In order to combust the feed that is supplied to the gasifier, there must be a continuous addition of heat in the gasifier. To determine the energy added an energy-balance must be made.

Two losses can be pointed out. The synthesis gas will leave the gasifier with a certain temperature and there will be losses through the walls of the gasifier. The loss through the wall depends on the insulation. This loss depends only on the thickness of the insulation-layer and the choice of insulation-material. Because of the high-temperature at the gasifier-wall, the insulation-layer must be built with two components. Directly at the gasifier-wall, a small layer of ceramic wool (~25 mm) must be positioned. The layer around the ceramic wool, consisting of rock wool will be placed to reduce heat-loss through the wall further. The second energy loss consists of the energy taken out by the synthesis gas. Both losses have to be compensated for by the energy generated in the gasifier, usually by the heat of reaction (combustion of the feed). This means for every ton of feed put into the gasifier, between 32.6% and 39.4% is needed to compensate the losses (Van Kasteren et al., 2005).

For pyrolysis oil already some refinery schemes have been proposed that include reforming. Pyrolysis oil can in principle be steam-reformed directly via gas phase reforming (pressure 1-30 bar) in a stand alone application. However, more integrated process schemes would allow synergy and more favorable economics for both gas phase and high-pressure (liquid or supercritical) reforming. Three examples are briefly discussed below.

Co-reforming of biomass in fossil fuel based reformers

For reducing the net CO2 emissions of commercial gas (natural and associated) and naphtha reformers and lowering the investment costs and implementation barrier, co-reforming of biomass and a fossil feedstock seems very attractive. Pyrolysis oil (or its fractions) and glycerol (from bio-diesel production via transesterification) are interesting candidates as they can easily be pressurized and are relatively clean feedstocks. Biogas (essentially CH4 and CO2) from digesters could be interesting only if they are available in significant quantities at the location of the reformer. The installation of a dedicated pre-reformer for the bio-feedstock would be desirable for the following reasons: (i) minimal changes have to be made to the existing reformer since a methane and hydrogen-rich gas is being fed instead of the original oxygenated compound. (ii) Since co-reforming is considered, the full amount of steam needed for the reformer can be fed to the pre-reformer. The pre-reformer can then operate on very high S/C ratios which is beneficial for the gas chemical equilibrium and minimizes coke formation for which oxygenated compound have a higher tendency to compared to its fossil counterparts. (iii) Impurities like sulfur in the bio-based feedstock are bound to the pre-reformer catalyst acting as a guard bed for the subsequent reformer. Fossil fuel impurities are removed prior to entering the reformer, but this is often not possible for the bio-feedstock due to its high reactivity. In this way, only the pre-reformer catalyst has to be regenerated

periodically. A conceptual scheme of bio-liquid co-reforming is given in Figure 20.4.

22.4.1 Introduction

For the recovery of minor components a lot of research has been reported. However, industrial applications of the described processes are not much published, except for in the patent literature.

In the production of biodiesel/biofuel from DD, its heterogeneous composition (FFA, acylglycerols, sterols, squalene and tocopherols) should be considered, not only for the selection of a good conversion process but also for the valorization of all the compounds which would make the process economically more interesting.

For their purification, different aspects should be considered. Since bioactive compounds, such as tocopherols, phytosterols and squalene, are minor components in DD, their enrichment is vital before they can be effectively fractionated and separated into an individual compound. The main challenge is to separate them from each other, especially in the case of the following pairs of components: tocopherol-squalene, tocopherol-fatty acids, tocopherol-sterol and sterol-squalene.

Numerous methods have been proposed for treating DD to isolate one or more compounds. In general, the selective separation of compounds in DD is based on differences in their chemical and physical properties such as solubility, polarity, molecular weight or differences in volatility.

Molecular distillation or short-path distillation is by far the preferred method for isolating both thermosensitive and high molecular weight compounds (Top et al., 1993; Shimada et al, 2000; Watanabe et al., 2004; Martins et al., 2005; Nagao et al., 2005). However, the disadvantages of this method are that the equipment is expensive and the operation cost is high. Furthermore, the sterols are one of the major components of DD and cannot be removed by molecular distillation because their molecular weights and vapor pressure are similar to those of tocopherols. A conventional method to concentrate tocopherols using molecular and vacuum distillation after removing the sterols via alcohol recrystalization requires several steps such as solvent recovery and purification, and further requires high amounts of solvents and energy.

