature from 1050ºC to 1200ºC in both pure air and

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steam gasification. However, it can be seen that the combined use of steam and air leads

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to a maximum in the BET surface at intermediate temperatures (1050ºC). In addition, it

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is worth pointing that values of specific surface area are in general lower than 70 m2/g,

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which makes unsuitable the application of gasification char (obtained under the

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experimental conditions tested) as activated carbon without further activation (e.g. via

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steam gasification and CO2).

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Results of BET surface are consistent with the images obtained by SEM microscopy,

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shown in Figure 9. Firstly, it can be observed the development of a network of bigger

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pores due to the coalescence of smaller pores when increasing the overpressure caused

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by the release of volatiles (Uzun et al., 2010), and even the presence of fractures due to

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the thermal contractions and expansions suffered by the fuel particle during the

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devolatilisation stage, which speed up gasification since they favour the reactants

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diffusion within the particle (Haas et al., 2009)(Mermoud et al., 2006). In addition,

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chars obtained at 1200ºC are in general more porous than those produced at 750ºC,

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which in turn have a more amorphous structure. As for the effect of the composition of

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the gasifying agent, high-temperature air-gasification char shows a more opened pore

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structure (which indicates a more intense volatile release and a higher extent of

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heterogeneous reactions) than char from steam gasification.

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Figure 8. Effect of temperature and steam content in the gasifying agent on the BET

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specific surface area of the char.

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Figure 9. Effect of temperature and steam content in the gasifying agent on the
morphology of the char.

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Figure 10. Effect of temperature and steam content in the gasifying agent on the

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proximate analysis (in dry basis) of the char.

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Figure 10 displays the effect of temperature on the proximate analysis of the

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gasification char. In the cases of air-steam and steam gasification, a decrease of volatile

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matter and an increase in the ash content of the gasification residual char are evident at

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higher temperatures. On the contrary, this effect is less clear for air gasification.

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Consistently, as shown in Figure 11, where char reactivity R is plotted against

358

conversion X (both data obtained from TGA under air atmosphere), chars obtained at

359

lower temperatures are significantly more reactive than those produced at higher

360

temperatures regardless the composition of the gasifying agent, which indicates that a

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higher fraction of organic matter remains in the residual char. This latter effect confirms

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the increase in the degree of conversion from biomass to producer gas when increasing

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the process temperature, in consistency with previous results of tar production

364

(Hernández et al., 2013). It is interesting to observe that the reactivity of low-

365

temperature chars increases with the steam content of gasifying agent (becoming even

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more reactive than the initial solid biomass for 100% steam gasification), although this

367

effect is lessened as temperature increases. These results are consistent with those found

368

in the literature. Actually, most authors agree that char reactivity decreases at higher

369

temperatures, due to the increase in the order of the carbonaceous structure via thermal

370

annealing and the removal of edge atoms, dislocations and heteroatoms (associated to

371

active sites) (Lin et al., 2011)(Min et al., 2011)(Tremel et al., 2012).

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Figure 11. Effect of temperature and the steam content in the gasifying agent on the

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reactivity (under air atmosphere) of biomass and gasification chars.

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Figure 12 shows the FTIR spectra obtained at different temperatures and compositions

377

of the gasifying agents. Results from FTIR spectroscopy are consistent with the rest of

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char characterisation previously analysed, as well as with results found in the literature.

379

When increasing temperature from 750ºC to 1200ºC, a loss of OH- (2400-3650 cm-1),

380

NH- (3100-3500 cm-1), and C=O (1650-1830cm-1) functionalities can be observed,

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leading to a decrease in reactivity. According to other authors, the number of

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ramification chains is reduced at higher temperatures, whereas the proportion of

383

benzenic rings and backbone chains increases (Min et al., 2011). It can be observed that

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char produced at 750ºC has a high number of bands between ~2900 and 4000 cm-1,

385

which correspond to CH, CH2 and CH3 aliphatic groups (2800-300 cm-1), aromatic

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groups (~ 3000 cm-1), and OH or NH groups (around 3100-3800 cm-1). In addition,

