Category Archives: Handbook of biofuels production

Utilisation of biodiesel B5 based cat-fish fat in diesel engines

In Vietnam, the master project of biofuels production until 2015, a vision to 2025 has been approved by decision 177/2007/QD-TT g of the government.33 According to this decision, in 2010, biodiesel production will be 50 thousand tons/year; in 2015, biofuels production will be enough for 5 million ton gasohol E5 and biodiesel B5; and until 2025, ethanol and biodiesel production will be 1.8 million tons that meets 5% of the national fuel requirement.

In order to complete the biodiesel production process and to utilise biodiesel B5 (blend of 5% biodiesel and 95% market diesel) in engines, a national research project, code DTDL.2007G/19, has been set up and run by Vietnamese Institute of Industrial Chemistry. Institute of Transportation Engineering, Hanoi University of Technology, is one of the collaborative institutions. The project was aiming biodiesel based cat-fish fat production and application of B5 fuel in diesel engines.34

Catalysts used in catalytic pyrolysis

The composition and structure of catalysts for catalytic pyrolysis are usually based on either conventional petroleum reforming or cracking catalysts or research materials derived there from. These systems have distinct advantages in terms of scale, availability of supply and cost since they are manufactured on the scale of millions of tonnes per annum. Conventional petroleum catalysts are roughly divided into two classes, alumina-silicates for cracking (i. e. formation of small chain hydrocarbons from long chain hydrocarbons)160 and transition metal catalysts for reforming (isomerisation and aromatisation).161 Note that because of the complex nature of these reactions, separating the effects of the catalysts into the simple roles indicated here is overly simplified. The alumina-silicate materials can be divided into several different classes as described below. Petroleum cracking catalysts tend to be alumina or silica supported precious metals or nickel based materials and because of the cost and lifetime of the precious metal catalysts in pyrolysis environments, investigation of these precious metal catalysts is limited to mainly academic work. Cost is an important issue in pyrolysis. Environments are harsh and both poisoning and sintering mitigate against achievement of long catalyst lifetimes. As a possible solution to this low cost materials are of importance and many studies are being carried out in commonly found inorganics such as naturally occurring zeolites (a complex aluminosilicate as described below) and clays. Because of their low cost and plentiful supply, carbonate materials are often used as catalysts in pyrolysis although their use has been largely superseded by the use of aluminosilicate materials. The various types of catalyst are described below. All of these would have a different range in functionality and this should be used to match the various pyrolysis materials used. For example for heavy oils a catalyst with combined cracking, isomerisation and aromatisation catalyst may be used. For a biomass pyrolysis a catalyst with strong de-hydroxylation capability might be preferred. For these reasons, a great deal of research effort is placed in developing the catalysts for a particular raw material as well as the pyrolysis reactor used.

Kinetic modeling

The kinetic model has two components: hydrodynamics and reaction kinetics. Both affect the overall gasification and are briefly explained below.

Hydrodynamics

A typical fluidized bed gasifier is divided into two zones, the dense zone and the freeboard zone. For the dense zone, the designer needs to compute the size of bubbles, bubble velocity, bubble fraction, and gas exchange between the bubble and the emulsion. Empirical relations for these are available in numerous studies, including Kunii and Levenspiel (1991). The solid distribution profile along the height of the freeboard may be determined using decay factors similar to that given by Kunii and Levenspiel (1991). Gas exchange in this zone is based on being a plug flow riser. More details are given in Nemtsov and Zabaniotou (2008).

Reaction kinetics

Gasification reactions proceed at a finite speed, which is largely governed by the reaction rate of the char, as it is the slowest of all reactions. Pyrolysis and gas — phase reactions are at least an order of magnitude faster than the char conversion. So, the time taken for heating up and devolatilizing of the fuel is much shorter than the time taken for gasification of the char remaining. Thus, the gasification rate of the char is the controlling parameter. The conversion of the porous char particle may be modeled assuming the process to follow either shrinking particle
(diminishing size), shrinking core (diminishing size of the unreacted core), or progressive conversion (diminishing density).