Other separation processes are based on differences in their polarity like solvent extraction (Brown and Smith, 1964; Chu et al., 2002; Gunawan et al., 2008; Leng et al., 2008). The advantage of solvent extraction over molecular distillation is that it operates under atmospheric pressure and lower temperature, and requires simpler equipment. Although modified solvent extraction is capable of separating tocopherols, free phytosterols, FFA and acylglycerols from DD on laboratory scale, it is difficult to apply this method in large-scale operations.

Other processes involve supercritical fluid extraction (Lee et al., 1991; Bondioli et al., 1993; Stoldt and Brunner, 1998; Stoldt and Brunner, 1999; Chia-Cheng et al., 2000; King and Dunford, 2002; Mendes et al., 2002; Mojca et al., 2003; Nagesha et al., 2003; Pereira et al., 2004; Liu et al., 2006; Vazquez et al., 2006; Fang et al., 2007; Vazquez et al., 2007; Fornari et al., 2009; Torres et al., 2009; Martinez-Correa et al., 2010), crystallization (De Areal Rothes and Verhe, 2005; Pan et al., 2005), crystallization and/or membrane separation (Lin et al., 2007), treatment with urea (Sampathkumar, 1986; Maza, 1992) and batch adsorption (Chu et al., 2004; Fabian et al., 2009).

Solvent extraction and crystallization are mainly used to recover sterols over tocopherols. This process has the advantage of not causing tocopherol oxidation

and does not use high pressure, but the amount of solvent required is still very high and the use of such quantities does not lead to an environmentally friendly process. It is also noted that extraction with solvent requires laborious manipulations. Research in this area is not very extensive due to the low recovery and low purity of sterols and tocopherols (Lin and Koseoglu, 2003; Moreira and Baltanas, 2004).

Besides the target compounds (sterols, tocopherols and squalene), the DD contains acylglycerols and FFA which make the purification more difficult. Consequently, it is necessary to modify DD with esterification and/or alcoholysis reactions using chemical or enzymatic means, which convert most of the fatty acids, free sterols and acylglycerols to FAME and esterified sterols.

A schematic representation of the most used pathway to separate the minor components is shown in Fig. 22.4.

The main barriers for applying fermentative hydrogen production as outlined in any economic analysis are the low yield of hydrogen, low production rates and the cost of the feedstock. In order to make the biohydrogen economy viable there is a major challenge to increase the yield and production rates through:

1. Overcoming the light saturation effect. In the light-driven processes the conversion efficiency of the solar energy to hydrogen is estimated to be 10% (Hallenbeck and Benemann, 2002). This estimation is considered to be optimistic since it is based on data obtained under low light conditions that favor the dark reactions, the rate of which is limiting. Under full sunshine, the mechanism for electron transfer in algae is ten times slower than that of the light capture. As a result 90% of the photons captured decay as heat or fluorescence. The so-called light-saturation effect also applies to the photosynthetic bacteria. To overcome the light-saturation effect, the efficiency of the process can be increased through application of suitable mixing patterns that would reduce the time of exposure to the intense light and increase the nutrients transfer, design of reactor configurations that would dilute the light fall on the algal surface, as well as development of algal mutants that would absorb and waste fewer photons (Hallenbeck and Benemann, 2002).

2. Improving the dark fermentation technology. Rapid gas removal and separation as well as bioreactor design enhance the yield and production rates of hydrogen. To keep CO2 and H2 at low concentration, rapid removal of these two gases is required and H2 purification to concentrate and remove CO traces that would contaminate PEMFCs is necessary. Techniques of removal of H2 and CO2 have already been presented in Section 13.3.6.

3. Improving the CO-water shift reaction. Levin et al. (2004) consider the CO­water shift reaction carried out by certain heterotrophic bacteria promising. However, in the case of the CO-water shift reaction, the supply of the CO gas in a large volume reactor may require new bioreactor design to facilitate the mass transfer and contact between bacteria and the gas.

4. Integration of bioprocesses. Integrated strategies consist of two steps, with the first one being the fermentative hydrogen production, and the second one being either photobiological hydrogen production or methane production or MEC for hydrogen production as already discussed in Section 13.3.7.