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there are absorption bands corresponding to C=C and C≡C aromatic rings (1400- 1600

388

cm-1), and to O-CH3 and C-OH groups (1150 cm-1). This variety and high number of

389

functionalities present in char indicates the lower extent of the devolatilisation and

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heterogeneous reactions (combustion, gasification) of the carbonaceous solid fraction at

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lower temperatures. However, when increasing temperature to 1200ºC, there is a clear

392

reduction of all the absorption bands, thus showing a higher fuel conversion to producer

393

gas. The existence of a nearly plain spectrum indicates an aromatic polymeric structure

394

of carbon atoms, where all polar functional groups have been removed or decomposed

395

(Sharma el al, 2001)(Asadullah et al., 2010)(Hu et al., 2008). The role of steam in the

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modification of char structure is also evident, probably due to its effect on moderating

397

the reaction temperature (Tay et al., 2013).

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Figure 12. Effect of temperature on FTIR char structure: (a) air gasification; (b) 54.6%

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air/ 45.4% steam gasification; (c) steam gasification.

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To conclude, the effect of temperature on the composition of inorganic elements

403

contained in char (obtained by ICP-OES spectroscopy) was analysed in the case of air-

404

gasification char (Figure 13 left). Firstly, it is remarkable to show that the main

405

inorganic elements are Ca, K, and Si, with significant amounts of Mg and P. This

406

composition would make gasification char suitable as soil conditioner and fertilizer, as

407

proved by other authors (San Miguel et al., 2012)(Nguyen et al., 2013), although it must

408

be taken into account that the toxic organic compounds adsorbed in the char surface

409

(PAHs) should be previously removed in order to comply with biochar guidelines

410

(European Biochar Foundation, 2012). The inorganic composition of char together with

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the open pore structure (without sintering) observed at high-temperatures could suggest

412

its use as catalyst support (despite its low specific surface area and very limited micro-

413

pore structure). Moreover, it is worth observing that the concentration of all major

414

inorganic elements is significantly higher in the gasification char than in the original

415

biomass. Two simultaneous factors are responsible for this trend: on the one hand, the

416

progressive conversion of the organic fraction of char to gas (as previously analysed in

417

this section), and, on the other hand, the tendency of inorganic matter to be retained in

418

the carbonaceous matrix or released into the gas phase. In order to separate both effects,

419

the retention level (defined as the ratio between the concentration of a specific

420

compound in the char with respect to the concentration in the original biomass) of some

421

of the most important inorganic elements, in % wt. in char, has been calculated. As can

422

be observed in Figure 13 (right), most inorganic elements have a high retention level,

423

around 70-80%. On the contrary, elements such as K and Si (which are actually some of

424

the most troublesome inorganic compounds of biomass) show lower retention values,

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around 50-60%, which warns about possible problems that the vaporised fraction of

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these elements may cause. The vaporisation of inorganic compounds (except for Ca) is

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427

favoured at higher temperatures, in consistency with the results of other authors (Maiti

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et al., 2006)(Joyce et al., 2006).

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Figure 13. Effect of temperature on the concentration of inorganic elements and the

431

inorganic retention level in air-gasification char.

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As for the effect of the addition of steam, Figure 14 shows the results obtained at a

434

constant temperature of 1200ºC. As can be seen, steam gasification slightly favours the

435

retention of all the inorganic elements as compared to air gasification, which might be

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probably due to the decrease of the temperature within the reactor when adding steam

437

into the process (since char-steam reactions are strongly endothermic). These results are

438

consistent with the conclusions of other authors, who determined that steam contributes

439

to the retention of elements such as calcium and magnesium (Tay et al., 2013).

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Figure 14. Effect of steam content in the gasifying agent on the inorganic retention level

442

in gasification char (T = 1200ºC).