Biomass-to-liquids-Fischer-Tropsch final fuel products

As analysed in detail in the previous section, the BTL-FT process — as any XTL process — can yield a different range of products, ranging from chemicals and gasoline range hydrocarbons to middle distillate range alkanes, based on the FT synthesis reaction operating conditions, choice of catalyst and reactor type. The BTL-FT process has been, however, mainly studied so far with the aim to maximize the production of diesel range products due to two main reasons: the decisive shift of EU towards a diesel economy and the increasing EU diesel deficit in terms of refining capacity (European Biodiesel Board, 2008). International Energy Agency (IEA) figures presented in Fig. 19.13 clearly show the upward trend of the diesel demand in the EU compared to the downward trend of gasoline consumption. In addition, EU car registration figures show that the majority of new cars purchased are diesel cars (70% of new cars in France, Italy, Belgium are diesel cars) (ACEA, 2008). In this context, interest in the BTL-FT process lays in the production of renewable, high-quality middle distillate fuels via the LTFT synthesis reaction to diesel, naphtha and FT waxes and subsequent upgrading of the FT waxes to premium diesel. With such BTL-FT configuration, BTL naphtha is produced both as a straight-run and as a co-product of the FT wax upgrading. We will, therefore, focus on the properties and combustion characteristics of the two main BTL-FT final fuel products: diesel and naphtha.

image137

19.13 Evolution of diesel and gasoline demand in EU 27 (IEA data).

Choice of raw materials

Once one is able to valorise residue streams, there will be a challenge to look for even better raw materials that contain a higher proportion of the previously called residue stream. Finally, one can try to optimise the culture on the field or even the genetic optimisation of the crop that contains more of that previously called residue component, and at the same time improve the crops processability by genetic means.

Good examples of genetic modification of primary crops have been obtained for the amino acid lysine in crops like potato (Voorst, 1999) and corn (Houmard, 2007). These improvements were initiated because of the application as animal feed ingredient. Recently, itaconic acid has been expressed in potato with the aim to use this as building block for the synthesis of methacrylate.

Primary residue streams such as wheat straw and other agricultural residues can be used for their cellulose content, but sometimes they still consist of other valuable components such as proteins as is the case for sugar beet or sugar cane leaves (trash), for the leaves of cassave, and other crops. Until now these plant components almost never had an economic application, but now have potentially become an interesting alternative for the fossil based resources.

DDGS and rapeseed meal are good examples of so-called secondary residue streams, i. e. residues that result from industrial processes (often in the food industry), and recently result from the biofuels industry. These fractions have similar compositions as primary residues with the logistic advantage that these streams are available from one point source. These streams therefore often have already some applications as e. g. compound feed components. It is not only that separation of components gives a higher value to each single component but also the removal of a component such as phosphate or potassium, that in too high concentrations have a negative value in actual compound feed applications, can help to increase the economic feasibility of biorefining primary and secondary residue streams.

Catalytic cracking of triglycerides molecules over acid catalysts: general reaction pathway

First studies dealing with the catalytic cracking of triglycerides molecules date from 1979 over ZSM-5 catalyst (Weisz et al, 1979). In this pioneering work, the authors performed the catalytic cracking of several vegetable oils achieving complete conversions of them in a mixture of paraffinic, olefinic and, above all, aromatic hydrocarbons (ca. 42-78%). After this initial work, a huge amount of work dealing with this topic has been reported in the literature over different acid catalysts: zeolitic molecular sieves (such as HZSM-5, H-Y and H-mordenite) (Bhatia et al., 1998; Idem et al, 1997; Katikaneni et al, 1995b, 1995c, 1996; Leng et al., 1999; Milne et al, 1990; Ooi et al, 2005; Prasad and Bakhshi, 1985; Prasad et al., 1986a, 1986b; Twaiq et al, 1999), Al-containing mesostructured materials (Al-MCM-41 and Al-SBA-15) (Bhatia et al., 2009; Demirbas, 2009; Idem et al., 1997; Ooi and Bhatia, 2007; Ooi et al., 2004, 2005; Twaiq et al., 2003a, 2003b) and amorphous materials (alumino-silicates, pillared clays and alumina) (Boocock et al., 1992; Katikaneni et al., 1995b, 1995c; Idem et al., 1997; Vonghia et al., 1995).