Apart from the limiting factors concerning the biohydrogen process technology, there are other important parameters that affect the economy of hydrogen. For example, the limited availability of infrastructure for the transport, distribution and storage of hydrogen. The traditional options for hydrogen storage are cylinders of pressurized or liquid gas which is very problematic in the case of hydrogen gas. Although hydrogen has a very good ratio of energy to weight, it has a poor ratio of energy to volume compared to other fuels (hydrocarbons), therefore large tanks are required. Application of pressure to reduce the volume or liquefaction may result in smaller tanks but these technologies are energy consuming. On the other hand, for transportation use, storage meets limitations of volume and weight, while sufficient fuels must be available to secure the vehicle autonomy over long distances compared to the gasoline. Another option of hydrogen storage is the physical (adsorption on metal hydrides) and chemical storage (formation of alkali metal hydrides). Nanostructured materials are another promising alternative since they ensure high capacity. Transfer through a pipeline grid is another option but there is a question about whether the existing gas pipeline systems can be used for hydrogen supply. The quality and condition of the material of the pipeline should be checked since any metallic components may be affected by the hydrogen. Parts of the pipeline as the welds, valves and flanges should also be checked for their ability to hold the hydrogen.

13.4 Future trends

Today, hydrogen is used mainly in the petrochemical industry or as a feedstock in the industry, but not as an energy carrier. For this, the development of the supply sector (production and distribution-transportation-storage) and the end-user application should evolve simultaneously. The high investment costs required for this venture would be counterbalanced by strong driving motivations such as (Groot, 2003):

• The use of hydrogen in an efficient and clean electricity process such as PEM fuel cells.

• The use of hydrogen as a fuel for vehicles (storage of hydrogen is an issue, especially for the small compact vehicles).

• The use of hydrogen as an energy carrier through conversion of the electrical energy generated by renewable resources (solar energy, wind power) to chemical energy in the form of hydrogen (via electrolysis). This will replace the need to store large amounts of electricity directly.

Gasifiers have been employed worldwide for a wide range of feedstocks, such as coal, biomass, and various waste materials. Figure 16.1 shows the status of gasification, showing its present and planned capacities. At present the energy production from gasification is around 60 GWth. In coming years, the energy production from gasification is set to grow rapidly reaching around 150 GWth by 2014, which is 1.5 times more than what is used at present.

Figure 16.2 shows the application of gasification. Presently, the product gas from the gasification process is used for chemical production and makes little contribution to power generation. However, growth of gasification in the future is predicted to be towards power generation.

Gasification will be a ‘breakthrough’ technology, as it combines the economic advantages of coal with the environmental benefits of natural gas. Because of its

huge resources, coal will remain a primary energy for power generation. However, environmental concerns will restrict its use. A safe route for the power company will then be to gasify the coal and use the syngas for power generation. Development of IGCC will be seen to increase in the near future as it is proved to be commercially and technically more attractive than convectional power generation. In the coming decades, a first and second generation IGCC plant is projected to be in the market. Gasification also offers an opportunity to capture carbon dioxide at a significantly lower cost as compared with other fossil-fuel — based technologies.

Natural gas has been used for power generation but in greater part for chemical production. In the US more than 70% of chemicals are derived from natural gas

and every $1.00 increase in the cost of natural gas adds $3.7 billion in costs to the industry (http://www. clean-energy. us/facts/gasification. htm). At present, the demand in the US for natural gas has already exceeded the supply, which predicts there will be a rise in price and the chemical industry will look for an alternative option. The only way to replace natural gas is to produce syngas through gasification and then use it for chemical production instead of natural gas. Another major area of gasification will be production of hydrogen, which will eventually reduce the use of petroleum in vehicles.

Waste is considered no longer as a waste; rather it is looked at as an alternative energy source. A commonly used methods for disposing of municipal solid waste is incineration, which is facing huge criticism for not being environmentally friendly, as it emits high amounts of harmful dioxins, as well as NOx and SOx. Gasification, on the other hand, for being more efficient in breaking down hazardous dioxins and furans into simple gases, has already been seen as an alternative to incineration. Indirect co-firing using a biomass gasifier and then combusting the product gas in a boiler has given new direction to the co-firing system.

Gasification has the advantage of being fuel flexible, as it can take different types of fuel. That is important when fuel prices are volatile and its availability is not reliable.

A technical challenge to gasification could be tar formation. But with the use of a catalyst and some modification in design, the tar can be effectively controlled. With stringent environmental regulations, its cost advantages will be better.

