Gasification output greatly depends on the properties of the feedstock used, as well as on the operating conditions. Some of the factors are listed below:

1

Ultimate analysis of the feedstock: This determines the chemical composition of the type of fuels. Figure 16.4 shows the C-H-O diagram, which demonstrates that liquid fuel only consists of carbon and hydrogen. In solid fuel, carbon and charcoal have less oxygen and biomass has a higher percentage of oxygen. It

also shows the transition from gasification to combustion. The line H2O-CO2 is the axis line for combustion; beyond this all fuel gets combusted. The H-CO line is the axis line for gasification.

2 Moisture content, volatile matter, and ash content of the feedstock: Through proximate analysis of the fuel one can identify the moisture content, volatile matter, and ash content of the feedstock. Fuel with low moisture content is desirable because higher moisture content requires more energy to evaporate liquid forms of moisture. In other words, for a given heat input, high moisture fuel will result in a lower temperature, which will effect the composition of gas produced, resulting in a lower heating value gas. Quaak et al. (1998) suggested that, for downdraft gasification, the moisture content of the feedstock should be less than 25%.

During pyrolysis, the volatile matter in a feedstock is released. This volatile matter mainly consists of organic compounds, commonly known as tar. Tar is classified as primary, secondary, and tertiary. The change of tar from one form to another depends on the temperature. Higher temperatures result in tertiary tar. Tar generally condenses in the cooler part of the gasifier and poses many operating problems, like choking of the pipes. If tar cannot be cracked well, it will cause a problem when the producer gas is used in an engine. Thus, a lower volatile feedstock is better suited for engine applications. However, proper design of the gasifier could also help in tar reduction; for example, using a two-stage gasification process could potentially reduce tar because of a higher oxidation temperature (Bhattacharya et al., 2001).

Ash is the mineral content in fuel or feedstock remaining after combustion. In the gasification process, the remaining material is not only ash but also unburned carbon. Ash interferes with the gasification process in two ways:

(a) It fuses together to form slag and this clinker stops or inhibits the downward flow of the biomass feed in a moving bed gasifier. Even if it does not fuse together, it could offer mass transfer resistance to fuel particles undergoing gasification.

(b) Some inorganic constituents of the ash have an important catalytic effect on the gasification reaction rate of the char.

High ash content feedstock means the ash must be continuously removed from the gasifier. In addition, it is possible that agglomeration can take place inside the gasifier when using high ash content feedstock. However, the melting point of the ash depends on the mineral compositions in it.

3 Size and size distribution of feedstock: The size and size distribution affects the pressure drop through the bed of the gasifier. Low particle size can increase pressure drop across the gasifier. However, large feedstock particles also need more time for complete gasification. Moreover, obstruction of the feedstock flow can take place in the case of using large feedstock particles in a moving bed gasifier.

4 Bulk density of the feedstock: Fuel with higher bulk density is preferable because it has a higher energy content per unit volume, and is easier to transport and handle. Low bulk density fuel may create a problem with improper flow under gravity, mostly in the case of a fixed bed gasifier. This improper flow may result in low calorific value gaseous fuel, difficulty in transporting it from one place to another, and fuel handling that requires a larger space.

5 Energy content of feedstock: The energy content refers to the heating value that affects the energy output of the gasifier. Considering the energy balance, if an adiabatic gasification process is assumed, the reaction temperature of the process depends on the heating value of the feedstock used. For charcoal downdraft gasification, the temperature in the combustion zone is higher than 1100°C, compared with 970°C in the case of wood gasification using the same reactor (Bui, 1996).

6 Temperature: Temperature governs most of the reactions taking place during gasification and the composition of the output completely depends on it. High temperatures — above 800°C — favors the water shift reaction, resulting in higher carbon monoxide in product gas, while temperatures around 650- 800°C favor water gas shift reactions, resulting in higher hydrogen production for the case of steam gasification.

7 Reactor type: The choice of the types of reactor depends on many factors, one of which could be the type of fuel to be handled. If the fuel is low quality coal, then an entrained bed gasifier can be used. For a wide variety of fuel, a fluidized bed gasifier can be used as it can handle different types of fuel in a wide range of particle sizes. However, if the fuel is of low bulk density and high moisture content and the application small scale, then a fixed bed gasifier will be suitable.