Products usually obtained by means of the catalytic cracking of vegetable oils and animal fats are depicted in Fig. 15.3. They are usually grouped in an ‘organic liquid product’ (gasoline, kerosene and diesel fractions), gaseous products (hydrocarbons C1-C5, CO, CO2), water and coke. The oxygen initially present in the feedstock is removed as water (which is easily isolated), CO and CO2. Therefore, there is not a remarkable presence of oxygenated hydrocarbons in the final organic cracking products.

The catalyst properties (e. g. crystalline nature, shape selective effect), the reaction conditions (temperature, pressure, space velocity, presence of steam, type of reactor. . .) and the nature of feedstocks, dramatically influence the conversion and yield towards the different reaction products. Generally, the presence of zeolites increases the yields towards the OLP fraction, whereas amorphous catalysts predominantly produce high amount of gases (Idem et al., 1997; Katikaneni et al, 1995c). Co-feeding steam during the reaction process helps to increase both the olefinic compounds formation and the durability of the catalyst. This fact takes place because the presence of steam diminishes the coke formation and thus the catalyst deactivation (Katikaneni et al., 1995b). The use of a fluidized bed instead a fixed bed reduces generally the selectivity towards the OLP fraction

image78

15.3 Simplified scheme of products coming from the catalytic cracking of triglyceride molecules over an acid catalyst.

due to the shorter contact time that diminishes the possibility of forming liquid hydrocarbons from the olefins C2-C5 oligomerization reactions (Katikaneni et al., 1997). In all the different studies, an OLP with a high concentration of aromatics has been obtained (over 50%) as well as a high triglyceride conversion (> 80%). Furthermore, the almost null presence of oxygenated hydrocarbons in the final cracking products is confirmed by the different performed studies (Katikaneni et al., 1995c, 1997; Leng et al., 1999; Twaiq et al., 2003a). The different authors have shown that although the initial decomposition of triglyceride molecule is mainly a thermal process, in the subsequent secondary cracking reactions (hydrogen transfer, isomerization, oligomerization, b-scission, aromatization), the acid catalyst has a crucial role (Twaiq et al., 2003a). Table 15.1 summarizes the most relevant work dealing with the catalytic cracking of triglyceride molecules indicating type of feedstock, reaction conditions and catalyst. As observed, most of the studies have been performed in fixed bed reactors, in a range of temperatures generally between 300 and 500°C and with liquid space velocities ranging from 2 to 4/hour.

The general reaction pathway of the acid-catalyzed cracking of a triglyceride molecule is depicted in Fig. 15.4. Once the triglyceride molecule has been primarily decomposed to heavy oxygenated hydrocarbons such as fatty acids, ketones, aldehydes and esters, their reactions to reach other products start by means of the breaking of the C-O and C-C bonds by b-scission reactions. The breaking of the bonds C-O and C-C follows two competitive routes: (1)

Подпись:Reference Bhatia et al. (2009)

Boocock et al. (1992) Chew et al. (2009) Dandik et al. (1998) Haag et al. (1980)

Idem et al. (1997)

Katikaneni et al. (1995b)

Подпись:Katikaneni et al. (1997) Leng et al. (1999)

Подпись: 402

Table 15.1 Continued

Reference

Feedstock

Type of reactor

Reaction conditions

Catalyst

Ooi et al. (2004)

Fatty acids from palm oil

Fixed bed

Atmospheric pressure, T = 400-450°C, WVSH: 2.5-4.5/h

HZSM-5, MCM-41

Ooi et al. (2005)

Palm oil

Fixed bed

Atmospheric pressure, T = 450°C, WVSH: 2.5/h

HZSM-5, MCM-41, SBA-15

Prasad et al. (1986a)

Rapeseed oil

Fixed bed

Atmospheric pressure, T = 340-400°C, WVSH: 2-4/h

HZSM-5

Prasad et al. (1986b)

Rapeseed oil

Fixed bed

Atmospheric pressure, T = 340-400°C, WVSH: 2-4/h

HZSM-5

Twaiq et al. (1999)

Palm oil

Fixed bed

Atmospheric pressure, T = 350-400°C, WVSH: 1-4/h

HZSM-5, beta, USY

Twaiq et al. (2003b)