19.3.1 Gasifiers

Gasification converts biomass into a gaseous mixture of syngas consisting of hydrogen, carbon monoxide, methane and carbon dioxide. The gasification of biomass is a crucial matter for the application of the BTL process, as BTL-FT technology has not been established mainly due to difficulties in syngas production/ cleaning-up from biomass. Moreover, almost 75% of the investment costs in a B TL plant are in the pre-treatment, gasification and gas cleaning section; therefore, the gasification pressure and medium greatly influence the economy of both gasifier and downstream equipment (Hamelinck et al, 2004). There are many technologies available for syngas production, as presented in Fig. 19.2 (Balat et al., 2009). Biomass gasifiers can be classified as air-blown, oxygen-blown or steam-blown, as atmospheric or pressurized, as slagging or non-slagging, as fixed bed updraft/downdraft, fluidized bed or entrained flow, and as allothermal (indirect heating) or autothermal (direct heating by combustion of part of the feedstock). A detailed description of the biomass gasification technology and the different types of gasifiers is given in Chapter 16 of the present book, which is dedicated to the production of bio-syngas via gasification. Therefore, attention in the present paragraph is paid to the gasification technology suitable for integration in a BTL-FT plant for the production of liquid fuels.

Fixed bed gasifiers have a relatively low throughput and therefore for large-scale applications, as in the case of BTL, with very strict requirements concerning the purity of the syngas, are considered unsuitable (Wang et al, 2008). On the basis of

throughput, complexity, cost and efficiency issues, circulating fluidised bed (CFB) (Hamelinck et al, 2004; Tijmensen et al, 2002; Wang et al., 2008; Zhang, in press) and entrained flow gasifiers (van der Drift et al., 2004) are very suitable for large — scale syngas production. Examples of CFB gasifiers employed for the gasification of biomass that have reached a certain degree of commercialization are the Lurgi CFB process, the Foster Wheeler gasifier, the VVBGC gasifier constructed under the EU-funded project Chrisgas, the UCG (Ultra Clean Gas) programmed by VTT, etc. (Higman and van der Burgt, 2008). Slagging entrained flow gasifier manufacturers are Shell, Texaco, Krupp-Uhde, Future-Energy (formerly Babcock Borsig Power and Noell), E-gas (formerly Destec and Dow), MHI (Mitsubishi Heavy Industries), Hitachi and CHOREN (formerly UET) (van der Drift et al, 2004).

The biomass gasification technology most close to commercialization for syngas production in a BTL-FT plant is the CHOREN Carbo-V patented biomass gasification process (Fig. 19.3). The process is a good example of the application of entrained flow gasifiers in the BTL process and is being used in the first demonstration, that is 15 000 tons per year BTL plant in Freiberg, Germany, coupled with the Shell SMDS FT process (Rudloff, 2005). The CHOREN Carbo-V patented gasification process consists of three stages: low-temperature, high-temperature and endothermic entrained-bed gasification (Rudloff, 2005). During the first stage, the biomass is continuously carbonized through partial oxidation with oxygen at temperatures between 400°C and 500°C, that is, it is broken down to a tar-containing gas (volatile parts) and solid carbon (char). The tar-containing gas is then fed to the high-temperature gasifier, where it is partially oxidized using oxygen as the gasification agent. The heat, which is released as a

result of the oxidation process, warms up the carbonization gas to temperatures that exceed the ash melting point of the fuels that have been used, that is 1300°C-1500°C. At these temperatures, any unwanted longer-chain hydrocarbons, for example tar and even methane, are broken down. The gas that is produced primarily consists of carbon monoxide, hydrogen, carbon dioxide and steam. The char from the low-temperature gasifier is cooled, ground down to pulverized fuel and is then blown into the stream of hot gas coming from the combustion chamber in the entrained flow gasifier. A huge amount of heat is absorbed when gasifying the char, and this allows lowering the temperature of the gas to 800°C-900°C in a matter of seconds. This ‘chemical quenching’ process produces a tar-free gas with a low methane content and high proportions of combustible carbon monoxide and hydrogen.

International Energy Agency (IEA) Bioenergy Task 42 has developed the following definition for biorefinery (IEA Bioenergy, 2010):

Biorefining is the sustainable processing of biomass into a spectrum of bio-based

products (food, feed, chemicals and/or materials) and bioenergy (biofuels, power

and/or heat).

This means that biorefinery can be a concept, a facility, a process, a plant or even a cluster of facilities.