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Therefore, given the favourable inorganic composition but the high content of adsorbed

445

volatile matter, one of the most suitable applications of the produced char might be the

446

total conversion of the organic fraction in a combustor or by recirculating it into the

447

gasifier, and the subsequent use of the ash as fertiliser additive. However, the inorganic

448

fraction of the char could cause operating problems, especially in fixed/moving bed- or

449

fluidised bed equipment. In order to assess the tendency to fouling and sintering of the

450

biomass ash and char, the alkali index (AI, kg/GJ), was calculated according to (Eq. 3).

451

HHV is the higher heating value in GJ/kg, Yash is the mass fraction of ash in the sample

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452

(biomass or char), and YK 2O and YNa2O express the mass fraction of potassium and

453

sodium oxides in the ash, respectively.

AI =

(

1
Yash YK 2 O + YNa 2 O
HHV

)

(Eq.3)

Tables 4 and 5 show the alkali index results. As can be observed, the alkali index values

455

of gasification char are in all cases significantly higher than that of biomass, and all

456

values are far above the limit considered as troublesome (0.17 kg/GJ) (Jenkins et al.,

457

1998). Therefore, the use of both dealcoholised marc of grape and the resulting char in

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fixed- or fluidised bed combustors and gasifiers is likely to cause slagging and fouling

459

problems. Moreover, no significant differences in alkali indexes can be observed

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between chars obtained at different gasification temperatures (Table 4), whereas char

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from steam gasification has a significantly higher alkali index than char from air

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gasification (Table 5).

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Table 4. Effect of gasification temperature on the alkali index (air gasification).

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Table 5. Effect of the steam content in the gasifying agent on the alkali index (T =

466

1200ºC).

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4. CONCLUSIONS

Operating the gasifier at intermediate Frg values (Frg ~ 3.5) leads to a trade-off between

469

fuel conversion and char specific surface area. However, gasification chars have a low

470

specific surface area (< 70 g/m2), which discourages its application as activated carbon

471

without further activation. In most cases, char has been determined to be less reactive

472

than the initial biomass (except for the case of low-temperature steam gasification).

473

The results point at the positive effect of higher temperatures on the gasification process

474

regardless the gasifying agent used, as can be derived from the decrease of char

475

reactivity, or the loss of FTIR functionalities. Phenomena such as pore opening and

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widening, fractures and coalescence of adjoining pores have been observed in SEM

477

images. Inorganic elements have in general a high retention level in the carbonaceous

478

matrix (~70-80%), which decreases at higher temperatures and/or lower steam contents.

479

However, the lower retention of elements such as K and Si (~50-60%) warns about

480

possible problems caused by the vaporised fraction of these compounds. The high

481

volatile content could make gasification char unsuitable for soil conditioning

482

applications. Regarding the use in a combustor or the recirculation to the gasifier, the

483

alkali index of both the initial biomass and the gasification chars are far above the limit

484

considered as troublesome, thus being likely to cause slagging and fouling (more critical

485

in fixed- or fluidised bed systems). However, the inorganic composition of char together

486

with the open pore structure could suggest its use as catalyst support (despite its low

487

specific surface area and very limited micro-pore structure) for tar removal applications.

488

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ACKNOWLEDGEMENTS

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The Ministry of Education and Science of the Government of Castilla-La Mancha

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(Spain) is gratefully acknowledged for their financial support through the GENERBIO

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Research Project (Reference POII10-0128-01789). The authors also thank Dr.

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Guadalupe Aranda for her contribution during the work developed.

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LIST OF TABLES
Table 1. Characterisation of dealcoholised marc of grape.

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Ultimate analysis (% wt., dry basis)
C
H
N
S
O
52.06 ± 0.13
5.75 ± 0.10
2.05 ± 0.04
0.14 ± 0.01 32.06 ± 0.27
Proximate analysis (% wt., dry basis)
Volatile matter
Fixed carbon
Ash
64.02 ± 1.06
28.04 ± 1.10
7.94 ± 0.06
Lower heating value (MJ/kg, dry basis) 20.4 ± 0.02
Specific surface area BET (m2/g)
2.56 ± 0.98
Apparent density (kg/m3)

629.2 ± 6.36