Palm oil

Fixed bed

Atmospheric pressure, T = 450°C, WVSH: 2.5/h

AI-MCM-41

Weisz et al. (1979)

Corn oil, peanut oil

Fixed bed

Atmospheric pressure, T = 400-500°C, QFeed = 2 ml/h

HZSM-5

 

image135
image136 image79

CO

____ >. Thermal cracking

——- >■ Catalytic cracking

Oligomerization

Light olefins C2—C5 —————- ► Olefins C2—C1 g

15.4 General reaction mechanism for the catalytic cracking of triglyceride molecules over acid catalysts.

decarboxylation (CO2) and decarbonylation (CO) reactions followed by C-C bond cleavage of the resulting hydrocarbon radicals or (2) C-C bond cleavage within the hydrocarbon section of the oxygenated hydrocarbon molecule followed by decarboxylation and decarbonylation of the resulting short-chain molecule (Idem et al, 1996). The occurrence of these different reaction routes depends on the double bonds in the initial oxygenated hydrocarbon. Whereas C-C bond breaking in the a and b positions is favoured in the presence of unsaturated hydrocarbon molecules, decarboxylation and decarbonylation reactions take place before C-C bond cleavage for saturated oxygenated hydrocarbons because, in a saturated hydrocarbon chain, the less endothermic bonding is the one associated with the b position of the carbonyl group (Osmont et al., 2007). Different subsequent cracking reactions finally yield CO, CO2 and water, as the main oxygenated compounds, and a mixture of hydrocarbons produced by different reactions such as b-scission, hydrogen transfer, isomerization, cyclization
or aromatization, some of them possible because there is an acid catalyst present in the reaction system. Furthermore, coke is formed by means of polymerization reactions (Maher and Bressler, 2007).

Distillation unit

The incoming liquid will consist of two main components: water and ethanol. These can be separated in a standard distillation tower. As ethanol will be used as a fuel, the ethanol coming out of the distiller may only contain 7 vol-% water (Thuijl et al., 2003). Therefore, this will be the goal of our distiller. The outgoing stream of water will still contain ethanol. To keep efficiency as high as possible, this stream should be inserted in the fermentation reactor.

Recycle

The water stream coming out of the distillation unit still has valuable components in it. To preserve these, this stream is used to be the water stream in the fermentation reactor. Because the reaction of the bacteria is dependent on pH as well as on the quantity of nutrient in the flow this should be kept at highest performance

image118

17.4 Picture of monolith reactor used for Syn gas fermentation (Salim et al., 2008).

levels. This can be done by adding nutrients and controlling the pH of the recycle stream.

Figure 17.5 shows an overview of the total process design and Table 17.2 shows the mass balance for 30 000 ton per year input of wood chips to ethanol plant (Van Kasteren et al., 2005).

Apart from ethanol acetic acid is also produced. An important issue for the whole process is the energy required for the distillation of the ethanol/water mixture.

Table 17.3 shows the energy requirements. The concentration of ethanol, which can be reached is crucial to the efficiency and economics of the process.

At least 3-5 wt% ethanol in water should be reached otherwise the energy requirements to separate the ethanol from the water become too high.

Conclusion is that the feasibility of the process is determined by the ethanol concentration reached. Coskata (www. coskata. com) is trying to solve this problem via the use of a membrane.

Подпись: 474

image119

Exhaust Ash Exhaust Water

exhaust

 

Подпись: © Woodhead Publishing Limited, 2011

77.5 Process scheme biomass to ethanol plant (Van Kasteren et a/., 2005).

Table 17.2 Mass balance of the integrated system

Stream

Quantity (kg/hour)

Quantity (ton/year)

IN

Wood

4167

31252

O2

70

525 for combustion

OUT

Ethanol

1175

8812

(97% ethanol)

Solids

117

888 with 75 ash

H2O out

761

5698

(3.1% ethanol, 17% AcOH, 80% H2O)

Liquid

0.8

6

Gases 1

2183

16373 mainly CO2

Source: Van Kasteren et al. (2005)