A main driver for the establishment of biorefineries is the sustainability aspect. All biorefineries should be assessed for the entire value chain on their environmental, economic and social sustainability. This assessment should also take into account the possible consequences due to the competition for food and biomass resources, the impact on water use and quality, changes in land-use, soil carbon stock balance and fertility, net balance of greenhouse gases, impact on biodiversity, potential toxicological risks and energy efficiency. Impacts on international and regional dynamics, end-users and consumer needs, and investment feasibility are also important aspects to take into consideration.

A biorefinery is the integral upstream, midstream and downstream processing of biomass into a range of products. A biorefinery can use all kinds of biomass, including wood and agricultural crops, organic residues (both plant and animal derived), forest residues and aquatic biomass (algae and sea weeds). A biorefinery should produce a spectrum of marketable products and energy. The products can be both intermediates and final products, and include food, feed, materials and chemicals; whereas energy includes fuels, power and/or heat.

The main focus of biorefinery systems which will come into operation within the next years is on the production of transportation biofuels. The selection of the most interesting biofuels is based on the possibility that they can be mixed with gasoline, diesel and natural gas, reflecting the main advantage of using the already existing infrastructure in the transportation sector. IEA Bioenergy Task 42 has defined that both multiple energetic and non-energetic outlets need to be produced to become a true biorefinery. The volume and prices of present and forecasted products should be market competitive.

Generally, both Energy-driven and Product-driven Biorefineries can be distinguished. In Energy-driven Biorefineries, the biomass is primarily used for the production of secondary energy carriers (biofuels, power and/or heat); process residues are sold as feed (current situation), or even better are upgraded to added — value bio-based products to optimise economics and environmental benefits of the full biomass supply chain. In Product-driven Biorefineries, the biomass is fractionised into a portfolio of bio-based products with maximal added-value and overall environmental benefits after which the process residues are used for power and/or heat production for both internal use and selling of the surplus to national grids.

A biorefinery is not a completely new concept. Many of the traditional biomass converting technologies such as the sugar, starch and pulp and paper industry used aspects connected with a biorefinery approach. However, several economic and environmental drivers such as global warming, energy conservation, security of supply and agricultural policies have also directed those industries to further improve their operations in a biorefinery manner. This should result in improved integration and optimisation aspects of all the biorefinery subsystems.

Palm (Elaeis guineensis) is the most potential source of edible oils. Palm oil is now already produced and marketed in very large quantities, because it is edible and is high yielding (+/—3 ton/hectare/year). Direct injection (DI) diesel engine performance, exhaust gas emissions and some of fuel properties have been studied for biodiesel from CPO and Refined Bleached Deodorized Palm Oil (RBDPO), and these fuels blended with diesel fuel.21 It was found that both of biodiesel fuels and their blended fuels with diesel oil had increased BSFC levels, while the exhaust emissions (CO, CO2, HC and smoke) were better than for diesel fuel. Both DI and IDI22 engines were used for this research. These fuels were also used for a 2200 km fleet road test with two passenger cars and two trucks and compared with the performance of neat petrodiesel fuel.5 Parameters evaluated before and after road testing were fuel consumption, exhaust gas emissions, fuel injection equipment and engine lubricant.

Physic nut (Jatropha curcas) is one of the most potential sources of non edible plant oil. Physic nut seed oil is practically unexploited commercially, although it has the potential to replace or substitute palm oil as the raw material for biodiesel during the periods of high food sector demand.

The effect of biodiesel fuel from Jatropha curcas oil in DI diesel engines on the components of the engine influenced by fuel before (injection pump, injector) and after the combustion process (piston crown, cylinder head) was studied.23,25 The test bed procedure used was that commonly used for injection cleanliness evaluation adopted by World-Wide Fuel Charter (December 2002).26 Exhaust gas emissions such as NOx, CO, BSFC and engine lubricant before and after the test were also measured.

A single cylinder DI diesel engine fuelled with pure biodiesel from physic nut oil and blends (B10, B20, B50) with diesel fuel was used to compare engine performance and engine exhaust gas emission by comparison with diesel fuel.27 The results from this research show that biodiesel fuel from physic nut oil and its blends with diesel can give comparable engine performance for parameters torque (T), fuel volumetric consumption (FVC), brake specific energy consumption (BSEC) and thermal efficiency (ne). Engine exhaust gas emissions of total hydrocarbon (THC), CO and smoke emissions were reduced significantly when engine was run with biodiesel fuel. Biodiesel use resulted in slight increases of NOx emission.