19.4.1 FT catalysts

The main requirement for a good FT catalyst is high hydrogenation activity in order to catalyze the hydrogenation of CO to higher hydrocarbons. The only metals with sufficiently high hydrogenation activity to warrant application in FT synthesis are four transition metals of the VIII group of the periodic table: Fe, Co, Ni and Ru. Although Ru exhibits the highest hydrogenation activity, its extremely high price and low availability render it unsuitable for large-scale applications such as the FT process. Nickel, on the other hand, is essentially a methanation catalyst, its application leading to the undesired production of large amounts of methane. Therefore, Fe and Co are the only industrially relevant catalysts that are currently commercially used in FT. The choice of catalyst depends primarily on the FT operating mode. Fe-based catalysts are suitable for the high temperature Fischer-Tropsch (HTFT) operating mode that takes place in the 300-350°C temperature range and is used for the production of gasoline and linear low molecular mass olefins. Both Fe and Co catalysts can be used for the low temperature Fischer-Tropsch (LTFT) that operates in the 200-240°C range and produces high molecular mass linear waxes (Dry, 2002). Moreover, the choice of metal also depends on the feedstock used for the FT synthesis. As Fe, unlike Co, catalyzes the WGS reaction, it is usually used for hydrogen-poor synthesis gas, most especially that from coal (~0.7 H2/CO molar ratio), to increase via the WGS reaction the hydrogen content of syngas to the optimum 2 H2/CO ratio of the FT reaction. Cobalt is, therefore, the catalyst of choice for GTL processes, using natural gas as feedstock. Whether the catalysts are Fe or Co, FT catalysts are notorious for their sensitivity towards sulphur and their permanent poisoning by sulphur compounds. As aforementioned, syngas requirements for FT synthesis ask for a sulphur content of below 0.05 ppm (Dry, 1990).

An extensive amount of research has been performed on several aspects of the Fe and Co catalysts, including fundamental, basic and applied research. These efforts include investigation of the effect of promoters, supports, additives, pre­treatments, preparation and generally all chemical and physical properties of the materials in order to increase catalyst activity, enhance selectivity to the desired products, inhibit formation of unwanted products, especially methane, and improve resistance to sulphur poisoning. A summary of improved, modified Fe and Co catalysts employed in industry for the FT process is presented in Table 19.2 (Bartholomew, 1990).

Iron catalysts

Iron-based catalysts are used in both LTFT and HTFT process modes. Precipitated iron catalysts, used in fixed bed or slurry reactors for the production of waxes, are prepared by precipitation and have a high surface area. A silica support is commonly used with added alumina to prevent sintering. HTFT catalysts for fluidized bed applications must be more resistant to attrition. Fused iron catalysts, prepared by fusion, satisfy this requirement (Olah and Molnar, 2003). For both types of iron-based catalysts, the basicity of the surface is of vital importance. The probability of chain growth increases with alkali promotion in the order Li, Na, K and Rb (Dry, 2002), as alkalis tend to increase the strength of CO chemisorption and enhance its decomposition to C and O atoms. Due to the high price of Rb, K

is used in practice as a promoter for iron catalysts. Copper is also typically added to enhance the reduction of iron oxide to metallic iron during the catalyst pre­treatment step (Adesina, 1996). Under steady-state FT conditions, the Fe catalyst consists of a mixture of iron carbides and re-oxidized Fe3O4 phase, active for the WGS reaction (Adesina, 1996).

21.2.1 Bioethanol

Current bioethanol production technologies are based on the conversion of carbohydrates derived from sugar cane, sugar beet, maize or cereals (i. e. wheat, barley) into ethanol. In addition, bioethanol can be derived from a number of other agricultural commodities such as cassava, or from residues or waste streams from other agro-industrial processes, including cane or beet molasses and starchy residues.

A number of by-products or co-products are produced during the conversion of biomass to ethanol. Most prominent by-product from ethanol production from corn, wheat or barley is so-called DDGS, which is a protein-rich fibrous residue that is primarily sold as animal feed. DDGS is formed by combining insoluble residues from the fermentation step with soluble residual streams from the distillation step, and drying the combined product. The market price of DDGS devaluated in the last 20 years due to increased production volumes saturating the feed market. Other high added-value products need to be found for DGGS to maintain its co-product status.

A common by-product of sugarcane derived ethanol is bagasse, which is the fibrous residue of the sugar cane stem after extraction of soluble sugars. Bagasse is commonly used to generate electric power and heat at the sugar mill facility to supply the energy needed for the bioconversion process.

Upgrading of process residues like DDGS and bagasse to higher added-value bio-based products (i. e. chemicals, materials) — turning the processes into biofuel — driven biorefineries — maximises the sustainable valorisation of the raw biomass materials, increasing the market competitiveness of the bioethanol produced. DDGS is high in protein content (over 30%) which, if isolated, can be used potentially for the production of chemical precursors (Brehmer, 2008). Bagasse can also have many other applications such as the production of fibre boards or the production of high added-value specialty chemicals, i. e. xylitol from xylose-rich effluents from acid hydrolysis of sugarcane (Baudel, 2005).

Another by-product of bioethanol production is CO2, which in certain cases is marketed as gas for industrial use.

Properties of biodiesel B100 produced under industrial pilot scale at the Institute of Industrial Chemistry are given in Table 23.6. It is shown in this table that the

produced biodiesel B100 meets all requirements of Vietnam standard on Biodiesel B100 (TCVN7717-07).35 The cloud point of 10°C of biodiesel B100 requires an additive to reduce for storage in biodiesel ‘neat’ form; however, within the pilot scale production, the fuel is stored in B5 form so this matter was not mentioned.

Following a specific blending procedure, biodiesel B5 blend (5% B100 and 95% market diesel) was produced. This biodiesel B5 meets almost all limits of the proposed Biodiesel B5 standard described in Table 23.7. In addition, due to low percentage of biodiesel B100 in the mixture, the B5 fuel has quite close properties with those of market diesel and standard limits of petrodiesel given in TCVN 5689-2005.36 The cetane number, flash point and kinematic viscosity are in turn of 54, 79 and 3.91, slightly higher than those of market diesel (in turn of 51, 78 and 3.87). These properties of Biodiesel B5 analysed within the mentioned national research project have contributed remarkably for Directorate for Standards, Metrology and Quality to develop B5 fuel standard in Vietnam.

For many years it has been known that acid sites are required for catalytic cracking and in the work of Thomas these were thought to be predominantly formed at aluminium sites connected through oxygen ions to silica.145 These sites are known to promote carbonium ion mechanisms and are probably the major reason for lowering the pyrolysis temperature. Such sites are not present on alumina. However, activated alumina has become an important adsorbent and catalyst and contains both Lewis (electron pair accepting) and Brpnsted acid (releases H+) sites. It is made by thermal de-hydroxylation of Boehmite, an aluminium oxide hydroxide (y-AlO(OH)) and yields a highly porous powder of surface area > 200 m2/g. The product alumina is normally in the у or n crystal structure which are conducive to generating high surface areas.162 The main role of activated alumina catalysts appears to be de-hydroxylation of hydrocarbons.163 This has been shown for the catalytic pyrolysis of a series of vegetable oils where the products were essentially linear hydrocarbons containing no oxygen.164 Chang et al. have reported that alumina has poor cracking and hydrogenation ability, consequently, the yield of low molecular weight products from wood biomass is small.136 For a study of the effect of a catalyst on the products of thermal pyrolysis, Samolada et al. studied a range of catalysts on the treatment of a synthetic bio — oil.133 They found that whilst y-alumina had little cracking ability beyond that expected of thermal treatment it did, however, notably improve the quality of the pyrolysis oil (via de-hydroxylation). a-alumina (low surface area) used as a catalyst exhibited little of this improvement of the bio-oil material.

16.6.1 Designing of fixed bed gasifier

One-dimensional modeling is generally employed to study the fixed bed gasifier. It is not only simple but also provides a better understanding of the engineering design and process optimization for the fixed bed gasifier. The modeling of updraft gasifier explained below is taken from De Souza (2004).

Figure 16.12 shows the model chart for the updraft gasifier. Here the gasifier is divided into two segments: gas and solid, flowing in a counter current direction.

There is continuous exchange of mass and heat along the interface of these two phases in the radial direction. The flow of each phase is assumed to be in plug flow mode. Also, there is no momentum transfer between the phases, which means the velocity profile of one phase is not affected by another. Thus, for the model chart developed, the following equation can be written to represent the mass and energy balance.

Mass balance for gas

.. dP,

Mass balance for solid:

where RMGj and RMSj is the rate of production if positive and the rate of consumption if negative. p represents the density of the respective phase. u represents the velocity of the respective phase.

The mass flux of chemical species j in gas in the z or axial direction is given by

And the mass flux of chemical species j in the solid is given by

where SS and SG are the fraction of area available for the flow of the solid and the gas phases, respectively, and can be defined in terms of the voidage of the reactor,

e=lk, A-I, A = _L.

V ’ Sa є Ss~ 1-е

where VG is the volume occupied by the gas and V is the total bed volume.

The energy balance equation for the gaseous phase and the solid phase can be written as:

Fccc dz — £S(f<j. g + Rc. c + R*.C + R«.c)’

Fscs dz — 0 £^{Rq, s + Rc„s + R/ij + R/u)’

where Rq is the energy source or sink, RC is the convective heat transfer, Rh is the enthalpy addition (or subtraction) from one phase to another due to mass transfer between the phases, Rr is the radiation heat transfer between the phases. Thus, solving equations [16.28] to [16.34], one can get the composition of the gas and also the conversion of the solid fuel.

BTL-FT diesel is a renewable fuel of excellent quality compared to both fossil — derived diesel and first-generation biodiesel produced via the transesterification of vegetable oils. BTL-FT synthetic fuel consists mainly of linear paraffinic hydrocarbons with almost zero aromatics and sulphur compounds. The physical properties of BTL diesel presented in Table 19.5 (Rantanen et al, 2005) demonstrate its very high cetane number that can reach up to 75, much higher than conventional diesel. The big advantage of BTL diesel is that it is directly usable in the present day in transportation sector, and furthermore, it may be suitable for future fuel cell vehicles via on-board reforming since it is free of sulphur. It is fully blendable with conventional diesel and compatible with current diesel

engines and with common materials used in the tank system and the engine components. This constitutes a great plus, as the fuel can be used today using the current distribution and retail infrastructure.

Due to its bio-origin, the BTL diesel has much lower CO2 emissions than fossil — derived fuels. Moreover, it shows considerably improved emission behaviour. BTL diesel fuels have been tested by Volkswagen AG and DaimlerChrysler AG in modern, state-of-the-art passenger cars, as part of the EU-funded IP RENEW project that explored technology routes for the production of B TL fuels (RENEW, 2008). The vehicles were equipped with different types of exhaust gas after­treatment system, oxidation catalytic converters (oxycats), which reduce CO and hydrocarbon emissions and are the most common technique in the existing fleet and additional diesel particulate filter (DPF), the after-treatment technology of future diesel passenger cars. The reductions of the different emissions with the BTL diesel compared to conventional diesel are summarized in Table 19.6. Great emission reductions were achieved with no special adaptation of the engine. The BTL diesel causes a significant reduction of CO and hydrocarbon emissions, a medium reduction of particulate emissions and only a slight reduction of NOx (nitrogen oxides) emissions. The next lines of the table present emission reductions with different after-treatment technologies and optimization of the engine operation with special software. It can be generally seen that a further reduction of particulates or a significant reduction of NOx can be realized. In general, the BTL diesel manages to reduce not only CO2 but also the emissions of most air pollutants. What is also important is that the BTL fuel exhibited at least the same fuel consumption as conventional fuels when compared on an energetic basis (RENEW, 2008). With adapted engines, the improved combustion process could also lead to better efficiency and thus reduced fuel consumption.

Economy-of-scale contributes to profitability because less investment is required per unit product manufactured. This economy-of-scale is often explained because the volume of a reactor increases with the power of three while the investment itself increases often with the power of two since this is dependent on the outer surface of the reactor itself (Lange, 2001). When major heat exchange capacities are required, the need for larger factories becomes apparent because the surface of heat exchangers needs to be in correlation with the volume of the reactor or in other words with the amount of heat that is produced in the volume of the reactor. If one could circumvent the need for heat exchangers, then the need for building large units will diminish and many small units can do the job that initially was done by the large factory. This will enable a totally different architecture of the processes and certainly of the logistics of biomass value chains.

Raw materials that contain a lot of water, and that are perishable, and before were not attractive to be transported to a large factory, can nowadays be (pre) processed at small scale. Cassave roots are a good example of this (Sanders et al, 2005). Ten small scale mobile factories of 4000 tons of starch product each are in operation at different locations in Nigeria. Also residues like beet leaf or carrot leaf can now be pre-processed, as is the case for meadow grass. It will be understood that for other crops that contain a lot of water like potato and grass, small scale processing will have a lot of advantages. Developments of small scale production of ethanol from corn is in progress in The Netherlands, where because of the reduction of unit operations that need heat exchange, the capital cost per litre of ethanol is not higher for these small units than for the large scale corn to ethanol plants in the USA, that operate at 100 times larger scale (Sanders et al., 2008).

The presence of an FCC catalyst solves the problems related to the thermal cracking of vegetable oils and animal fats. FCC catalysts are very effective in removing oxygen from biomass by transformation into CO2, CO and water without using hydrogen and allowing the control of the final product distribution. Thus, as mentioned above, the FCC unit of a refinery seems to be the most appropriate system for the co-processing of this renewable raw material. Moreover, physical properties of triglyceride-based feedstocks are close to those found in typical refining streams that are usually fed to the FCC unit (H/C mass ratio, density, viscosity. . .) as well as the fact of the high miscibility. Co-processing of triglyceride-based biomass in the FCC unit not only would help to achieve the bio-component target fixed by the EU directive (Commission Directive 2009/28/ EC) but also to the improvement of some properties in the final FCC products. Processing these renewable materials in a refinery would lead to a lower content in metals (such as nickel or vanadium) and heteroatoms (such as sulphur or nitrogen) in the final products due to the fact that this feedstock does not contain those metals and heteroatoms in their composition. Moreover, they are formed by paraffinic and olefinic hydrocarbons, more crackable than the aromatic compounds present in the typical streams usually fed to the FCC unit, which tend to remain as unaltered compounds in the low-value heaviest fractions. Other benefits would be a slight increase in the coke production, which could help to maintain the thermal balance between the reactor and the regenerator in the FCC unit; higher olefin production in the gas fraction, which favours the application of these compounds to produce polymers, alkylates and tertiary ethers; and an increase in the amount of gasoline and in its octane number due to enhancement of aromatization reactions and olefins production.

S. R.A. KERSTEN and D. KNEZEVIC, University of Twente, The Netherlands and R. H. VENDERBOSCH, BTG Biomass Technology Group B. V., The Netherlands and University of Twente, The Netherlands

Abstract: The topic of this chapter is hydrothermal conversion of biomass, a thermo-chemical technique especially suitable for conversion of wet biomass streams. The chapter deals with the process chemistry and the product distribution with special attention to de-oxygenation reactions and char formation. Furthermore, the main characteristics of the reaction products are discussed. The chapter shows typical process layouts and gives a brief historical overview of the research and the status of the most important industrial and laboratory scale activities. Finally, a critical view on the status of the technology is offered.

Key words: hydrothermal conversion, liquefaction of biomass, hot pressurized water, deoxygenation, char formation.

18.1 Introduction

Hydrothermal conversion (short form ‘HTC’) is a thermo-chemical conversion technique in which sub — or super-critical water is used as a reaction medium and/ or as a solvent. Although principally all biomass can be used as feedstock, wet (biomass and waste) streams are the obvious choice of feeds from an energetic point of view. The proposed operating regime is broad and ranges from 250°C to 450°C and from 80 to 300 bar, and depends on the type of products it is aimed at. HTC can be used for gasification and liquefaction (see Fig. 18.1), catalytically and non-catalytically. By using (noble) metal catalysts, complete conversion of biomass to methane-rich gas can be reached under hydrothermal conditions. At more severe temperatures (500-700°C), hydrogen-rich gas can also be obtained. Gasification in hot compressed water is discussed in Chapter 20 of this book.

In this present chapter, we will focus on hydrothermal liquefaction (short form ‘HTL’), the conversion of wet streams into condensed products. Typically, HTL is carried out in sub-critical water (temperature < 374°C). Under these conditions, biomass is converted into various components, which, upon cooling to ambient conditions, constitute three different phases: an aqueous phase (comprising water plus dissolved organics), a hydrophobic phase and a gas phase. Extraction of the hydrophobic reaction product results in a solvent-soluble (oil) and a solvent-insoluble part. Acetone is often used as a solvent. The solvent-insoluble product has a char-like appearance

and is solid at room temperature. It is a direct remainder of the feedstock (char formed in the pyrolysis reactions) and a product of secondary reactions of liquid products.1,2 The hydrophobic product has a considerably lower oxygen content (typically 10-30 wt.%) and, consequently, a higher heating value than the feedstock. A direct application of HTL oil (short form ‘HLO’) and solids is as a fuel. In this process option, HTL yields hydrophobic organic products that are easy to separate from the water phase and can be fed as fuel in boilers, furnaces or gasifiers.3,4 After fractionation by suitable solvents, the solvent-soluble fraction (oil) is considered for upgrading to transportation fuel precursors, for example by catalytic hydro-deoxygenation.5-8

For detailed overviews on HTL, the reader can refer to Moffatt and Overend,8 Bouvier et al.,9 Stevens,10 Solantausta et al.,11 Venderbosch et al.12 and Peterson et al.13

In this chapter, the process layout, chemistry, product characteristics and product distribution are further detailed. Process development and demonstration activities are described as well as the current research focus. The chapter ends with conclusion and future prospects.