Category Archives: Handbook of biofuels production
This work sets out a comprehensive review of catalytic pyrolysis centred on the production of fuel oils for use in transportation and energy production. An overview of pyrolysis economics is given and the environmental requirement to generate fuels that are environmentally benign. It should be stressed that catalytic pyrolysis should be viewed as a ‘refinement’ of thermal pyrolysis. The products of conventional thermal pyrolysis are a bio-oil that can be combusted in turbines and boilers but has less value for transportation because of its stability and quality. Catalytic pyrolysis can be viewed as a technique to upgrade the pyrolysis products to transportation fuel quality. This is important because transport accounts for around 70% of all fossil fuel use. Pyrolysis is a sustainable technology using waste materials, fast-growing low value crops and other organic materials such as polymers that can only be recycled at considerable cost. It can be almost carbon neutral and through combustion of waste pyrolysis products such as char and gas the process costs can be reduced to effectively zero (since pyrolysis is an endothermic process). Methods and techniques in the general area of pyrolysis are reviewed in order to introduce the technology and science of catalysed pyrolysis. A thorough review of the science of catalytic pyrolysis, the process methodology and the catalyst and feedstocks are provided.
The development of catalytic pyrolysis into a common and widespread commercial technology is reliant on a number of factors. The construction of pyrolysis plants is capital intensive and profitability requires the products to be competitive against fossil fuel prices or to be preferentially marketed with proactive government subsidies. However, it is clear that the increasing cost and shortages of crude oil will necessitate the development of new fuels akin to petrol/gasoline and diesel. It seems likely that the depletion of fossil fuel sources coupled to increased energy demand will ensure the uptake of new and emerging technologies such as pyrolysis. Whilst it is generally accepted that bio-fuels will become an increasingly important component of global energy strategy, there are a number of parallel and competitive technologies for generation of biofuels. Pyrolysis has considerable advantages over some of these competitive techniques such as fermentation and bio-degradation because it is closer to market and is based on well-established and large scale methods used in the petroleum industry. Pyrolysis also offers considerable advantage in that it does not place further pressures on food security as it does not require sugar-rich crops. It should also be noted that catalytic pyrolysis products are a direct replacement for current energy/transportation fuels and do not necessitate any costly technological development of turbines, boilers or internal combustion engines. In many cases and in properly controlled processes, the catalytic pyrolysis products are indistinguishable from conventional petroleum products and can be distributed through existing infrastructure and retailers. This is a major cost advantage over alternative energy sources such as hydrogen. The choice of biofuel technology will also be partly dependent on the local environment and it is likely that pyrolysis will not be a universal solution. For example in a country that has no facilities for cost-effective recycling of waste polymers, pyrolysis may be a very attractive possibility reducing land-fill, transportation and energy costs. Pyrolysis may also be a preferred option if there are large areas of non-arable land where low-value, fast-growing, sustainable energy crops such as miscanthus and switch grass can be readily grown and harvested. Further, areas highly dependent on forestry and agriculture where significant amounts of waste are generated may find pyrolysis a useful technology. One further advantage of pyrolysis is that it is highly scalable and plants can be designed and constructed to process tonnes to thousands of tonnes of feedstock per day.
It should not be thought that catalytic pyrolysis is unproven on the commercial scale; it is at an advanced stage of development and, in all likelihood, will become ever more important. Commercial scale plants operate in China due to a shortage of crude oil and the poor quality of China’s oil stocks. The uptake of pyrolysis technology in China has been reviewed.141 Progress in pyrolysis has been rapid, Envergent Technologies now offers commercial technology to prospective partners.230 Envergent Technologies is a Honeywell company that combines pyrolysis expertise (Ensyn Corp.) with petroleum refining and process technology from UOP which have been leaders in refining and catalyst technologies for over 100 years. Evergent offers a fast pyrolysis process for biomass (forestry, paper manufacture and agricultural waste materials) via a circulating transported FBR system similar to the one used in conventional petroleum cracking technologies. The production of transportation grade fuels is via a secondary upgrading process using hydroprocessing technology. This technology is expected to be available for licensing of large scale production (2000 tonnes per day) from 2012. In November 2009, Envergent Technologies announced a partnership with the Italian power company Industria e Innovazione for the development of a facility to convert biomass (pine forest residue and waste wood from construction) into pyrolysis oil for renewable power generation. Whilst the planned plant is only of the scale 150 tonnes per day, it represents a major step in commercialising pyrolysis. It thus seems that pyrolysis and catalytic pyrolysis will truly be an emerging technology. Further research and development are required to maximise yields from many sources and provide catalysts of improved efficacy but the technique and methods have been established for both commercial and environmental exploitation.