Much research has been focused on the use of biodiesel and its blends in stationary DI diesel engines. Only a few studies on use of biodiesel and its blends in automotive diesel engines or indirect injection diesel engines have been done. The effects of biodiesel and its blends on an automotive IDI diesel engine by comparison with local commercial diesel fuel28 were studied in an experiment. Jatropha curcas methyl ester (JCME) and its blends had slightly lower torque, power output and thermal efficiency, but slightly higher BSFC than diesel fuel. In exhaust gas emission tests JCME and its blends significantly reduced HC, CO and Bosch Smoke Number but NOx emission slightly increased. The results indicated that B10 was the optimum fuel for the test engine.

A similar study carried out using both palm oil methyl ester (POME) and JCME with a DI engine yielded similar results.29 Coconut (Cocos nucifera) is an edible oil, but because it is widely distributed all over Indonesia in areas where it is often difficult to provide fossil fuels which are consequently high in price, it even becomes feasible to use coconut oil for fuel. Coconut methyl ester (CME) was field tested in vehicle and fishing boat engines as a fuel for use in remote areas.30 In the vehicle road test, B30 CME was used as fuel for 15 000 km, and in fishing boat engine, B100 CME was used for 200 hours. Results indicated that as long as the biodiesel quality was according to Indonesian Biodiesel standard SNI 04-7182-2006, there were no significant differences in engine operation during the test by comparison with diesel.

Kapok nut (Ceiba Pentandra L.) is a non edible oil. Kapok trees are also widely distributed throughout Indonesia but not utilised as an energy source.31 Biodiesel from Kapok seed oil was tested with a DI diesel using standard test procedures including engine injector nozzle coking test CEC F-23-A-01. Fuel consumption and smoke emissions increased. Nozzle tip deposits were very thick, presumably caused by the content of cyclopropenoid. Hydrogenation would be required to crack the cyclopropenoid structure before transesterification to solve this problem.

Mixed biodiesel.32 There is considerable potential for ASEAN to produce and supply various biodiesel products to the rest of the world due to its natural resource base; however, the use of biodiesel still presents a number of problems which need to be resolved, especially the high price of raw materials and the quality of biodiesel fuels. In view of these limitations, seeking ways to combine various biodiesel raw materials (e. g. edible and non-edible oils) is one strategy that could be used to solve the problems: reducing the economic cost, utilising the availability of raw materials and improving the quality of biodiesel fuels particularly cetane number, oxidation stability and cold flow properties. In this study, four biodiesel fuels were mixed to create three biodiesel fuel mixtures in differing weight ratios as follows: (1) 70% Jatropha curcas oil methyl-ester (JME) with 30% palm oil methyl ester (PME), (2) 70% JME with 30% coconut oil methyl ester (CME), and (3) 75% soybean oil methyl ester (SME) with 25% PME. Three kinds of mixed biodiesel fuels in form of B10 and B100 together with conventional diesel fuel have been tested in a DI diesel engine. Via analysing process based on the in-cylinder pressure data and rate of heat release, the obtained results showed that biodiesel fuel mixtures had similar cetane number to diesel fuel; this is the main factor to explain why three biodiesel fuel mixtures were selected to simulate the current used fuel — diesel fuel. Moreover, all mixed biodiesel fuels were comparable with conventional diesel fuel in performance and combustion efficiency and exhaust gas emissions were reduced significantly (e. g. THC, CO and PM). Especially, the reduction of NOx is an interesting issue in this study; this reduction could be explained by the rate of heat release obtained and the use of antioxidant BHA.

There is no uniform acceptance for the mechanisms involved in catalytic pyrolysis. This is probably because of the complexity of the process and the range of products

