Mania Abdollahi-Neisiani, Jean-Philippe Laviolette, Rouzbeh Jafari and Jamal Chaouki

Abstract Bioreflnery is the object of significant research and development efforts due to the scarcity of economically viable crude oil, renewable energy source, and its environmental benefits. This has prompted chemical corpo-rations to look for alternative sources of carbon and hydrogen to produce chemicals, biologics, and other products such as biomass and waste matter. Two main reaction pathways are currently explored for biorefinery: thermochemical and biochemical. The thermo­chemical pathway proposes significantly higher reaction rates compared to current biological processes that use non-genetically modified organisms. One of the thermo­chemical pathways for biomass conversion is gasification which is a decomposition of solid fuels at high temperatures and oxygen-lean atmosphere. The successful development of biomass gasification processes requires addressing several critical technical difficulties including biomass diversity, feedstock treatment, gasification mechanism and reactions, gasifier types, and their performances. This chapter re­views key features of biomass gasification as a pretreatment for biorefining which can be used as a practical guide for gasification process. This chapter consists of six sections that include types of biomass for gasification, their properties, and pretreat­ment steps; gasification mechanism and reactions; syngas cleaning and conditioning; different gasifier, their characteristics, and modeling.

Keywords Biomass gasification ■ Pretreatment ■ Gasifier ■ Gas cleaning ■ Tar removal ■ Catalytic gasification

J. Chaouki (H) ■ M. Abdollahi-Neisiani ■ J.-P. Laviolette ■ R. Jafari Department of Chemical Engineering, Ecole Polytechnique de Montreal, C. P. 6079, succ. Centre Ville, Montreal, H3C 3A7, Canada e-mail: jamal. chaouki@polymtl. ca

M. Abdollahi-Neisiani

e-mail: mania. abdollahineisiani@polymtl. ca

J.-P. Laviolette

e-mail: jean-philippe. laviolette@polymtl. ca R. Jafari

e-mail: rouzbeh. jafari@polymtl. ca

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_10, © Springer-Verlag Berlin Heidelberg 2013

10.1 Introduction

The beginning of industrial civilization has triggered the frantic use of non-renewable fossil-fuel resources (coal first, followed by oil and gas), which has grown worldwide ever since. Today, the price of fossil fuels is increasing due to depleting “conven­tional” resources, rising demands from developing countries and the establishment of a low-carbon economy. In this context, significant investments and research are focusing on the development of new processes to extract energy and goods from renewable resources, such as biomass.

Biomass is a carbonaceous matter, known as a renewable energy source from living or recently living organisms. Examples include forest residue, agricultural wastes, and even, municipal solid waste. To convert biomass, two main reaction pathways are currently considered: biochemical and thermochemical. Gasification is a thermo-chemical pathway, which transfers the combustion value of the solid fuel to the gas phase whose composition maximizes its chemical energy rather than sensible heat. Syngas, a mixture of CO and H2, is one of the products of the gasification process, which could be used as a fuel or building block for many hydrocarbons. The products derived from syngas can be divided into three categories: (1) chemicals, such as ammonia and methanol; (2) transportation fuels, such as synthetic natural gas and synthetic diesel; (3) and energy feedstock, such as methane. Currently, syngas is produced mainly from fossil fuels; however, there is a growing interest in generating “green chemicals” and “green fuels” from the gasification process.

To successfully design an industrial gasification process, a thorough knowledge of biomass pretreatment, gasification reaction kinetics, and reactor technologies is essential. This chapter discusses the subject of biomass gasification through a detailed review of the scientific and industrial literature.

The equilibrium model predicts the maximum yield when the reactants are in contact for an infinite time without taking into account the reactor type and size [76]. In reality, the products leave the reactor before having the opportunity to reach equilibrium so this type of model only provides the ideal yield. For practical applications, therefore, the use of the kinetic model is more realistic. At higher temperatures (>1,500 K), however, the use of the equilibrium model is more effective. There are two types of equilibrium modeling approaches: (1) stoichiometric or the use of the equilibrium constant; (2) non-stoichiometric or the minimization of Gibbs free energy method. In the stoichiometric model, all the chemical reactions and species involved are considered. For a known reaction mechanism this method predicts the maximum yield of all the products and the possible limiting behavior of the reactor. In the

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and Mahinpey developed a model capable of predicting the performance of an at­mospheric fluidized-bed gasifier [83]. They used both built-in Aspen Plus reactor models and external FORTRAN subroutines for hydrodynamics and kinetics to sim­ulate the gasification process. Other authors have worked with Aspen Plus to model the gasification process for coal and biomass. Yan and Rudolph developed a model for a compartmented fluidised-bed coal gasifier process [86], Sudiro et al. modeled the gasification process to obtain synthetic natural gas from petcoke [87]. Abdeloua — hed proposed a compressive model for dual fluidized bed gasifier modeling with ASPEN Plus [88]. A comprehensive review on Biomass gasification simulation also provided by Puig-Arnavat et al. [90] (Fig. 10.3)

The fruits and vegetables harvest and transformation processes yield many wastes: trimmings, hulls and shells. In 1995, the US Department of agriculture estimated that over 250 million dry tons of agricultural crops and residue were generated over a year in the country [23]. The chemical composition of agricultural crops and residues is very similar to that of perennial herbaceous plants (see Sect. 11.2.1.4). The interest in these feedstocks is reflected in the abundant literature found on agricultural crops and residues pyrolysis [24-26].

11.2.1.5 Animal Manures

Animal manures are used as fertilizers: their high urea, phosphorus and organic con­tents enrich soils dedicated to agriculture. Cattles are the main manure producers with production of over 200 million dry tons a year in the US (commercial broilers are showing comparable numbers) [23]. Because manure has a heterogeneous com­position, thermal decomposition has gained interest to recover that feedstock. Cattle manure is more difficult to collect than poultry manure [10]. Therefore, poultry ma­nure is considered as a good candidate for industrial pyrolysis, and the scientific literature has been mostly focused on this type of manure.

Development of the biofuel production processes had begun in the nineteenth century; in the beginning of the twentieth century interest to the biofuels had died out because of the rapid growth of cheaper fossil fuels usage; developments in this field have been resumed again due to the oil crisis in 1973 [12]. Today, the development of biofuels production and using technologies is driven by: increase in prices of energy resources, fossil fuels depletion, and also CO2 emission issue.

Energy crops are plants which have been cultivated as a source of energy. Basi­cally they are represented as herbaceous or woody fast-growing plants, for example switchgrass [13] and willow [14]. Algae are one of the most promising biofuels. Fertile lands are not required for their cultivation, and they grow in virtually any kind of water [15].

Brazil was the major producer of biofuels until 2000; however, by 2008 its world output has increased from ~20 billion liters a year to almost 75 billion, basically due to the rapid development of the bio-energy technologies in the USA and Canada [16].

One of the most promising sources of biomass is Panicum virgatum, well-known as switchgrass. Let us examine it as a characteristic representative of energy crops.

The economical conversion of lignocellulosic biomass to higher value products re­quires efficient recovery of cellulose, lignin, and hemicellulose components during the fractionation process. Organosolv pretreatment can meet the requirement due to the selective dissolving ability of the solvent employed in the process. In addition to the cellulose-rich materials, it produces organosolv lignin and hemicellulose sugars that is environmentally friendly.

The hemicellulose sugars recovered from the water-soluble stream can be con­centrated for fermentation using special organisms to convert the five-carbon aldose to ethanol or other products [33]. Organosolv lignin is a suitable feedstock for the production of phenolic resins/adhesives [73], antioxidant [74], bio-based polymer

3.04% mannose, 0.93 % galactose, 0.48 % arabinose)

composites [75], and even hydrocarbon products for blending with gasoline [76], owing to its unique high purity, low molecular weight, and abundance of reactive groups [21, 77].

However, the water-insoluble property of organosolv lignin may limit its appli­cations [78]. Therefore, conversion of the lignin dissolved in the organic solvents by catalyst or organic-solvent-stable enzymes may be a potential technological approach to resolve the problem [79-81].

16.3.1.1 Milling

Milling is a mechanical process of pretreatment that breaks down the structure of lig — nocellulosic materials and reduces the crystallinity of the cellulose [14]. During ball milling, biomass is grounded with the contact of the balls inside a cycle machine to get the uniform particles size [25]. This method can be considered environment-friendly due the absence of added chemicals in this process that produce toxic substances

[14] . A disadvantage of milling is the high power required by the machines and consequent high energy costs [14]. Buaban et al. [26] reported that the increase of the time of milling increased the amounts of the sugars (glucose, 89.2 ± 0.7 % and xylose, 77.2 ± 0.9 %) after 4 h of milling.

16.3.1.2 Irradiation

Gamma-rays-mediated pretreatment is irradiation pretreatment which allows the breakdown of beta-1, 4glycosidic linkages, thus enhancing the surface of area and crystallinity of cellulose [16]. It is a physical pretreatment which increase the surface area, consequently reducing the crystallinity. This method is expensive for large-scale operations with considerable environmental and safety concerns [16].

16.3.1.3 Microwave Irradiation

The use of high-energy radiation such as microwave causes one or more changes in the characteristics of cellulosic biomass, such as an increase in surface area, reduction in the degrees of polymerization, and crystallinity of cellulose and hemicelluloses, and the partial depolymerization of lignin [27]. However, irradiation pretreatment have the disadvantage of high energy consumption, and the methods are slow and expensive. Microwave irradiation process acts under the structural change in cellulose with the occurrence of the lignin and hemicellulose degradation, thus increasing the enzymatic accessibility [14]. To further improve the sugar yield after pretreatment, microwave radiation process was combined with chemicals. Binod et al. [28] tested different microwave pretreatment conditions for SB and reported highest reducing sugar yield (0.83 g/g dry biomass) in the microwave-alkali pretreatment followed by acid pretreatment.

Pretreatment of feedstocks with adsorbents such as magnesium silicates (such as Magnesol 600R, The Dallas Group, USA) were found to be very effective in remov­ing FFAs. D-SOL has been introduced successfully at commercial scales for food frying operations to remove FFAs from frying oil. At 2 % additive concentration, the Magnesol 600R reduced FFAs from 3.8 % to around 1.24 %. When a blend of chicken fat and vegetable oil with FFAs concentration of 1.45 % was tested with Magnesol, all concentrations of the 600R product reduced the FFAs to below 1 %. This means, the 600R is a low-cost solution for reducing FFAs levels of <4.0 % which works out to be about 5 % per gallon per 1% FFAs reduction [35].

Bentonite clay on the other hand, reduced calcium, magnesium, and phosphorus in feedstocks better than the Magnesol 600R. If a plant relies on a proprietary catalyst, the producer is tied-in to one manufacturer. It is possible that as the market matures different catalyst manufactures will offer drop-in alternatives [36].

18.6.1 Immobilized Enzyme-Catalyzed Reduction of FFAs

Enzyme is a new biocatalyst to the biodiesel industry. Lipases belonging to the enzyme group of hydrolases are capable of converting FFAs in an esterification reaction with methanol to biodiesel and water byproduct. If used properly, the use

Table 18.2 Advantages of the enzymatic process over the chemical process Chemical Enzymatic

of lipases is cost effective and environment friendly. Lipases can be easily used for lowering the FFAs in different feedstock through esterification with methanol to form FFAs and water [37]. The use of such type of biocatalyst would provide an elegant solution for reducing the environmental impact of yellow grease collected from restaurants, brown grease (>90 % FFAs) and fat collected in municipal and industrial waste-water treatment plants [38, 39].

Most recently TransBiodiesel, Israel has developed and commercialized unique immobilized biocatalysts for the conversion of crude and low-grade feedstocks to biodiesel. The developed biocatalysts are capable of converting any grade of veg­etable oil and animal fat to biodiesel with minimal waste products [40, 41]. The biocatalysts would act on any oil feedstock with any level of FFA-containing oil including crude vegetable oils, vegetable oil distillates, yellow and brown greases, and virgin oil, and to reduce their FFAs content to lower than 1 %. These feedstocks with high FFAs levels are much cheaper feedstocks than virgin plant oils (40-60 % cheaper). It is estimated that 20-40 % of the operational costs alone can be saved when dealing with the enzymes developed by TransBiodiesel (www. transbiodiesel. com).

The proposed enzyme technology offers biodiesel manufacturers flexibility in their choice for feedstocks which might contain FFAs in the range of 0-100 %. It allows biodiesel manufacturers to expand their feedstocks selection from expensive virgin oil (approx $1,100/t) to yellow grease ($700/ton) to inexpensive brown-grease feedstock obtained from waste-water treatment plants ($300/t). The major advantages of the enzymatic process over the chemical processes are summarized in Table 18.2.

It has been demonstrated that feedstocks need not be FFAs free in the enzymatic process, and de-hydrated feedstock is not a requirement as in the case of the chemical process. Operating at a relatively low temperature and with no need to neutralize acid, TransBiodiesel’s enzymatic process produces remarkably clear biodiesel and high-quality glycerol that needs little refining because enzymes are used at room temperatures (20-30°C) without any other acids or bases.

TransBiodiesel has two main enzymes TransZyme and EsterZyme. TransZyme is an immobilized lipase of high transesterification as well as esterification activity. TransZyme is capable of converting any type of feedstock, including virgin oils, crude plant oils, animal fats, waste-cooking oils, acid oils, and brown grease, regard­less of the FFAs content (0-100 %), to form biodiesel through transesterification and esterification processes simultaneously [40, 41]. TransZyme favors more transes­terification and esterification than hydrolysis even in the presence of 1-10 % water. TransZyme is also capable of transesterifying phospholipids and wax esters to form biodiesel and free alcohols allowing the use of crude unrefined vegetable oils. Due to the capability of the developed biocatalyst to transesterify phospholipids the overall yield of biodiesel production from crude plant oils would be increased by 1-3 %.

EsterZyme is an immobilized lipase of high esterification activity. It transforms free fatty acid in the presence of methanol (or other alcohols) and under reduced amount of water (preferably below 0.5 %) and glycerol into biodiesel and water byproduct [40, 41]. Furthermore, EsterZyme exhibits relatively high transesterifi­cation activity toward partial glycerides and wax esters and lower activity toward triglycerides. The biocatalyst can also be used for lowering the FFAs% in any type of feedstock down to 0-2 % starting from any type of feedstock containing FFAs from 3 % and up to 100 %.

Both enzymes developed by TransBiodiesel are suitable for use in batch and continuous reactors using stirred tank or packed column reactors (Fig. 18.6). While many plants using acid esterification and de-gum their feedstock, TransBiodiesel’s technology uses crude feedstock without resorting to de-gumming since gums don’t interfere with the enzymatic step.

The use of a catalyst for gasification is not essential, but it can increase gasification efficiency by reducing tar content or other unpleasant products, such as methane. The application of a catalyst promotes tar cracking at lower temperatures or promotes a steam-reforming reaction, which is a reaction between methane and steam in the temperature range of 700 °C-1,100 °C to produce syngas. The catalyst can be used directly in the gasifier or the secondary reactor downstream of the gasifier [47, 48]. There are different criteria for developing or choosing a proper catalyst, such as being inexpensive, effective, and resistant to attrition, carbon fouling, and sintering. Catalysts used in tar cracking can be classified into three main groups: alkali metals, non-metallic oxides, and supported metallic oxides.

The pyrolysis conditions affect the global kinetics by promoting specific elemental reactions. The main operational parameters for pyrolysis are temperature (and heating rate), pressure, co-feeding of different feedstocks and presence of catalysts.

11.3.3.1 Temperature and Heating Rate

Pyrolysis is governed by many parallel and series reactions characterized by their specific kinetics and the relative importance of each of these reactions will depend on the temperature of the system [41, 42]. Also, a slow heating process implies that the biomass remains at every temperature for a longer time period. As pyrolysis kinetics and heat transfer compete, pyrolysis occurs during the heating of the particles and might even be completed (at thermodynamic equilibrium) before reaching the temperature set-point. At low heating rate, more decomposition happens at low temperature such that more bio-char and less volatiles (condensable (bio-oil) and non-condensable gases) are produced.

One of the main mechanisms controlling the interaction between the temperature and the heating rate is the stabilization and reorganization of the macromolecular solids. Thermal decomposition brings lighter molecules to unbind from the solids (biomass or waste in the present case) to form a volatile phase. In parallel, this creates physicochemical instabilities that lead to a molecular rearrangement. The kinetics associated to these intra-molecular modifications then inhibits the volatile formation kinetics. If the heating rate is slow, stabilization occurs and higher char yield is obtained. On the other hand, heating faster will impede stabilization and volatile production will be promoted. Temperature has a different effect on the pyrolysis products. Char production decreases with increasing temperature and the yield of gas increases (both condensable and non-condensable). The extent of the gas thermal cracking determines the yield of non-condensable gas and average molecular weight of the volatile fraction. Thermal cracking kinetics becomes important with increasing temperature and gas residence time.

The important stage of experiment is determination of moisture and composition of investigated fuel. The fuel moisture is determined by the method of long-time drying at air ambient of 105 °C. The total carbon and hydrogen content in a fuel is determined by ISO 625-96 technique.

Before the experiment, the gasifier shaft is completely filled with char coal. This kind of solid fuel is the most suitable for a stage of preliminary heating up as it possesses the low content of volatile matter.

Ignition of one of the gasifier’s plasma torches is considered as the beginning of the experiment (timing is counted from this point). Supply of air plasma improves heating and firing of fuel. Heating up the reactor shaft takes at least 8 h and comprises several stages differing by oxidant flow rate. Gasifier lining has very high thermal inertia that is why practically all experiments are carried out at quasi-stationary mode.

The analysis of syngas composition begins at the final stage of preheating and the charcoal feeding is replaced by the investigated material feeding. The transient process from the charcoal gasification to the investigated fuel gasification starts when the investigated material reaches drying and pyrolysis zone. The transient process could be considered completely finished when the whole gasifier shaft is filled by this fuel and the products of its gasification. When the experimental program is completed the fuel feeding into the reactor stops and starts after-burning of fed materials and their remnants, and when they are burned out the installation cools.

Experiments on plasma gasification have been carried out for such fuels as char­coal, wood (of various types: woodchips, pressed sawdust, and chocks), coal, lignite, Refuse Derived Fuel (RDF), and others.