In this route methanol is converted into acetic acid, which can be hydrogenated to ethanol. Monsanto (1968) commercialised this process for the production of
Table 17.1 Production of ethanol via synthesis gas based routes
acetic acid from methanol. The hydrogenation of acetic acid is possible but has to take place at high pressures and the mixture is highly corrosive which does not make it an attractive process.
Alternatively Davy McKee has patented the conversion of acetic acid with ethanol to ethyl acetate (temperature 175°C, pressure 7 MPa), which can be hydrogenated to two ethanol molecules thus rendering a net production of ethanol (Bradley et al, 1983). The last reaction can take place at 200°C with a Cu/ZnO catalyst.
The total reaction scheme is:
CH3OH + CO ^ CH3COOH
CH3COOH + C2H5OH ^ CH3COOC2H5 + H2O
CH3COOC2H5 + H2 ^ 2 C2H5OH
The net reaction is:
CH3OH + CO + H2 ^ C2H5OH + H2O
A variation on this concept has been developed by the Halcon SD group (Porcelli and Juran, 1985). In this process methyl acetate is carbonylated instead of methanol. The resulting anhydride forms together with ethanol and methanol two different acetates. After separation the ethyl acetate is hydrogenated to ethanol and the methyl acetate is recycled to be carbonylated again.
CH3COOCH3 + CO ^ (CH3CO)2O
Alternatively ethanol can be produced via ethylene which can be converted to ethanol via the existing catalytic hydrolysis of ethylene. Overall yields are however not very high and it looks more promising nowadays to produce ethylene from ethanol via sugar fermentation processes making bio-ethylene production possible rather than the other way around.1
At first biomass gasification was primarily envisioned for heat and power production. Nowadays, the production of liquid fuels and chemicals via synthesis gas is also regarded as an interesting route. The developments in the coal and oil industry have led to three archetype (biomass) gasifiers, viz.: fixed bed, fluid bed and entrained flow. From extensions of these archetypes and combinations of them, several derived systems were developed such as slagging fixed beds, circulating fluid beds, twin reactors (indirect gasifiers), etc.1 Gasifiers operated below 900°C (low-temperature gasifiers) generate so-called fuel gas including tars. Tars are the Achilles heel of this technology; these poly-cyclic components cause, among other problems, fouling (condensation) in downstream units.9
Operation above 1300°C (high-temperature gasifiers) results in synthesis gas. Intermediate gasification temperatures of 900-1300°C are unfavorable because the ashes in the feed become partly molten/partly solid — a situation that is almost impossible to handle in a reactor. Both fuel gas and synthesis gas need cleaning (removal of e. g. S, Cl, and alkalis) before entering a catalytic downstream conversion step. Biomass gasification is basically the same technology as coal and oil gasification, except gasification (reforming) processes for very wet feeds which are developed especially for biomass. Differences are: (i) the oxygen content of biomass, (ii) the differences in ash (mineral) composition and amount, and (iii) the reactivity. The differences in reactivity become clear when analyzing the main gas-producing step: in coal gasification, gas is produced by the heterogonous reaction of solid carbon with H2O and/or CO2, while for a solid biomass the majority of the gas comes directly from depolymerization/devolatilization reactions of the feedstock. Complete reviews on biomass gasification and the associated problems are those of Beenackers and Van Swaaij,10 Maniatis,11 Knoef,12 and Stassen et al?
This chapter proposes to offer an overview of different processes used to convert DD to biodiesel/biofuel. Additionally, different processes to recover valuable minor components are described where conversion of FFA and/or acylglycerols to FAME (fatty acid methyl ester) was applied in order to facilitate their purification. However, most of the literature study targets either quality of biodiesel/biofuel or quality of the minor components and seldom offer aspects regarding the overall quality of obtained by-products.
An overview of the described routes for biodiesel/biofuel production was given by Echim et al. (2009). Biodiesel/biofuel can be produced from DD by direct esterification (Fig. 22.1) of the FFA or by conversion of FFA to acylglycerols prior to transesterification (Fig. 22.2).
Chemically catalyzed process
Soragna (2009, personal communication) described the industrial process for the conversion of FFA into FAME using heterogeneous catalyst, called FACT (Fatty Acid Conversion Technology). This technology is an alternative option compared to the classical technology using homogeneous catalyst, consisting of a continuous countercurrent multiple step esterification using solid catalyst in fixed bed reactors, at 90°C and 0.35 MPa. Production of biodiesel/biofuel from feedstocks with high acidity by direct conversion was registered as a ‘stand-alone process’ Fig. 22.3(a).