formed which make quantitative analysis difficult. It is difficult to experimentally resolve intermediates in the reaction because of the number of different products formed and the conditions within reactors are not always amenable to characterisation by either ex situ or in situ methods. Further, the reaction temperature and pressure as well as the nature of the catalyst may change the nature of the reaction. Two mechanisms have become widely suggested as being the basis of pyrolysis. These are a free-radical mechanism and a carbonium ion mechanism and both of these are now very well established in terms of catalytic assisted cracking of hydrocarbons145 and have been used to describe the mechanism of catalytic pyrolysis of biomass,146 heavy oils,147 natural oils149 as well as various polymers.150,151 Although these mechanistic models have been proposed since the 1940s and earlier, there has been little additional detail provided because of the experimental and analysis problems described earlier although Sakata et al. have extended the older models for the catalysed decomposition of polyethylene.152 The free-radical mechanism is based on a number of steps where high temperature homolytic reactions create free radicals; these free radicals are unstable and will have tendency to crack at b-bonds to form smaller hydrocarbons. Combination of free radicals allows recombination and, thus, some isomerisation. Water elimination, aromatisation, carbon oxide formation and hydrogen production follow from reaction of these radicals with each other or other hydrocarbon molecules. It is generally believed that the catalyst has a more profound effect on the initial free-radical generation step than the subsequent reactions since free-radical reactions are kinetically fast.

The carbonium ion reaction is similarly complex, involving carbocation formation and has been developed from early concepts into two distinctly different mechanisms. It should be noted that there is a confusion on the term carbonium ion. In the pyrolysis literature, few authors use the term correctly because the term is generally used to indicate any positively charged carbon atom. However, a carbonium ion is properly defined as penta — or tetracoordinated carbocation such as R5C+.153 The first carbocation mechanism is described as monomolecular cracking.154 Here, a penta-coordinated carbonium ion is formed from an alkane or alkane containing molecule group and this subsequently undergoes cracking and evolution of an alkene containing molecule or hydrogen. This reaction is considered relatively slow at lower pyrolysis temperatures and the second type of carbocation mediated reaction is probably more important; this is known as the bimolecular or b-cracking mechanism.155 This process is initiated by a carbenium ion (a trivalent carbocation of type R3C+) which subsequently undergoes hydrogen or hydride transfer followed by b-bond scission. Since the scission occurs with formation of an additional adsorbed carbenium ion, the mechanism is generally considered to be much faster than the monomolecular route.156

Separating the mechanism into two quite separate routes, free-radical and carbocation mediated, is probably not possible and both reaction mechanisms may contribute to the product formation although the relative importance of each is likely to vary with temperature. It has been found that the free-radical reaction will dominate at higher temperatures.147 A term RM has been used to describe the relative contributions of free-radical and carbocation mediated reactions in the cracking of heavy oils.147,148 RM can be estimated from the isobutane to normal butane ratio. When the RM term (greater than 1.5) is high, the reaction will be dominated by the carbocation mechanism and when low (below 0.5) by the free — radical reaction mechanism. Intermediate values indicate that both reaction mechanisms are important. This may be rationalised in terms of the shorter lifetime of free-radical species which mitigate more complex rearrangements.

Whatever the nature of the reaction mechanism, it is clear that the catalysed pyrolysis of complex organics must involve a number of different reactions.146 Following an early work by Chang and Wan in 1947157 for the decomposition of triglycerides (see later chapters in this book for a more detailed review of the catalysis of triglycerides) these will involve reaction steps similar to those below (not inclusive of all possible reactions):

1 Degradation of the complex reactants to yield acrolein (CH2=CHCHO) plus various complex fatty acids and ketenes as well as other similar compounds (e. g. RCOOH, RCH=CO where R is an alkyl group).