Table 17.3 Energy balance of the integrated gasification-fermentation system

Item

Required energy MWth

Produced

MWth

Drying and grinding

Gasification unit

0.5 MWe

Gasification

0.52

Cooler

2.22

Fermentation

Fermentation unit

1 MWe

Flash

0.07

Distillation column

Distillation unit

6-12

Total electrical energy required (MWe)

1,5

Source: Van Kasteren et al. (2005)

Description of the ongoing research and status of proposed and tested technologies for biomass reforming

20.5.1 Adding reform catalysts to biomass gasifiers

Catalytic biomass gasification research18 has focused on tar removal and hydrocarbon conversion to CO/H2. Dedicated efforts to develop catalysts for biomass gasification is in its infant stages, and the strategy till now has been to use catalysts (i) off the shelf, commercial, not so cheap, methane steam reforming catalysts, (ii) cheaper materials, dolomite based clays, alkalis salts (Na, K, chlorides). Catalysts are used mostly in fluidized bed reactors, such as bubbling or circulating fluidized beds. Because both the feedstock and the catalyst are solids, the catalyst acts only on the gases and vapors produced. When using a fluidized bed reactor the mechanical strength of the catalyst will be very important, besides activity. Unlike Fluid Catalytic Cracking (FCC) catalysts (also a fluid bed process) where the catalyst is a single structure, steam reforming catalysts have a metal supported on a carrier with promoters which makes the catalyst much more vulnerable for attrition. Tests in a fluidized bed with crushed commercial steam reforming catalysts (Sud Chemie C11-NK and ICI 46-1 S) showed a weight loss due to attrition of 28-33% after 48 hours of testing.19 Stronger fluidizable steam reforming catalysts have been developed. NREL19 developed pure (99.5 wt%) alumina and alumina based (>90 wt%, rest being MgO, SiO2 and K2O) fluidizable supports which had a lower surface area than commercial ones (1.4-2.7 m2/g versus 9.7 m2/g commercial) but a very low attrition rate (0.01 wt%/h versus 0.41­0.69 wt%/h commercial). Dolomite CaMg(CO3)2 has gained the most attention as it is very cheap and easy to apply.20,21 Although its calcined form can convert tars to a large extent it is more often used as a tar-reducer, a guard material, allowing the usage of more active but also more sensitive catalysts downstream.22 Dolomite is not able to effectively convert methane and suffers from attrition.21,23 Olivine23,24 is much more resistant to attrition than dolomite with a somewhat lower activity for tar destruction.

Nickel on alumina based catalysts have been used in the industry for naphtha and natural gas reforming for many years and it was therefore also logical to test them for biomass gasification applications. Baker et al.25 employed several Ni-based catalysts in a fluidized bed. They observed rapid deactivation which was ascribed to carbon fouling. The mineral olivine, which mostly contains SiO4, Mg and Fe with trace elements of Ni, Ca, Al, and Cr, has been proposed as support for nickel based steam reforming catalysts by Courson et al.26,21 The mineral has superior strength and a mild catalytic activity of its own. When the calcination temperature for NiO on olivine is varied three different connections can be made: (i) the Ni is freely deposited onto the support (~900°C) (ii) the Ni is strongly linked to the olivine (~1100°C) and (iii) the Ni is integrated in the olivine structure (~1400°C). The Ni-olivine which was calcined at 1100°C was found to be the most active for dry reforming of methane.21 The catalyst was also tested in a fluidized bed gasifier where it showed a higher tar conversion relative to normal olivine as shown in Table 20.1.28 However, especially the methane was still present in high amounts. The attrition rate of the Ni-olivine was around 0.025 kg/kg of dry fuel. Glass-ceramic catalysts have been proposed by Felix et al.29 Via controlled crystallization of a mixed melt (in the case for steam reforming Li2O-Al2O3-SiO2 with 15 wt% NiO and traces of MgO) a very strong material is produced which is claimed to be more resistant to attrition than olivine. Steam reforming of an artificial syngas (vol%: 16 H2, 8 CO, 12 CO2, 4 CH4, 16 H2O, 44 N2 and 600-700 ppmv of naphthalene) resulted in a

Table 20.1 Results of fluidized bed pilot biomass gasifiers using olivine and Ni-olivine.28

Olivine

Ni-olivine

Temperature (°C)

850

838

Steam/Fuel (kgH2O/kg dry fuel) Dry gas composition (vol%)

0.63

0.63

H2

38.9

43.9

CO

29.1

27.2

CO2

17.5

18.8

CH4

11.4

8.3

C2H4

2.0

1.3

LHV of product gas (MJ/Nm3)

13.8

12.4

Gas production (Nm3/kg)

0.95

0.99

Tar production (g/Nm3, dry gas)

12.7

1.2

‘steady-state’ relative conversion of ~70-80% naphthalene and 5-10% methane at 800°C.

To the best of our knowledge, no fluid bed catalyst has been developed which has a similar activity and stability compared to fixed bed catalysts.

The current status of catalysts (natural materials or Ni-based) in biomass gasifiers is that they can lower the tar and the higher hydrocarbon content of the gas, which lowers the load on downstream tar removal and upgrading units. There are, however, still operational problems. Catalyst deactivation, catalyst make-up and fluidization problems still need research attention before these dolomite and olivine catalysts could be effectively employed. In practice, tars and hydrocarbons are actually dealt with predominantly downstream of the gasifier.

. Separation of minor components by solvent extraction

Another method implied for the separation of minor components is solvent extraction, after which the components of interest were further purified by distillation or chromatography (Gunawan et al, 2008). The pretreatment of the feedstock mainly consisted of hydrolysis and neutralization for the concentration of the target minor compound (Chu et al, 2002; Leng et al., 2008). However, esterification was also used to facilitate the separation in polar and non-polar components (Brown and Smith, 1964).

Gunawan et al. (2008) reported that the separation results of a modified Soxhlet extraction are comparable to those obtained from molecular distillation. The purpose of their work was to isolate and purify natural FASEs from SODD by a suitable method without degradation of the FASEs.

A modified Soxhlet extraction with hexane was first employed to separate SODD into two fractions based on differences in the polarities of the constituent compounds (Fig. 22.7). The resulting nonpolar lipid fraction (NPLF) was rich in hydrocarbons and FASEs, whereas the polar lipid fraction was rich in FFAs and acylglycerols. The NPLF was then fractionated by a modified silica gel column chromatography to yield FASE-rich fraction. FASEs with high purity were finally obtained by solvent extraction.

By combining a modified Soxhlet extraction, a modified silica gel column chromatography and water/acetone extractions, the FASE fraction with high purity (87%) and high total recovery (85%) could be obtained from SODD with an initial FASE content of 4%. According to the results, this separation process can yield the FASE fraction from SODD without degradation of the FASEs. The advantage of the process is that, starting with SODD, high-purity squalene and FASEs can be obtained. In addition, the polar fraction (PLF in Fig. 22.7) contains

image164

22.7 Flow chart showing the separation and purification of FASEs from SODD (from Gunawan et al., 2008).

most of the tocopherols and free physterols and can be further processed to obtain pure tocopherols and free phytosterols.

Leng et al. (2008) described a process to recover squalene from PFAD using commercial immobilized lipase. The PFAD was hydrolyzed and neutralized, and then squalene was concentrated after a second neutralization and extracted with hexane. In this study, an RSM (response surface methodology) was used to evaluate the effects of several variables (reaction time, water content and lipase concentration) on the enzymatic hydrolysis.

Chu et al. (2002) separated tocopherols from PFAD by extraction with hexane after pre-concentration using an enzymatic hydrolysis-neutralization method. Acylglycerols in PFAD were hydrolyzed using a commercial immobilized thermal- stable lipase to liberate FFA and was subsequently treated with alkali. Removal of the FFA salts resulted in concentration of tocopherols. Factors affecting the degree of hydrolysis were studied to reach a better understanding of the recovery of tocopherols from PFAD. It was observed that the FFA levels in PFAD remained unchanged, but the tocopherols concentration decreased when the reaction was prolonged to 7 h. This was explained by the possibility that tocopherols might have been oxidized due to the long period of heating at 65°C. Increase of water content in the reaction mixture from 20% to 50% increased both the FFA levels and tocopherols concentration significantly (p < 0.05). However, a further increase of water content in the mixture significantly (p < 0.05) decreased the FFA levels and the tocopherols concentration.

Winton and Smith (1964) describes a process for the separation of sterols and tocopherols that involves the treatment of DD with a strong acid to convert FFA into esters, followed by the liquid-liquid extraction with a polar and the nonpolar solvent. The obtained polar liquid solution contains mainly sterols and tocopherols and nonpolar liquid solution is rich in esters and TAG. However, under the concept of the invention, there must be a sufficient immiscibility not only of the solvents but also of the solutions formed after admixture of the solvents with the sludge to result in two liquid phases. The process comprised an additional step of separating the polar liquid solution or extract fraction into a sterols product and a tocopherol concentrate. This was obtained by concentrating the solution to the point of incipient precipitation of sterols or complete removal of the solvent by vacuum distillation, followed by crystallization and filtration.

Pyrolysis: a brief background

Pyrolysis is a term used to describe the effect of heat on a substance such that no or little external oxidation or hydrolysis occurs. In this way, the pyrolysis can occur in an ambient environment provided combustion does not occur. Recently, the term pyrolysis has been associated with the development of an alternative means to recover energy from organic materials and is one of several possible strategies to develop energy sources from renewable sources rather than fossil fuels. There are biochemical methods for producing bioethanol as an energy source and these are based on either direct fermentation techniques for sugar rich crops or fermentation combined with chemical treatments for more woody and low-sugar plants.18 This traditional sort of fermentation process to produce bioethanol is often described as a first generation biofuel technology. Other first generation biofuel technologies include transesterification (e. g. the reaction of vegetable oil with an alcohol) and biological anaerobic digestion of biomass. Despite the modern policy driven trends towards the use of bioethanol as a fuel or fuel additive19 there is a conflict between the use of what are essentially edible crops and food security.20 The use of lignocellulosic materials as a potential biofuel source would do much to prevent fears on shortages of food that might arise as arable land is used to generate fuels rather than foods. Despite the clear need to develop low quality crop fermentation, the science is not facile and chemical/bio pre-treatments and designer yeasts are being developed to allow this technology to be delivered.21

Pyrolysis is one of a related series of thermo-chemical methods to extract energy from organics that rely on heating them and effecting a conversion of the materials. The most direct thermo-chemical method is combustion where the materials are heated in excess oxygen to form carbon dioxide, water and heat (from the highly exothermic reaction). The combustion reaction may be catalysed to maintain lower flame temperatures thereby limiting oxidation of nitrogen and the production of nitrous oxide pollutants.22-24 Energy can be recovered from the exothermicity of the combustion reaction in several ways. These include heat to drive turbines, the volume expansion deriving from the liquid expansion to gas as well as from high pressure steam raised in the combustion. Partial oxidation, when the oxygen used is significantly below that needed for complete oxidation, is described as gasification as it yields light fuel gases and CO and H2 known as syn(thesis) gas (from its use in the industrial synthesis of methanol). Catalysts and conditions (flow, pressure, contact times) can be used to control the gas products that result from this controlled oxidation.25

Pyrolysis is the heating of the organic materials (biomass, waste food, waste plastic, etc.) in the absence (or at partial pressures and/or temperatures where reduction rather than oxidation is favoured) of oxygen. Pyrolysis produces a range of products including solids (char), which is charcoal like and can be used for solid combustion systems, liquid (tar) and gaseous products both of which can also be used for energy storage, generation and transportation although the char usually requires further upgrading for optimum use.26-29 The volatile, but readily condensable, components are sometimes described by the term bio-oil or pyrolysis-oil. These terms are used because they reflect the possible use of the product as a replacement for petroleum in automotive and energy applications. The mechanisms involved in pyrolysis are exceedingly complex involving free — radical reactions.30 They are briefly summarised below.

As detailed above, pyrolysis is a complex process with products varying considerably according to the temperature and pressure used. This is because the reaction involves several different chemical reactions in both the gas and condensed phases and there are further heat and mass transport limitations which prevent an accurate representation by equilibria.31 One of the challenges in delivering cost-effective commercial technologies is modelling these complex kinetic processes which are necessary for the design of efficient plant.32 Because of this, the reaction products depend not only on the temperature and pressure but also on the rate of increase of temperature and the residence time in a reactor.

The main product of a pyrolysis reaction can be written as:

Organic feedstock ^ char + volatiles

The organic feedstock (see below) can be anything from biomass (plants and other vegetation), vegetable oils, food waste, waste polymers, animal fats, etc.

The char is a carbon-rich, low hydrogen, ash containing solid.33 It has many applications; as the name suggests as a coke substitute,34 an advanced adsorbent35 and as a soil additive to increase fertility.36 The char can also be gasified by suitable oxidants and catalysts to provide a route to syn gas.37,38 The volatiles are essentially a mixture of compounds that are condensable (in usual conditions) or non-condensable. The non-condensables are the basic components of syn gas, H2, CO, CO2 together with methane and are normally referred to as gas or syn gas.39 In general, the relative amount of the gas component compared to the condensable and solid (char) content increases with temperature.18,40,41 The gas component is, possibly, an important part of the strategy in a move towards a ‘hydrogen economy’ but there are many competitive processes to be considered.42 One of the more promising methods is the combination of pyrolysis with water gasification.43

In most cases, the most sought after component of a pyrolysis reaction is generally the condensable or liquid fraction because of its potential as a possible fuel in power stations or internal combustion engines.44,45 The liquid fraction (or bio-oil) that results from pyrolysis is a complex mixture of chemicals and contains a range of molecular weights from light hydrocarbons through to molecular weights of 200+.46 The elemental composition of the liquid (which is often described as a biofuel because of the uses outlined above) is close to that of the biomass (or other material) feedstock.47 The bio-oil is a mixture of an aqueous phase containing a variety of low molecular weight oxygenated organic compounds (methanol, ethanol, acetic acid, acetone, etc.) and a hydrophobic non-aqueous component consisting of heavier molecular weight oxygenates (e. g. alcohols, phenols, cresols), aromatics (e. g. benzene, toluene, indene) and polycyclic aromatic hydrocarbons (PAHs — e. g. naphthalenes, anthracenes).48 The bio-oil, as-produced, can be burned in engines and turbines directly. However, it is relatively unstable, acidic (and, therefore, corrosive), of relatively low calorific value compared to petroleum oils and viscous. This ensures that it has limited application for direct use in turbines or engines49 and much work has been carried out into developing methods whereby the product oil can be upgraded for practical use and this is an important application of catalytic pyrolysis.50

As briefly mentioned above, the temperature at which pyrolysis takes place plays an important role in the product distribution obtained from the pyrolysis reaction.51,52 At lower temperatures (<600 K), formation of char is favoured whilst at higher temperatures (>800 K), increased reaction rates and the breakdown of C-C bonds lead to gas formation. Intermediate temperatures favour oil production. Experimentally, much time is spent varying temperatures and conditions in order to define conditions for optimum product distribution. As well as temperature, ‘residence time’ plays a major role in defining the product distribution. In a simple batch reactor, residence time has little meaning being simply the time over which the reaction is run. Simple batch reactors are of little practical use and, instead, flow-through reactors are used for most of the research being carried out both academically and industrially. In flow-through or continuous reactors, great care is required to control residence time and this is most important in reactions of this type where the products are kinetically not equilibrium defined. Whilst full discussion of this subject is beyond the scope of this review, it is generally found: that high residence times at lower temperatures favour char production, high residence times at higher temperature favour gas synthesis whilst low residence times at higher temperatures favour liquid production.26 The requirement to control product distribution coupled to a need to generate technologies that can process industrially significant amounts of feedstock has led to several forms of pyrolysis which are often differentiated in the literature. They are differentiated by the heating rate used and the residence time in the reactor chamber. In a basic reactor for industrial use, the aim will be for the feedstock to pass through a heated zone, complete reaction and subsequently pass through separators and secondary reactors such as reformers and crackers to upgrade products.50 The residence time of the reactor will be defined by the time feedstock spends passing through the pyrolysis chamber heated zone. The heating rate is defined as the time taken to reach maximum temperature although equilibrium with the chamber temperature is not always possible and effectively the feedstock maximum temperature may be less than the actual chamber temperature. The products and unused feedstock will be separated via cooling and condensation with an appropriate recycle if needed. A complete description of reactor design is given elsewhere.53