For feedstocks with medium/high acidity an ‘integrated process’ was applied (Fig. 22.3b) where a transesterification step for the conversion of the acylglycerols was also included. The FFAs were distilled off and further esterified to FAME before the transesterification of the residual acylglycerols.
The advantage of these processes is the possibility to process high diversity acidity feedstocks (up to 100%) with a conversion of up to 99.8% without limitation in capacity, no usage of liquid acids, higher quality by-products and mild operating conditions.
22.1 Production of biodiesel/biofuel by direct conversion route (from Echim et al., 2009).
Verhe et al. (2008) reported a process of converting the DD to biodiesel using sulfuric acid as catalyst, at 75°C for 5 h. The FFA and MAG have undergone esterification, resulting in methyl esters. The crude biodiesel was further washed, dried and distilled in order to increase the quality of the methyl esters. The distillation pitch was further processed for the recovery of sterols and tocopherols.
An extensive study was carried out by Chongkhong et al. (2007) on the palm fatty acid distillate (PFAD) (93% FFA), as feedstock for a batch and continuous production of biodiesel. For the continuous process (CSTR), the amount of FFA was reduced from 93% to less than 2% at the end of the esterification process. A further treatment consisting of neutralization of the FFA and transesterification of the glycerides was required in order to obtain biodiesel which complies with the specifications.
Facioli and Barrera-Arellano (2002) described a process to obtain ethyl esters from soybean oil deodorizer distillates (SODD) using concentrated H2SO4 as catalyst. The DD contained 47% FFA, 26% acylglycerols and 26% unsaponifiable matter.
22.2 Production of biodiesel/biofuel via acylglycerols route (from Echim et al., 2009).
A conversion of 94% of the fatty acids to ethyl esters was achieved. However, the acylglycerols were not affected and the losses of tocopherols were around 5.5%. A molar excess of ethanol in relation to SODD:FFA was found to be necessary to obtain the best conversion.
Hammond and Tong (2005) described a three-stage acid catalyzed esterification. The reaction mixture was centrifuged, the supernatant lipid phase was separated from the sludge (glycerol, water, acid and methanol), and further reacted with methanol and acid. The maximum FAME conversion obtained for 12-tested acid oils averaged 81%. However, the ester phase could not be increased above 85% even after a fourth-stage reaction or if a basic catalyst was used in large excess. Unknown materials were reported in both FAME and in the sludge phase having a hydrophobic and hydrophilic behavior, respectively. The former compound caused an increase of the biodiesel viscosity and is hypothetically attributed to the presence of polymers.
The polymers might have been formed during the soap acidulation process or during the esterification reaction, due to the limited supply of methanol and the long reaction time. The compound could not be further removed by distillation.
Up to now, a continuous scaled-up process for sustainable fermentative H2 production has not been reported in the literature. Only very few studies, are available so far, regarding the fermentation of sugars to hydrogen, at pilot scale. Ren et al. (2006) performed a pilot scale study in a continuous flow anaerobic fermentative reactor with an active volume of 1.48 m3 and using molasses as substrate. The reactor operated under the organic loading rates of 3.11-85.57 kg COD/m3 reactor/d and produced 5.57 m3 H2/m3 reactor/d or 8240 L H2/d with a hydrogen yield of 26.13 mol/kg COD removed. The effluent which was produced, contained primarily acetate and ethanol and was as high as 3000 L/d. This, rich in acetate, effluent could be further exploited for hydrogen production through a subsequent photoeterotrophic stage, which could increase hydrogen production by 317%.
Vatsala et al. (2008) evaluated the feasibility of hydrogen production from a sugar cane distillery effluent using co-cultures of Citrobacter freundii 01, Enterobacter aerogenes E10 and Rhodopseudomonas palustris P2, at 100 m3 scale. The reactor operated at batch mode for 40 hours, and the hydrogen production was 21.38 kg with an average yield of 2.76 mol H2/mol glucose and a rate of 0.53 kg/100 m3/h. The results showed that distillery effluent could be used as a source of hydrogen providing insights into treatment for industrial exploitation.
Since data for real applications are not available so far, we can design such a process based on the respective lab-scale experiments. The problem is that the hydrogen productivity and yields depend significantly on the prevailing conditions, the feedstocks as well as the inoculum used. However, from laboratory-scale work on continuous processes, it could be suggested that such a process may operate at a mesophilic temperature, at a pH around 5.5 and an HRT approximately 8-12 hours, for simple substrates. Higher HRTs are indicative for complex carbohydrate-rich feedstocks. Finally, such a process may use as microbial inoculum, heat-treated sludge form aerobic or anaerobic process or the indigenous microbial species available in the feedstock/waste, which has often proved to work optimally (Antonopoulou et al., 2008a; 2008b).
A. DUTTA, University of Guelph, Canada and
B. ACHARYA, Dalhousie University, Canada
Abstract: Gasification is becoming a most attractive conversion technology for energy production from fossil fuels, as well as an alternative source of biomass. This chapter gives an insight into gasification, beginning with a general introduction. It discusses different types of gasifier, as well as some of the innovative approaches. The last section of this chapter reviews design methods for different types of gasifier.
Key words: gasification, gasifier types, gasifier design methods, gasifier modeling.
Gasification, once extensively used for transportation and lighting during the Second World War, lost its merits because cheap and easy fuels were commercialized for power production. At present oil reserves are diminishing and coal combustion is creating the problem of environmental contamination with greenhouse gases. Gasification technology is again getting new life. Its growth in the past has been slow but future predictions show a sharp rise. It has become more modern and sophisticated, such that technically it can easily compete with the existing power generation technologies. Rises in fossil fuel prices, their scarcity and penalties for environmental contamination could be other forces driving the economics of gasification and making the technology more attractive, technically, as well as economically.
Gasification is a thermo-chemical process that converts solid carbonaceous feed into a gaseous fuel product in the presence of steam and/or sub-stoichiometric oxygen. The result of gasification is the producer gas, containing carbon monoxide, hydrogen, methane, and some other inert gases. When it is mixed with air, the producer gas can be used in gasoline or diesel engines with little modification. The gaseous product is applied mainly as fuel gas for electricity generation and direct heating. It can also be used as a synthetic gas in the process industry to produce methanol or ammonia. The idea of gasification power generation fits well with the decentralized energy generation concept. A small-scale gasifier system (10-30 KW) would be appropriate for many applications in villages in developing countries.
Theoretically, almost all kinds of biomass can be gasified but, practically, various properties of the materials impose limitations on the quality of gases that can be
produced. Higher volatile matter results in higher tar content, an undesirable product of gasification. Similarly, large particle size, higher moisture content, and ash content pose a lot of technical challenges. The key to successful design of a gasifier is to understand the properties and thermal behavior of the fuel fed to the gasifier.
Already in 1944, Berl34 reported that the biomass could be converted in hot compressed water into a petroleum-like product. In the 1970s and 1980s, the interest in alternative energy sources, such as biomass, was high due to the oil crises. Liquefaction research was started in 1971 by the US Bureau of Mines,27 with conversion of carbohydrates in hot compressed water in the presence of CO and Na2CO3. This combination of CO and Na2CO3 was reported in early HTC
developments to produce in-situ hydrogen,27 but Molton et al.24 showed that the use of CO in combination with alkali only leads to a limited increase in the oil yield.
Early work by the US Bureau of Mines led to the development of an 18 kg wood per hour process development unit (Albany pilot plant).35 In this installation, Douglas fir was liquefied, first using the product oil itself (the ‘Pittsburgh Energy Research Center/PERC process’) and later using water (the ‘Lawrence Berkeley Laboratories/LBL process’) as a carrier (see Fig. 18.5). For the LBL process, slurries, formed from acid pre-hydrolyzed wood chips and water, were used as
feedstocks. Operating problems led to several process modifications. However, not all issues were completely successfully resolved.36 This, along with a large number of parameters that needed to be studied,37 caused a shift to research in a much smaller scale (continuous 1 l autoclave).37,38
HTL, using biomass/water slurries of high organic/water ratios, was studied at the University of Arizona39-41 and the University of Saskatchewan42-44 by using special feeding systems.
Another important development involved sewage sludge treatment in the so-called sludge-to-oil reactor system (STORS). This process was developed using autoclaves and continuous installation with the capacity of 30 kg of concentrated sewage sludge (20 wt.% solids) per hour in the Battelle Pacific Northwest laboratories of the US Department of Energy.45 Sodium carbonate was employed as a catalyst.
After a period of reduced attention, the interest in conversion of biomass into energy carriers was renewed in the mid-1990s driven by political, environmental and economical incentives. For example, work on the Hydro-Thermal Upgrading (HTU®) process, developed during 1980s in the Shell Laboratories in Amsterdam, was restarted using a bench-scale experimental setup (10 kg water-biomass slurry per hour)5 and a pilot plant (20 kg dry matter per hour).4 To the best of the authors’ knowledge, this plant is now mothballed.
Several demonstration and (semi) commercial activities can be identified as well. A five 5-ton per day STORS process demonstration plant was built in Japan, with the aim of converting sewage sludge into a combustible energy carrier (see Fig. 18.5).46 After a successful municipal wastewater treatment STORS demo project in Colton, California, ThermoEnergy (USA) has patented the improved wastewater treatment process marketed under the name ‘Thermofuel process’. EnerTech Environmental Inc. (USA) is also developing a process for converting sewage sludge into a solid energy carrier, the ‘Slurrycarb process’. The company operates a 1-ton per day process development unit, a 20-ton per day process demonstration unit in cooperation with Mitsubishi Corporation in Ube City (Japan), and is currently commissioning a commercial-scale facility in Rialto, California. When completed, the installation will convert more than 880 wet tons of bio-solids per day from five municipalities in the Los Angeles area into approximately 170 tons per day of the product called E-Fuel.
Changing World Technologies was developing a so-called thermodepolymerization and chemical reformer process for conversion of turkey waste (carcasses) to fuel products and fertilizer. The company used a 15-ton per day pilot plant and a 200-ton per day processing unit (the Renewable Environmental Solution unit in Carthage, Missouri).
From this overview, it appears that the HTL of specific feedstocks to hydrophobic fuels for combustion (specifically solids) is nearing commercial operation. On the other hand, application of HTL for broader range of feedstocks and for production of transportation fuel precursors is still in the development stage.
Next to the pilot plant studies, a significant amount of laboratory-scale research was performed over the last four decades.23,26,47-52 In the past and also currently, this research has been dominated by chemical and kinetic studies. Mostly, these investigations use model components instead of real biomass. Recently, several complete reviews have appeared on these items.13,29-31 There is hardly any process development research ongoing. Also, the link between the insights gained by the chemical research with possibilities for process improvement is not well worked out. It is interesting to note that several research groups have realized that the knowledge obtained from HTL research is very useful for the development of other processes such as high-pressure thermal treatment of bio-liquids (HPTT),53,54 hydrodeoxygenation (HDO)55-58 and solvolysis.59,60 In particular, the knowledge of polymerization reactions of biomass’ decay products in HTL has provided, and can still further offer, many insights in the mechanisms and problems in HPTT, HDO and solvolysis.
For high temperatures (>500°C), alkalis have been proposed as catalysts.18 Alkalis promote the water gas shift and methanation reactions leading to more hydrogen or methane production and a carbon monoxide lean gas. The studies on whether or not alkalis enhance the extent of gasification are contradictory.64,65 Recovery of alkalis from the process may be a problem, because alkalis hardly dissolve in supercritical water. Antal et al.66 reported that leading the effluent of their empty tube reactor over a fixed bed of activated carbon derived from coconut increased the extent of gasification from 0.7 to 1.0. Kersten et al.65 used the Ru/TiO2 of PNNL and found complete gasification of glucose (1-17 wt% solutions) at 600°C and approximately 60 seconds residence time. The produced gas was at chemical equilibrium. The reaction is much faster at 600°C compared to 350°C, which is beneficial for the size of the reactor. However, no information is yet available concerning the stability of catalysts in the high temperature range supercritical water.
Reported problems with respect to the catalysts are poisoning through trace components such as sulfur, magnesium, calcium and the growth of the active metal crystals during operation (sintering). A general problem of the near and super critical region is that it enhances leaching of the catalytic active phases and degeneration of the support. Furthermore, if coke is formed on the surface of the catalysts, the high H2O concentration helps in keeping it clean via gasification. In accordance with that it was found that coke formation on the catalyst surface is a minor problem.67
Biomass can be converted via reforming into synthesis gas, H2/CO2 gas, and CH4/CO2 gas. The technology is in the R&D stage with some pilot work ongoing.
Applying natural catalytic materials (dolomite, olivine) in biomass gasifiers can lower the tar and the higher hydrocarbon content of the gas, thus reducing the load on downstream tar removal and reforming units. Engineered catalysts (primarily Nickel based) for inside gasifiers seem to be a dead end as there are too strong cooking, attrition, and poisoning issues. Tars and hydrocarbons can be removed downstream of the gasifier in relatively standard fixed bed type reformers. It is however essential that the feed gas of the reformer is cleaned from e. g. S, Cl and tertiary tars. Another great challenge will be dealing with the impurities in synthesis gas made from biomass for upgrading in secondary conversions (FT, alcohols, etc.). Downstream upgrading of bio-based fuel gas is technologically feasible (e. g. see the Sasol process for coal gas), but an expensive alternative (e. g. cleaning and pressurization).
Bio-liquids such as pyrolysis oil and its fractions and aqueous waste streams from other (biological) biomass conversions are considered as interesting feedstocks for reforming. These liquids are easy and cheap to pressurize and contain less contaminants than raw biomass. Besides these technical advantages, bio-liquids support a logistic scheme in which the primary conversion can be performed near the source of the biomass feedstock (e. g., remote, rural areas), with large-scale production of the finished bio-fuels in refineries near the market. Reforming of bio-liquids can become an important element of bio-refineries for hydrogen production. For quick introduction and growth of large amounts of bio-fuels, it is essential to integrate and to partner with existing industries and markets. In case of reforming, this can be done by co-feeding natural gas and naphtha reformers with bio-liquids. Reforming of bio-liquids can be done in the gas/vapor phase, the liquid phase or in the supercritical phase. All technologies have potential, but there are still challenges ahead. Optimal process and reactor configurations still have to be developed. Important issues here are handling of coke formation, mineral deposition, catalyst make up, heat addition, and biomass feeding systems. For reforming in hot compressed water feeding of biomass slurries is a real challenge while for the vapor phase system controlled atomization still requires R&D. Dedicated catalyst systems will be mandatory in biomass reforming. Extensive prior knowledge and experience with coal, oil, and natural gas can be used to modify, adapt, or design efficient catalysts. Most importantly, integration of catalyst, reactor, and process design and engineering in an early stage is needed.
Early in the research stage CJO was considered to be suitable as a fuel oil based on its visual properties. The greatest difference between CJO and diesel oil is in viscosity. The high viscosity of CJO may contribute to the formation of carbon deposits in Compression Ignition Engines (CIE). Incomplete fuel combustion results in reduced engine life. Reducing the viscosity of CJO oil by preheating or dilution with diesel fuel was studied in engine tests.6,24 To investigate the suitability of CJO oil as alternative fuel and examine emissions, two tests of performance and exhaust gas emission, and a long-term durability test of CIE in a direct injection (DI) engine were conducted. In performance and exhaust gas emission tests, JO10 (blend of 10% CJO and 90% diesel) was similar to diesel fuel. Its oxygen content is an advantage in improving combustion. Exhaust gas emission increased slightly because its slightly higher viscosity influences fuel atomisation. JO10 is a promising alternative fuel because its performance and exhaust gas emission are similar to diesel fuel. JO100 gave lower performance and higher emission compared to diesel fuel because of its high viscosity. Using JO100 the engine was difficult to operate. The long-term durability test indicated that JO10 resulted in operational problems including increased exhaust gas emission (HC, particulate matter), injector coking, piston and liner erosion. Maintenance frequency would be increased substantially including changing or cleaning of the injector nozzles at 125 hour intervals, thus increasing the cost of operation. Dilution of lubricating oil and friction caused by ring sticking and deposits in the combustion chamber would reduce the lifetime of engine components. The main concern is the fuel quality and composition. The content of phosphorous compounds in JO10 was found to be significant affecting the combustion process and exhaust emission. A degumming process to reduce the phosphorous level is therefore required to improve the fuel quality of CJO.
Diesel engines can be operated on either PPO or biodiesel. The biodiesel process increases the cost of production as many processes are needed, whereas PPO only needs degumming to decrease phosphorous content and deacidification to decrease acid number. Potential resources of PPO in Indonesia include coconut, palm and jatropha as they are tropical plants with a high population throughout the country. Various PPOs have been investigated.7,8 Test fuels include pure coconut oil (PCO), pure palm oil (PPaO), pure jatropha oil (PJO)9 and diesel fuel for comparison. Each PPO was blended with diesel fuel with composition 50%-volume and heated to 60°C, to decrease the viscosity by 1/10. Trials using a small DI diesel engine for 17 hours endurance tests under various operating conditions were conducted according to engine test bed procedures for DI diesel and engine injector nozzle coking test. PPOs are characterised by high viscosity, low volatility and low energy content. All PPOs had higher brake specific fuel consumption (BSFC) before the endurance test by comparison with diesel fuel, but at the end of the test all PPOs had BSFC similar to diesel fuel due to decreased friction between engine components. However combustion of PPOs was not as complete as that of diesel fuel because of poorer spray characteristics, evidenced by low CO2 and high UHC, carbon monoxide (CO), O2 and opacity emissions. The phosphorus content, unsaturated fatty acid content and low combustion quality of PPO result in higher engine deposits than for diesel fuel. Even though the PPOs had been degummed the residual phosphorous content contributed to deposit formation. Deposits from PPOs were between 140% and 290% more than from diesel. However PPOs exhibited anti-wear properties on the plunger and injector due to the lubrication effects of the fatty acid content. PCO had the best anti-wear property of the test fuels.
Further investigation of the combustion and exhaust gas emissions of a DI CIE using Jatropha curcas L. oil as CJO (JO) and PJO/Degummed Jatropha Oil (DJO) was done.10 Of all the tested fuels, DJO10 was found to be closest to diesel fuel in performance, exhaust gas emission and its combustion process (ignition delay).
In addition a study of combustion of Jatropha curcas L. oil (crude, degummed, fatty acid methyl ester) as a fuel in a DI diesel engine was conducted.11,12 The summary of conclusion drawn from the experimental data was as follows:
• JO100 and DJO100 have low cetane indexes and very high viscosity. Lower engine performance and high exhaust gas emission were found. However, these fuels can be used in emergencies.
• Blends of JO10 and DJO10 improve engine performance and reduce exhaust gas emissions at low engine load. However, nitrogen oxides (NOx) emission tends to increase.
Catalysis is the foundation of the chemistry industry and is widely used in large scale synthesis of bulk chemicals and fine chemicals. It is the heart of the fertiliser, petroleum, polymer, inorganic and pharmaceutical industries amongst others and is of growing importance in environmental control including pollutant and waste mitigation, pollution ablation as well as in the generation of new alternative energy and fuel sources. A general review of catalysis is beyond the scope of this book but the reader is referred to a number of excellent books. Chorkendorff and Niemantsverdriet have recently reviewed the general area of catalysis96 and Ertl et al. have edited a comprehensive summary of the state-of-the art.97 Morris and others provide a preface to summarise recent work.98 The industrial perspective has been well reviewed by Rase99 whilst Cybulski and Moulijn have presented details of modern reactors and process design.100 The import subjects of catalyst preparation and synthesis, which are pivotal in determining cost-effectiveness of the process and the efficacy of the catalyst itself, have been thoroughly summarised.101 Finally, the theory of catalysis has been detailed in depth.102 Catalytic mechanisms are normally defined by the adsorption and specifically the chemisorption of molecules at the catalyst surface.103
Catalysis has been divided into two separate subject areas. The first of these is homogeneous catalysis where the reactants and the catalyst are in the same phases, e. g. ion catalysed reactions in solution. The second area is heterogeneous catalysis, principally used in the manufacture of very large quantities of chemicals, organic and inorganic materials.104 Here, the catalyst and reactants are in different phases most usually a solid catalyst and gas phase reactants. This is most useful for high throughput and is used for many of the very large scale processes carried out in industry including sulphuric acid, ammonia, methanol, nitric acid, ethylene oxide, cyanide synthesis as well as petroleum reforming and cracking, gasification, steam-reforming and water-gas shift processes.
As every high school student would know, a catalyst increases the rate of a chemical reaction without itself being consumed or altered in the reaction. This is a result of the catalyst interacting with the reactants to lower the activation energy barrier to the reaction. It does not alter the energetics (i. e. the free energy) of the reaction and so does not change the equilibrium between reactants and products directly. In the most obvious use of a catalyst, the rate of reaction is increased allowing increased rates of production at a particular temperature. Alternatively, the catalyst may be used by offsetting the increased rate of reactions that can be achieved (which can be orders of magnitude greater) against the process temperature used. This may be used to simply move a reaction into a feasible temperature range where reactor engineering becomes practical. It can also be used to reduce the energy input required by endothermic processes allowing more economic operation. A simple example would be methane combustion which can be catalysed by a number of precious metals as well as lanthanide oxide materials.22-24 The catalyst is used in gas turbines to lower combustion temperatures, thereby reducing the oxidation of nitrogen to nitrogen oxides. Similar reactions where that catalyst would be used to simply increase the rate of reaction in the formation of the thermodynamically stable product are the oxidation of CO and hydrocarbons to CO2 as shown below.105
Practically, catalysis is often used in more complex ways so as to produce significant amounts of a product that would not be obtainable in an un-catalysed process. An example is the partial oxidation of ethylene to ethylene oxide:106
H H yos
Y = c’ + 0.502 ^ H — C — C — H
H H H H
This is the reaction which is mildly exothermic and the ethylene oxide formed is a partial oxidation product. The catalyst is not used to increase the rate of formation but rather to provide an operable process window at lower temperatures because at higher temperatures combustion of ethylene is highly favoured and the ethylene oxide would be a very short-lived and unrecoverable intermediate. Industrially, the reaction is catalysed by silver supported on alumina although various promoters for the epoxidation reaction as well as total oxidation inhibitors are used as part of the catalyst formulation to ensure high selectivity. The reaction operates at around 250°C, a low enough temperature for the desired product to be separated.
As mentioned earlier, the Haber-Bosch process has been used for the synthesis of ammonia from nitrogen and hydrogen for many years:11
N2 + 3H2 ^ 3NH3
This is an exothermic, thermodynamically favoured process but is kinetically limited. The reaction is catalysed by a potassium promoted iron catalyst. In simplistic terms, the catalyst is used to allow low temperature dissociation of nitrogen, the main activation energy barrier in the reaction. However, this is an equilibrium process and as the temperature is increased, the equilibrium favours the reactants (le Chateliers principle). In this way, although a catalyst does not alter the equilibrium in a chemical process, in this case the catalyst allows a low temperature (400°C) to be used and, thus, a higher equilibrium concentration of the product ammonia to be achieved. An un-catalysed mixture of hydrogen and nitrogen would reach the same equilibrium concentrations but would take considerably longer and not be consistent with the continuous process needed for large volume manufacture. The use of catalysts to effectively control equilibrium and rate is further illustrated by the water gas shift reaction:107
CO + H2O ^ CO2 + H2
This process is also mildly exothermic and equilibrium limited and like ammonia synthesis, the equilibrium favours reactants at higher temperatures but is kinetically limited requiring a catalyst to be used to achieve reasonable rates at temperatures below 1000°C. In order to achieve a high rate of reaction it is carried out in two different stages. The first is high temperature shift (HTS) using an alumina supported nickel catalyst at around 400°C which yields high rates of reaction. The second, low temperature shift (LTS) process uses an alumina supported copper based catalyst at around 200°C — the lower temperature allows almost complete recovery of the valuable hydrogen product at the equilibrium limit. The shift reaction(s) also demonstrates how important the synthesis of the catalyst is in these high volume processes since that catalyst must have sufficient activity and lifetime to be cost-effective. The subject of catalyst design and manufacture is briefly outlined in this review because in catalytic pyrolysis the catalysts used face very challenging environments.
In very general terms, the catalyst functions by reactant molecules interacting with the surface and becoming ‘activated’ in some way. Understanding how and where these reactions occur on the catalyst surface has been the subject of intense research and most researchers use an active site concept where specific arrangements of surface atoms or defects in the surface provide locations for adsorption/activating of reacting species.108 Very often, catalytic activity can be a unique property of a certain metal, oxide or combination. In order to maximise the number of collisions with the surface, it is usual to provide the active component with as high a surface area as is possible. This is normally achieved using an inorganic ‘ support’ over which the active component is ‘dispersed’.109 This allows very high surface areas of what are sometimes quite expensive materials (e. g. precious metals) to be achieved at relatively low amounts (below 5% by weight of the support) and at sustainable costs. The inorganic support is designed to be thermally robust and plays little part in the catalysis reactions although metal-support interactions between an active metal component and a support oxide have been well documented.110 In many cases the support is designed to be porous in order to provide as great a surface area as possible.111,112 Despite the fact that theoretically catalysts are unchanged by the reaction it promotes, their performance is often assessed by their lifetime in practical use. The lifetime is defined by the period in which its rate lies within an acceptable performance level (i. e. rate of production). Very often, the rate decreases with time and although this can be compensated by increasing reactor temperature, there is a point where practically an upper operating temperature is reached. A too higher temperature might be manifest as an unacceptable amount of side products — i. e. the reaction selectivity is compromised. Alternatively, the temperature may rise beyond process variables to protect plant and safety. This decrease in activity largely results from two different processes occurring during use. These are sintering and poisoning.113-115
Sintering is a complex process whereby loss of surface area is observed through thermal treatment which promotes mass transport and particle agglomeration or loss of pore structure. Pores play an important role in catalysis as they allow access to internal surfaces and can promote size controlled reactions where the molecules restrain the molecules that can enter the pore system. The process of sintering is thermodynamically favoured because it results in lower surface area and consequent decreases in the free energy of the system. Highly dispersed and supported active materials (as crystallites or particles) grow by a diffusion mechanism into larger particles. Other high temperature processes may also be responsible for sintering including solid-state reactions, new phase precipitation (e. g. in the thermal phase transformation of high surface area y-alumina to low surface area a-alumina) and the crystallisation of amorphous silica supports.
Poisoning generally describes the adsorption of strongly held species at the active catalyst sites. These passivate the surface to the desired catalyst mechanism. This results in deactivation as either reduction in production rate or as loss of selectivity to the required product. Common poisons include S, P and Cl. These are normally adsorbed from contaminants in the gas phase and sacrificial adsorbents can be used to reduce the concentration of these.116 More importantly for pyrolysis catalysts, carbon is also as an important poison and arises from hydrocarbon at the catalyst surface. Extensive carbon formation can leave to ‘coking’ where thick carbon deposits are formed. This coking can also be useful in oil chemistries because it can lead to useful liquid (resids) formation.117