2 Degradation of fatty acids and acrolein into carbon dioxide, water and alkanes, e. g.

RCOOH — CO2 + RH

2RCOOH — CO2 + RCOR

3 Breakdown of ketenes, ketones and acrolein into carbon monoxide, light hydrocarbons and alkenes

2RCH=CO — 2CO + RHC=CHR

CH2=CHCHO — CO + C2H4

RCOCH2R — R2 + CH2CO

2RCOCH2R — 2 R2 + CO + C2H4

4 Decomposition of alkanes into hydrogen and carbon (principal char forming route)

CnH2n+2 — nC + (n+1)H2

5 Formation of alkenes from alkanes

Cm, — CH + H

n 2n+2 n 2n 2

6 Division of alkanes and alkenes into smaller alkane, alkene and di-alkene molecules, e. g.

C m, — C H„ ^ + C H,

n 2n+2 n-m 2(n-m)+2 m 2m

7 Growth of longer chain alkanes

C H ол + C H — C

n 2n+2 m 2m n+m 2(n+m)+2

8 Isomerisation of alkanes and alkenes

9 Aromatisation of alkanes and alkenes via reaction mechanisms such as the Diels-Alder reaction,149 e. g.

CnH2n+2 — Cn-6H2(n-6)+1C6H5 + 4H2

10 Formation of alkynes from alkenes

C H — C H + H

n 2n m 2m-2 2

11 Hydrogenation of alkenes and alkynes, e. g.

CnH2n+2 + H2 — CnH2n+2

More generally, the types of reactions occurring in catalytic pyrolysis that are directly affected by the presence of the catalyst can be described in terms of more general mechanisms based on combinations of cracking, reforming and other reactions. Some of these are described above. The water gas-shift and similar reactions also clearly affect the gas composition. This more general description is useful because the pyrolysis temperature (whether pyrolysis and catalytic reactions are separate or integrated) is the most critical process parameter because the products of the thermal pyrolysis reaction are strongly dependent on the temperature and these gas products will strongly affect these catalysed reactions by affecting the equilibrium and relative rate of the catalysed reactions. In this way, the product distribution can vary considerably with temperature. These reactions are:

1 Classic catalytic reforming reactions such as isomerisation, cracking and aromatisation as described above. The role of the catalyst is based on dissociative chemisorption of the alkanes and alkenes forming chemisorbed hydrocarbon fragments and hydrogen. Recombination of fragments leads to formation of smaller hydrocarbons, isomers and aromatics as well as hydrogen.

2 Hydro-cracking where hydrogen produced in other reactions is used in the fragmentation of long hydrocarbon chains into smaller units, e. g.

C H + C H

n-m 2(n-m)+2 m 2m+2

3 Hydrogen can also be important in terms of dehydration reactions with the products of the thermal pyrolysis,158 e. g.

C6H8O4 + 6H2 ^ 6CH2 + 4H2O

C6H8O4 + 4.5H2 ^ 6CH15 + 4H2O

C6H8O4 + 3.6H2 ^ 6CH12 + 4H2O

where CH2, CH15 and CH12 represent the average stoichiometry of the alkane, alkene and aromatic hydrocarbon products respectively and C6H8O4 is an indicative formula for the pyrolysis oil.

4 In order to maintain the highest amount of oil product the catalytic pyrolysis process must be carefully controlled to minimise processes such as steam reforming, partial oxidation and auto-thermal reforming (which combines steam reforming and partial oxidation158,159) as these reactions lead to H2, CO2 and CO formation.

CnH2n+2 + 2nH2O ^ nCO2 + (3n+1)H2 (steam reforming)

CnH2n+2 + n/2O ^ nCO + (n+1)H2 (partial oxidation)

The other important method of controlling product is by careful choice of the catalyst as the chemical nature of the catalyst will define which of the individual reaction steps is most strongly affected. The catalysts used in pyrolysis are described in some detail below.

This model is based on the premise that at equilibrium stage, the total Gibbs free energy has to be minimized. The procedure mentioned below is developed for spouted bed gasification by Jarungthammachote and Dutta (2008) and for steam gasification with in-process carbon dioxide capture by Acharya and Dutta (2008). The total Gibbs free energy is given by:

N /=1

where ni = number of moles of species i,

Pi = chemical potential of species i given by,

( f ‘

Pi=G°i + RTIn -5-

I/ i)

f = fugacity of species i and Go; andfo i = standard Gibbs free energy and standard fugacity of species i.

Equation [16.20] can be written in terms of pressure as

[16.21]

where ф = fugacity coefficient.

For the ideal gas case at atmospheric conditions

P, = AGf + RT ln(y), [16.22]

where yi = mole fraction of gas species i

n

У =———————— 1——————- ‘

‘ Total moles in the mixture,/? .

7 total

AGof, i is the standard Gibbs free energy of formation of species i and is set equal to zero for all chemical elements.

Now, substituting equation [16.22] in equation [16.19], we get

[16.23]

The value of n; should be found such that the G* will be minimum. Lagrange multiplier methods can be used for this purpose. To use this method, the constraints need to be defined. Thus, the constraints can be defined in terms of the elemental balance on both the reactant and product side as:

N

Yjaljni=Aj, J = ,2,X—;K [16.24]

/ = 1

where aij = number of atoms of jth element in a mole of ith species. A} = total number of atoms of jth element in the reaction mixtures.

Thus, the Lagrange function (L) is defined as:

К f N

і V i=l

where Я = lagrangian multiplier. So, to find the extreme point,

Substituting the value of Gt from equation [16.23] to equation [16.25] and then taking its partial derivative as defined by equation [16.26]; the final equation will be of the form given by equation [16.27]: