Category Archives: Handbook of biofuels production

Flash pyrolysis

Flash pyrolysis is an extension to fast pyrolysis where heating rates reach around 1000°C/s. Flash pyrolysis has been reviewed several times.66-68 The residence time of the solid is less than a second and depending on the type of reactor the temperatures can be as low as 500 or as high as 1200°C. The very high rate of heating requires very rapid heat transfer from the reactor environment to the feedstock and, because of this, particle sizes of less than 0.5 mm but usually less than 100 um are required. The small particle sizes of feedstock also result in small particles of char and this is a major disadvantage of the technique.65 Great care must be taken to remove particles of char from the as-produced bio-oil because it can catalyse polymerisation of some of the products and increased viscosity of the bio-oil.65 The major advantage of flash pyrolysis is the improved energy efficiency of the process which can be in excess of 70%.69 Although we have not discussed the efficiency of pyrolysis processes here, it should be noted that pyrolysis is a strongly endothermic process and energy must be supplied to affect the heating in the reaction. The source of energy for heating is the feedstock itself, either before or after pyrolysis. The challenge for engineers is to configure reactor technologies to minimise losses through heat recovery and other methods to allow efficient heating of the feedstock during pyrolysis. There are various reactor technologies used in flash pyrolysis and are briefly introduced below.

1 Fluidised bed and circulating FBRs: These are probably closest to integration into large scale commercial use, and large scale pilot plants have been demonstrated.70

2 Entrained flow reactor: This reactor has been scaled to allow pyrolysis of 500 kg/h of feedstock.71 In this reactor a carrier gas and a combustion gas (to produce the pyrolysis temperature by combustion) are fed into a reactor tube and powdered feedstock is fed into the high flow gas stream. Whilst the design is relatively simple the use of carrier gas (usually nitrogen) is a disadvantage.

3 Vacuum pyrolysis: This is a relatively new technique where the sample is heated under vacuum; the vacuum removes pyrolysis generated volatiles which are then condensed to the bio-oil.72 This technique results in low residence times and also allows for rapid separation of the oil and char. Its major advantage is that it can operate at relatively low temperatures of 500°C.

4 Rotating cone reactor: This type of reactor was developed by scientists in The Netherlands.73,74 In the rotating cone reactor, biomass particles are fed to the bottom of a rotating cone with inert heat carrier particles and are pyrolysed whilst being transported spirally upwards along the cone wall. The advantage is the absence of a carrier gas and high oil yields.

5 Ablative pyrolysis: There are several versions of this methodology which varies considerably from the other techniques discussed. It consists of solid particles being exposed directly to heat via contact with a heated surface or radiatively.75 The action of pressing the particles against the hot surface reduces heat transfer requirements.

The final type of pyrolysis often differentiated in the literature is catalytic pyrolysis, the main subject of this article. In reality this is not a completely different form of pyrolysis and can be used with the same type of reactors, etc. outlined above. Catalytic pyrolysis is outlined in depth below.

Entrained bed gasifier

Most of the gasifiers developed since 1950 are of the entrained flow type. The advantages of using entrained flow gasifiers lie in their flexibility in handling any type of coal as feedstock to produce clean, tar-free product gas. With the development of the Integrated Gasification Combined Cycle (IGCC) as a prospect technology for overcoming greenhouse gas emission issues and being more efficient, use of entrained bed gasifiers will further increase in the future for power generation. Entrained bed gasifiers operating at high pressure can supply the product gas at high pressure to the IGCC system without additional compression.

In the entrained flow gasifier, a dry pulverized solid is gasified with oxygen (much less frequently, air) in co-current flow. The gasification reactions take place in a dense cloud of very fine particles (typically <100 pm). The much smaller biomass particles mean that the fuel must be pulverized, which requires somewhat more energy than for the other types of gasifiers. Entrained flow gasifiers operate at high temperature (1300-1500°C) and high pressure (20-50 bar), and thus high throughputs can be achieved (Drift et al., 2004). The high temperatures also mean that tar and methane are not present in the product gas. Thermal efficiency is, however, somewhat lower, as the gas must be cooled before it can be cleaned with existing technology. By far the greatest energy consumption related to entrained bed gasification is in the production of oxygen used for the gasification.

There are two types of entrained bed gasifiers: slagging and non-slagging. One differs from the other by the way in which the ash is removed from the system. If the ash is removed in molten form, then it is of the slagging type. If the ash is removed in solid form, then it is of the non-slagging type. To ensure the proper operation of the slagging type, the flow of molten ash should be 6% of the fuel flow. The non-slagging type is mostly favored if the ash content of the fuel is below 1% (Drift et al., 2004).

To feed the fuel at higher pressure, the size of particles needs to be very small. This limits the use of biomass as fuel, as it is fibrous in nature and very difficult to cut into smaller sizes. Also lower bulk density and low heating value reduces its suitability as fuel for entrained bed gasification. To use biomass as fuel, a larger amount of carrier gas is required. This means higher energy for compression of the gas and also a product gas with a poor heating value due to dilution with the carrier gas. In the case of pneumatic feeding, the power penalty is high. For instance, pressurizing biomass up to 40 bars using pneumatic feeding consumes

°.°25 kW^Wth wood reducing efficiency by °.°4 kWsyngas^Wth wood (Drift et aU

2004).

Figure 16.10 shows different types of the entrained bed gasifier. The Siemens EGB gasifier consists of a top fired reactor. The reactants are introduced into the reactor through the single centrally mounted burner. This process has some special advantages: it provides axis symmetrical construction, reducing equipment costs, flow of the reactant is from a single burner, thus reducing the number of points to be controlled, and, lastly, the product gas and slag flow in the same direction, reducing any potential blockage in a slag trap (Higman and Burgt, 2008).

Koppers-Totzek atmospheric process is the first entrained flow slagging gasifier operated in atmospheric pressure. This process has been commercially built mainly for the ammonia manufacturing process. It consists of two-side mounted burners, where a mixture of coal and oxygen are injected. The gas leaving at the top at a temperature around 1500°C is quenched with water first. The reactor has a steam jacket to protect the reactor shell from the high temperature (Higman and Burgt, 2008).

The E-Gas gasifier is a two-stage coal/water slurry feed entrained flow slagging gasifier. It is designed to use sub-bituminous coal. The coal slurry is fed in at the non-slagging stage, where the upward flowing gas gives heat to it, thus the gas exits at a lower temperature. The gas is then passed through a fired tube boiler and is filtered in a hot candle filter. The char is separated out at the hot candle filter and is again taken back to the slagging zone of the gasifier. The slag is quenched in a water bath at the bottom of the slagging reactor (Higman and Burgt, 2003).

The British Gas/Lurgi proposes a novel coal gasifier. It is a dry fed, pressurized, fixed bed slagging gasifier. Oxygen and steam are introduced into the gasifier vessel through sidewall-mounted tuyeres (lances) at the elevation, where combustion and slag formation occur. The coal mixture (coarse coal, fines, briquettes, and flux), which is introduced at the top of the gasifier via a lock hopper system, gradually descends through several process zones. Coal at the top of the bed is dried and devolatilized. The descending coal is transformed into char and then passes into the gasification (reaction) zone. Below this zone, any remaining carbon is oxidized, and the ash content of the coal is liquefied, forming slag. Slag is withdrawn from the slag pool by means of an opening in the hearth plate at the bottom of the gasifier vessel (Phillips).

The Hitachi gasifier is an oxygen blown entrained gasifier where the pulverized coal is fed at two stages. At the upper stage, two burners are arranged tangentially to feed the pulverized coal spirally into the gasifier. This gives a swirl motion to the coal, thus increasing the residence time. Oxygen in excess is supplied at the lower zone to melt the slag. In the upper stage, reaction occurs at relatively lower temperatures in the presence of less oxygen. Thus, the coal particles get de-volatized and the char formed moves down to be reacted with high temperature gas.

BFW

 

image158

Oxygen

 

image97

image160

(a) Koppers-Totzek EBG
(b) BLG slagging EBG

Oxygen, steam

 

Burner

 

Cooling screen

 

Pressure water inlet —I Quench ^ water!

 

Cooling jacket

 

Water

overflow

 

^ Granulated slag

 

Подпись: c

image162 image163 image164

Siemens EBG

Подпись: (e) E-gas EBG(d) MHI air blown EBG

16.10 Different types of entrained bed gasifier (a) and (c), Higman and Burgt (2008); (b), Basu (2006); (d), Higman and Burgt (2008); (e), EPRI/ Advanced Coal Generation).

Table 16.4 Comparison of different types of entrained bed gasifier (De Souza, 2004)

Entrained flow gasifier

Entrained flow moving bed gasifier

Koppers-

Texaco

Foster

Combustion

Lurgi Lurgi dry

Totzek

Wheeler

engineer

slagging ash

Steam/O2

Water/O2

Steam/air

Air

Steam/O2 Steam/O2

Pressure

Mpa

0.13

4

2.5

0.1

2.1

2.5

Combustion

°C

1925

1400

1370-1540

1750

2000

980-1370

temperature Gas exit

°C

1480

230 (after

925-1150

925

350-450

370-540

temperature

quenching)

Steam

kg/kg

0.4

0.5

0.05

0

1

4

(water)/

oxidant

Oxidant

kg/GJ

52

37

111

139

20

17

Coal

s

1

3

N/A

2.5

0.4

1

residence

time

Cold gas

%

75

75

90

69

90

80

efficiency

CO

%

53

53

29

23

61

18

CO2

%

10

12

3

5

3

30

H2

%

36

35

15

12

28

40

CH4

7

9

N2

%

4

1

1

1

GCV

MJ/

11.3

11.1

6.6

4.2

13.8

11.3

m3

The Shell Coal Gasification Process can gasify any type of coal that can be pulverized to the right size and pneumatically transported. Buggenum in The Netherlands was the first IGCC plant built using SCGP with a capacity of 2000 tons/day (see Table 16.4).

Reactors and process conditions

Several good reviews have been published in the last decades analysing the fundamentals and comparing different reactors for the FT synthesis (Dry, 1996; Dry, 2002; Geerlings et al., 1999; Guettel and Turek, 2009; Sie and Krishna, 1999). The heterogeneously catalyzed FT reaction is highly exothermic, with the heat released per reacted carbon atom averaging at about 146 kJ (Anderson, 1956), about an order of magnitude higher than heat released in processes typically applied in the oil industry (Sie and Krishna, 1999). Due to this extremely high
exothermicity, the rapid removal of heat is one of the major considerations in the design of FT reactors that have to be able to quickly extract the heat from the catalyst particles in order to avoid catalyst overheating and catalyst deactivation and at the same time maintain good temperature control. Moreover, the reaction usually takes place in a three-phase system, gas (CO, H2, steam and gaseous hydrocarbons), liquid hydrocarbons and solid catalysts, thus imposing great demands on the effectiveness of interfacial mass transfer in the reactor (Sie and Krishna, 1999). Last but not the least, the FT process is a capital-intensive process, and therefore, for both economic and logistic reasons, it is only economically favourable on a very large scale. Easy reactor scale-up is therefore a third important requirement when considering a reactor type for the FT process. Three main reactor types, discussed in the following paragraphs, have been commercialized or are thought as promising for industrial applications: multitubular fixed bed reactors, gas/solid fluidized bed reactors and three-phase slurry reactors.

Cellulosic ethanol

Figure 21.2 shows a general schematic of the conversion of lignocellulosic biomass to bioethanol. The process consists of a pre-treatment step, a hydrolysis step and a fermentation step, followed by distillation and dehydration. In this process, lignin is discharged as a by-product and can be used to generate electricity to supply the process with energy or to export to the electricity grid.

Pre-treatment is necessary to break open the lignocellulosic structures and to facilitate the separation of the main carbohydrate fractions hemicellulose and cellulose from lignin, in order to make these better accessible for hydrolysis, the next step in the process (Mosier et al., 2005). Pre-treatment is considered by

image153

many as the most costly step in lignocellulosic biomass conversion to ethanol. Pre-treatment may also significantly affect costs of subsequent steps in the process, including hydrolysis, fermentation as well as down-stream process steps (e. g. product separation). A variety of pre-treatment methods have been studied and some have been developed at pilot scale or demonstration scale. Current pre-treatment methods include: steam explosion, liquid hot water or dilute acid, lime, and ammonia pre-treatments (Maas, 2008). Pre-treatment methods using organic solvents such as ethanol or organic acids have been evaluated as well.

Hydrolysis is the process to convert the carbohydrate polymers cellulose and hemicellulose into fermentable sugars. Hydrolysis can be performed either chemically in a process involving the use of concentrated acids or enzymatically by using enzymes. Most pathways developed today are based on enzymatic hydrolysis by using cellulases and hemicellulases that are specifically developed for this purpose. Fermentation is the main process used to convert fermentable sugars, produced from the previous hydrolysis step, into ethanol. While in principal, the fermentation process is largely similar to that in the current ethanol production facilities, a major fraction of sugars produced from lignocellulosic are pentoses (5-carbon sugars such as xylose), which are difficult to ferment with standard industrial microorganisms. Therefore, a second important challenge in the conversion of lignocellulosic biomass to ethanol is the optimisation of ethanol-fermenting microorganisms that can convert all biomass-derived sugars, including xylose and arabinose. Furthermore, the efficient integration of various unit operations into one efficient facility is challenging. In some processes, the hydrolysis and fermentation steps are combined into one process which is often referred to as simultaneous saccharification and fermentation or SSF. Lignocellulosic biomass conversion to ethanol is currently in the pilot plant stage, with more than 30 pilot plants being operated or erected in both North America,
the EU and elsewhere (IEA Task 39, 2009). Furthermore, in the recent years two demonstration plants for lignocellulosic biomass conversion to ethanol were erected in Canada and in Spain. In addition, one demonstration plant for cellulosic ethanol was commissioned in Denmark, and further plants are in the planning phases. All three demonstration plants were designed to use wheat straw as primary feedstock.

Findings from durability test

Variance of engine power and fuel consumption in percent (%) comparing with those parameters before 300 hours durability test of each testing engine is depicted in Fig. 23.23. Where D243-B5 and D243-Do are in turn of the testing engine fuelled with biodiesel B5 and market diesel; D243-B5-150h means the testing engine D243 fuelled with biodiesel B5 after 150 hours. The same definitions are applied for D243-B5-300h, D243-Do-150h and D243-Do-300h.

As shown in Fig. 23.23 the engine power decreased and the fuel consumption increased after 150 hours and 300 hours durability test. Although the differences are not much due to short period running time, there is a clear consensus in the changes of engine power and fuel consumption. The fact that the engine fuelled with biodiesel B5 had lower changes of engine power and fuel consumption after 150 hours and 300 hours durability test is not relevant with other research results which showed higher engine wear when the engine was fuelled with biodiesel.1

Exhaust emissions were measured before, after 150 hours and after 300 hours durability test following R49 driving cycle. Results are given in Fig. 23.24.

It is shown in Fig. 23.24 that none of the emission components meets Euro2 emission standard limits. This reveals somehow the current emission quality of the diesel engine in Vietnam. The emission components HC, CO and PM were risen but NOx depleted with the test period. These results match with the deflection of engine power and fuel consumption as mentioned above, again longer testing period is needed to have better evaluation of engine durability.

Principally, as the wear of engine’s parts increased after a certain time of operation, compression pressure reduced and more combustion products blew to

D243-DO — 300 h D243-DO — 150 h D243-B5 — 300 h D243-B5 — 150 h

 

image194

Emissions deflection (%)

23.23

image195

Deflection in percent of engine power and fuel consumption during 300 hours durability test.

Deflection (%)

23.24 Deflection in percent of emission components during 300 hours durability test.

crankcase, the combustion process of the engine deteriorated causing worse engine’s performance, high hydrocarbon, carbon monoxide and particulate matter were formed, whilst nitrogen oxide reduced due to lower temperature.

There was no damage observed to the engine’s components during 300 hours durability test with biodiesel B5 fuel. The potential coking of the injector was not found as the kinematic viscosity of the biodiesel B5 fuel is almost equal to that of the market diesel. However, this has to be considered with higher biodiesel blends because high viscosity of the biodiesel causes larger fuel droplet sizes. The fuel droplet size is a function of surface tension, density and viscosity. Since the viscosity of biodiesel is high, the fuel droplets are large and hence may not be fully burned. The remaining biodiesel may then decompose at high temperatures (430-480°C) and form deposits.

Fluid catalytic cracking (FCC) catalysts

It is beyond the scope of this chapter to describe commercial FCC catalysts in depth. These types of catalysts are extensively detailed in the following chapter and the reader is referred to the data provided there. In this section we will mainly discuss the application of FCC catalysts that can be used directly in the pyrolysis process rather than upgrading of pyrolysis oils. Whilst many catalysts have shown promise, it has proved important to use robust commercially available catalysts in the development of catalytic pyrolysis as a commercially viable technology. The FCC process allows for efficient conversion of high-boiling point and high — molecular weight hydrocarbon fractions of crude oil into more valuable petrol fuel grades.206 Their use has been refined since their introduction in the 1960s (for petroleum refining) to allow for high performance, long-life and re-activation in fluidised bed systems. Scherzer has given an excellent view on the design of these catalysts as applied to zeolite-Y.207 How these catalysts deactivate through a combination of coking, poisoning and attrition and the deactivation of these catalysts is an area of great interest both industrially and academically.208 The synthesis of zeolite materials usually provides small particles that can not be readily sintered into larger materials because of their highly crystalline nature and these particles are too fine for commercial applications. The basic design of these commercial catalysts allows development of catalyst particles that can be readily supported in a fluidised bed, and an FCC catalyst usually consists of a mixture of activated alumina (as described earlier), the active zeolite, a binder (normally a silicate) and an inert matrix (a clay or related material; kaolin is often used). The alumina and the binder provide both mechanical and thermal robustness. The inert matrix allows the formation of larger particles (a few micron in diameter) and pellets (less than 100 micron diameter). Careful synthetic processing is required to allow the hydrocarbons access to and from the active phases within these complex systems. Coking is the major problem (deactivating the active sites as well as physically blocking pore systems) and in use the catalysts are continually re-circulated between the reactor (the riser) and the oxidising regeneration chamber.

FCC catalysts have been widely used for catalytic pyrolysis of polymers,209 biomass133 as well as various vegetable/plant oils.210 Samolada et al. found that FCC catalysts were effective in the pyrolysis of a bio-oil producing low coke and gas yields compared to several other zeolite and transition metal catalysts.133 The catalyst also effected the greatest degree of de-hydrolysis but the stability of the oil was somewhat lower than other catalysts.133 Ioannidou et al. found that FCC catalysts were effective in the pyrolysis of corn cobs and stalks providing a higher quality bio­oil than in the absence of catalyst.27 A similar finding was made by Antonakou and co-workers who found that the use of an FCC catalyst greatly improved the stability of the pyrolysis oil compared to thermal pyrolysis in its absence.142 Work by Lu et al. reports that FCC catalysts (for pyrolysis of biomass) based on a combination of HZSM-5 and y-Al2O3 are more effective in improving both isomerisation and aromatisation than a zeolite-Y based material.211 Zhang et al. have recently published excellent work on FCC catalysed pyrolysis of corn cobs.212 They compared different relative volumes of catalyst and biomass in a fluidised bed and found the ratio had a profound effect on the product distribution. Whilst fresh catalyst resulted in greater dehydration of the corn, used catalyst resulted in greater oil yields. It was also found that the improvement in stability of the product oil was related to the reduction of some active oxygenated hydrocarbon species that promoted polymerisation.212 The use of FCC catalysts to upgrade bio-oil produced by pyrolysis of lignin through the removal of polymerisation active phenols has recently been reported by Gayubo et al.213 These results all point to the effectiveness of these catalysts. It should be stressed that the majority of pilot-scale testing of these technologies for biomass pyrolysis has been largely dominated by these catalysts.

FCC type catalysts appear successful for pyrolysis of heavy oils but the amount used has to be carefully controlled in order to optimise the yield of oil and an ideal product distribution.147 In polyolefin pyrolysis FCC catalysts have been shown to be particularly effective with good production of lighter hydrocarbons and good aromatic content. Indeed, the performance of FCC catalysts appears to be significantly better than zeolite-Y or ZMS-5 with not only improved liquid yields but a greater proportion in the gasoline/petrol composition range.214 The reason for the more effective behaviour of these catalysts appears to be the bimodal pore size distribution arising from the combination of microporous and mesoporous structures exhibited by the different materials used in the formulation of these materials.215 One of the more consistent findings for these catalysts for polymer pyrolysis is that spent (i. e. after cycling through the reactor and regenerator in typical FBRs) materials have better than expected or even better performance characteristics than fresh catalysts and this appears to be true for a range of polymers and process conditions.216,217

Production of bioalcohols via gasification

J. M.N. VAN KASTEREN, Eindhoven University of Technology, The Netherlands

Abstract: This chapter discusses developments and possibilities in the field of alcohol production via synthesis gas based on biomass feedstocks. The most promising technologies based on biomass gasification are believed to be the catalytic and the biocatalytic routes. Biobased synthesis gas fermentation processes to methanol, ethanol and even butanol are being developed. The main bottleneck for these fermentation based processes are still the relatively low concentrations of alcohol in water (<5 wt%), which can be reached with bacteria. New alcohol-water separation processes are needed to make these processes become feasible.

Key words: gasification, bioalcohol, biocatalyst, fermentation, synthesis gas.

17.1 Introduction

Alcohol production via gasification is already a very well established process for methanol production. In fact most of the methanol produced nowadays is based on the catalytical conversion of synthesis gas (Ullmann’s Encyclopedia of Industrial Chemistry, 2001). The synthesis gas is being produced from fossil fuels, e. g. natural gas. For the higher alcohols these routes are not so common. Ethanol being the second largest alcohol produced is mainly produced via fermentation of sugars and for a smaller part via direct hydrolysis of ethylene. Research and developments are focused on production of ethanol directly from synthesis gas via a (bio)catalytical route making the synthesis gas route more interesting. Besides methanol and ethanol the most important alcohols are 1-propanol, 1-butanol, 2-methyl-1-propanol (isobutyl alcohol), the plasticiser alcohols (C6 — C11), and the fatty alcohols (C12 — C18), used for detergents. They are prepared mainly from olefins via the oxo synthesis, or by the Ziegler process (Ullmann’s Encyclopedia of Industrial Chemistry, 2001).

The aim of this chapter is to enlighten and discuss more the developments and possibilities in the field of alcohol production via synthesis gas based on biomass feedstocks. Environmental effects (greenhouse gas emissions), demand for independencies on fossil fuels and rising costs of fossil fuels have set an urge to diversify feedstocks and use biomass also as a chemical resource.

Since the 1970s the interest for the use of biomass as feedstock for chemicals and fuels has risen and resulted in an increase in the fermentation processes for ethanol especially as a fuel for automotive purposes. Also the increase in the use of biodiesel which contains 10 wt% methanol has increased the demand for biobased methanol.

Disadvantage of the fermentation routes of ethanol is that they can only convert sugar into ethanol, limiting the biomass feedstock from an economical and efficiency point of view to high yield sugar containing crops like sugar cane and sugar rich waste streams.

Gasification of biomass and subsequent conversion of the synthesis gas produced to alcohols would overcome these disadvantages. The problem with the present factories for methanol production is that they are based on a very large scale input (mainly natural gas) which means that very large amounts of biomass will have to be transported to one location. This is not economically attractive. Many options have been suggested such as to convert the biomass via digestion into methane which can be transported via pipelines to the factory. This means building up a pipe line infrastructure with subsequent high investment costs. Another route tried at a large methanol plant in Delfzijl in The Netherlands is to convert the glycerol byproduct from biodiesel production facilities into methanol via gasification (BioMCN, 2009). Disadvantage for this route is that there is a hydrogen shortage which has to be added from other sources.

The most promising technologies based on biomass gasification are believed to be the catalytic and the biocatalytic routes. The idea is in both case the same: convert synthesis gas via a (bio)catalyst into alcohols. Methanol is very difficult to achieve with biocatalyst because of its more toxic nature. For ethanol, this seems more promising and making smaller scale plants interesting (more fitting to the decentralised character of the biomass production process). Fermentation of the gasification product gas, however, is a rather new development.

Datar et al. (2004) have been working on the fermentation of producer gas, and have successfully produced ethanol. Figure 17.1 shows the schematic of the

biomass to ethanol process. The idea is to gasify the biomass to synthesis gas (CO + H2) and subsequently ferment this biosynthesis gas to ethanol via a direct fermentation process. The first step is to gasify the biomass input to synthesis gas.

The gasification/fermentation pathway is a very interesting alternative way of producing bioethanol. Via traditional fermentation processes, lignin, an important component of biomass cannot be fermented. Gasification and subsequent fermentation of the produced gas enables fermentation of all carbon and hydrogen containing material and also non biodegradable materials like plastics. The resulting higher feedstock efficiency should make the biobased, smaller scale processes economically feasible (Van Schijndel and Van Kasteren, 2004).

Production of biofuels via biomass reforming

G. VAN ROSSUM and S. R.A. KERSTEN, University of Twente, The Netherlands

Abstract: This chapter describes various technologies for biomass reforming for the production of high-value gases. These gas mixtures can be used for the production of fuels and chemicals or as a product itself (like hydrogen). Both ‘wet’ and ‘dry’ biomass conversion technologies are detailed with and without intermediate processing steps. Throughout the chapter, the conversion of biomass via fast pyrolysis and subsequent reforming is highlighted.

Key words: biomass, steam reforming, reforming in hot compressed water, pyrolysis oil, gasification.

20.1 Introduction

Reforming is a technology to upgrade biomass into tuned gas mixtures. Synthesis gas (H2/CO), H2/CO2 gas and CH4/CO2 gas are possible products.

A combination of hydrogen and carbon monoxide can be used for the manufacturing of ethers, alcohols and Fischer-Tropsch products. H2/CO2-rich gas is a feedstock for alcohol production. Hydrogen is an interesting fuel as such. There is also an increasing demand for hydrogen in the current petrochemical industry and it is envisaged that hydrogen will become of paramount importance to make biomass compatible with fossil refinery streams. Methane can be used as a substitute natural gas (SNG) for the grid or in compressed form (CNG) as motor fuel. The reform reactions of biomass (here represented by C6H10O4) can be described by the following conceptual stoichiometric equations:

C6H10O4 + 2H2O ^ 6CO + 7H2 [20.1]

C6H10O4 + 2CO2 ^ 8CO + 5H2 [20.2]

Reaction [20.1] is steam reforming and reaction [20.2] represents dry (CO2)

reforming. Like for fossil feedstock, both reactions require catalysts. The water-

gas-shift [20.3] and methanation [20.4] reactions will typically reach equilibrium over reform catalysts.

CO + H2O « CO2 + H2 [20.3]

CH4 + H2O « CO + 3H2 [20.4]

The proposed operating regime for biomass reforming is very broad and ranges from 230°C to 1000°C and 1 bar to 300 bar. Without a catalyst, the reaction of biomass and H2O/CO2 will yield a typical fuel gas at temperatures below 1000°C:

C6H10O4 + aH2O ^ bCO + cH2 + dCO2 + eCH4 + fCxHy + gH2O + tars [20.5]

Next to steam and dry reforming, auto-thermal reforming is also a well-known reaction system:

C6H10O4 + H2O + 0.5O2 ^ 6CO + 6H2 [20.6]

Gasification and reforming of fossil feedstock have been two separate developments. For biomass feedstock, gasification and reforming processes cannot be distinguished that easily. In this chapter, gasification will be used to denote the non-catalytic processes converting biomass into gas, and reforming will be used for catalytic biomass-to-gas technologies. Biomass can be raw biomass from the fields or biomass-derived products such as pyrolysis oil and aqueous by-products from biological conversion processes. For coal and heavy oil, gasification systems are now in operation and for natural gas, associated gas and naphtha reforming is used. Gasification systems for fossil fuels are thermal processes1 while fossil fuel reforming uses a catalyst (except for the Exxon and Kellog catalytic coal gasification processes, but these never reached commercial implementation2). On the other hand, biomass gasification and biomass reforming have always been interconnected technologies. Reforming activity has been introduced originally inside low-temperature (<900°C) biomass gasifiers to upgrade the product gas catalytically. There have been also attempts to create direct contact between solid biomass and catalysts (e. g. by impregnation), but this is outside the scope of this chapter.3 It has been attempted to add catalytic active materials to the gasifier and to use dedicated down-stream catalytic reactors to remove hydrocarbons (tars) and to upgrade the fuel gas to synthesis gas or hydrogen. Later, reforming systems have been proposed for liquid biomass streams and for very wet biomass feedstock. To understand the developments in biomass reforming, it is necessary to have some insight in ‘reforming of fossil fuel or feedstock’ (Section 20.2.1), ‘gasification of fossil fuel or feedstock’ (Section 20.2.2) and ‘biomass gasification’ (Section 20.2.3). For this reason we start this chapter with short accounts on these technologies.

As mentioned before, the proposed operating regime for biomass reforming is rather broad. This is mainly because two essentially different chemical processes are considered:

(1) Steam or dry reforming, using pressures up to 30 bar and temperatures of 350-1000°C. In this process the reactants and products are in the gas/vapor phase.4

(2) Aqueous phase or hot compressed water reforming (hereafter called reforming in hot compressed water).5 This process uses sub — or super-critical water as reaction medium. Temperatures in the range of 230-700°C are used. The
pressure is chosen in such a way that water is either in the liquid or supercritical state (Tc = 274°C, Pc = 220.6 bar). A typical operating pressure for temperatures above the critical temperature lies around 250 bar.

Next in this chapter, the chemical thermodynamics of biomass steam reforming (Section 20.3) are introduced. Because most reforming catalysts are designed to obtain chemical equilibrium (there are some recent developments6 that aim at designing catalysts that produce hydrogen by reforming in hot compressed water under conditions favoring methane thermodynamically), this thermodynamic analysis gives insight in the product distribution that can be obtained at different conditions. The biomass feedstock for reforming (Section 20.4.1) is briefly discussed as well as those bio-refinery concepts and processing schemes that include reforming pyrolysis oil or its fractions (Section 20.4.2).

image140
The heart of this chapter is the description of the ongoing research and status of proposed and tested technologies for reforming of biomass (see Figure 20.1), as summarized in the following sections:

20.5.2 Reforming of bio-liquids (e. g., pyrolysis oil and its fractions)

20.5.3 Reforming of gases/vapors produced by biomass gasifiers/evaporators.

20.5.4 Reforming of very wet biomass streams in hot compressed water.

These technologies will be compared and we will end with conclusions (Section 20.6) including R&D needs to mature these technologies.

Valorization of by-products for the production of biofuels

C. ECHIM, R. VERHP and C. STEVENS, Ghent University, Belgium and W. DE GREYT, Desmet Ballestra Group, Belgium

Abstract: The valorization of by-products helps to reduce waste, to minimize the footprint of the technology and to add value through the production of biodiesel as an energy carrier. Alternative resources such as deodorizer distillates can partially replace the traditional feedstocks for the production of biodiesel, but require application of new technologies and/or additional purification steps. This chapter proposes to offer an overview of different methodologies used to convert deodorized distillates to biodiesel/biofuels and to recover the valuable minor components such as sterols, squalene and tocopherols.

Key words: biodiesel production from deodorizer distillates, conversion routes for high-acidity feedstocks, recovery of the minor components.

22.1 Composition of deodorizer distillate

Crude vegetable oils contain triacylglycerols (TAG) as major component and various minor components such as diacylglycerols (DAG), monoacylglycerols (MAG), free fatty acids (FFA), phospholipids, tocopherols, sterols, squalene, color pigments, waxes, aldehydes, ketones, triterpene alcohols and metals which may affect the quality of the final product. These minor components are removed partially or entirely by either physical (RBD) or chemical (NBD) refining.

Deodorizer distillate (DD) is one of the side streams obtained in the final step of refining of vegetable oils used to remove odoriferous components and to reduce the free acidity in order to make the vegetable oils suitable for human consumption.

It was observed that the composition of DD is dependent on the oil source, the refining routes (physical or chemical) and the deodorizer operating conditions (De Greyt and Kellens, 2000; Kellens and De Greyt, 2000). Determination of the DD composition or stability was studied by different authors (Haas and Scott, 1996; Verleyen et al, 2001; Dumont and Narine, 2007; Dumont and Narine, 2008). DD obtained from physical (RBD) and chemical (NBD) refining of different feedstocks contains typically 30-90% FFA, an important unsaponifiable matter such as tocopherols, sterols and squalene (5-33%), but also acylglycerols (<1-14%) (Table 22.1).

Physical (RBD) and chemical (NBD) refining differ both in the composition of the deodorized oil and of the distillate.

It was observed that physically refined oils have a higher retention of unsaponifiables in the oil compared with the chemically refined oils. A sterol retention varying

Table 22.1 General composition of DD

Compounds

Deodorizer distillates

(%)

RBD*

NBD**

Water

Free fatty acids

80-90

30-60

Acylglycerols

<1-14

5-12

Phospholipids

Unsaponifiable matter

5-10

25-33

Source: Echim et al. (2009).

* RBD = physical refining (refined, bleached and deodorized) ** NBD = chemical refining (neutralized, bleached and deodorized)

between 68% and 90% in the physical and 79% and 87% in chemical refining is observed while tocopherol retention between 23% and 92% in the physical and 21% and 73% in chemical refining was found. The higher retention of unsaponifiables in the physically refined oil is attributed to the lower vapor pressure of these components due to the advance of FFA during deodorization (Verleyen et al., 2002).

DDs obtained from the chemical refining are rich in tocopherols and sterols and contain little FFA. On the contrary, distillates derived from physical refining contain mainly FFA and consequently little tocopherols and sterols representing little economic value.

Deodorization has an important effect on the overall refined oil quality and distillate composition. The last decade’s increased attention has therefore been paid to the optimization of the deodorizing process conditions and the development of improved deodorizing technology (Verleyen et al., 2002).

The development of a new type of scrubber operating at two different tempera­tures (dual condensation concept) allows the production of DDs with a unique com­position (high in tocopherols and sterols) and higher value (Kellens et al., 2005).

A modification to the condenser unit was made for the collection of the distillate in two separate fractions. The first fraction contains mainly FFA (80%) where the second fraction contains concentrated sterols and tocopherols (17% and 15%, respectively) and residual FFA (43%) with a similar composition as DD from chemical refining (Verleyen et al., 2002).

Techniques for improvement of fermentative hydrogen production

At present, development of a practical and efficient hydrogen generation process is a growing concern among the research community. In the last decade, several methods such as mutagenesis, genetic modification or metabolic pathway control have been shown to improve hydrogen yield in laboratory scale experiments. These methods are based on the metabolic pathway and the enzymes which are involved during the fermentative hydrogen production process. Hydrogen evolution follows the NADH (Nicotinamide Adenine Dinucleotide) pathway described by the reaction [13.10] which is catalyzed by the enzyme of hydrogenase. Increased hydrogen yields could be achieved by shifting the chemical reaction so as to increase the amount of NADH usable for hydrogen production:

NADH + H+ ^ NAD+ + H2 [13.10]

NADH is usually generated by the catabolism of glucose to pyruvate via glycolysis. In general, the yield of hydrogen produced upon mixed acid fermentation of carbohydrates is quite lower than the maximum theoretical yields since sugars fermentation, in addition to VFAs, also leads to the formation of various reduced end-products, such as ethanol, butanol and lactate. These compounds contain additional hydrogen atoms that are not liberated as gas. Therefore, in order to maximize the yield of hydrogen, bacterial metabolism should be directed away from alcohols and reduced acids and towards VFAs production. The conversion of pyruvate to ethanol, butanediol, and lactic acid involves oxidation of NADH. The concentration of NADH would increase if the formation of these alcoholic and acidic metabolites could be blocked (Das and Veziroglu, 2001). Kumar et al. (2001) reported enhanced hydrogen yields by blocking the pathways of organic acid formation using the proton-suicide technique with NaBr and NaBrO3. A similar enhancement of hydrogen yield using

E. aerogenes HU-101 was reported by blocking the formation of alcoholic and acidic metabolites by both allyl alcohol and the proton-suicide technique (Mahyudin et al., 1997).

Operation conditions such as pH, HRT, temperature and hydrogen partial pressure are reported to have a significant effect on metabolic balance. C. acetobutyricum has the ability to produce solvents at pH values lower than 5 and under phosphate and iron limiting conditions. In order to obtain high hydrogen yields using C. acetobutyricum, a pH above 5, phosphate and iron concentrations above the limiting levels and glucose concentration below 12.5% are recommended (Dabrock et al., 1992). In addition, clostridia produce VFAs and hydrogen in the exponential growth phase and rapid alcohol production occurs in late growth phase (Lay, 2000). In order to shift the metabolic pathway towards VFAs production and away from solventogenesis, an application of a low HRT should be essential.

It is also reported that a hydrogen partial pressure higher than 60-100 Pa inhibits the hydrogen production process and in order to obtain maximum hydrogen yields, the hydrogen produced should be removed from the reactor system. For this reason many approaches have been proposed. Mizuno et al. (2000) showed that gas sparging with nitrogen enhanced hydrogen yield, while Voolapalli and Stuckey (1998) developed an applicable technique based on a submerged silicone-membrane dissolved gas extraction system, removing hydrogen and carbon dioxide from the reactor volume. Another potentially efficient method for removing hydrogen from the gas stream based on a heated palladium-silver membrane reactor has been proposed by Nielsen et al. (2001).

Another strategy for enhancement of hydrogen production by existing pathways can be sought by increasing the flux through gene knockouts of competing pathways or increased homologous expression of enzymes involved in the hydrogen-generating pathways. Up to now, the majority of attempts in laboratories employ the metabolic engineering of E. coli, because its genome can be easily manipulated, its metabolism is the best understood of all bacteria and it readily degrades a variety of sugars. For example, Yoshida et al. (2005) performed genetic recombination of E. coli in conjunction with process manipulation to elevate the efficiency of hydrogen production in the resultant strain SR13. The genetic modification resulted in 2.8-fold increase in hydrogen productivity of SR13 compared with the wild type strain. However, it is still unclear what pathways function under what environmental conditions and what the substrate specificities of all hydrogenase-coupled pathways are involved in E. coli (Laurinavichene et al., 2002b).

Metabolic engineering of other native hydrogen-producing microorganisms has so far been limited because there is poor knowledge regarding the existing pathways involved in hydrogen-production system (Jones, 2008). Despite this fact, there are few recent noteworthy examples of improvement in fermentation — based hydrogen production by either genetic or chemical engineering strategies of mesophilic hydrogen producing strains. For example, mutants of both E. aerogenes and E. cloacae have been isolated after subjecting wild-type strains to chemically selective media, requiring alterations in fermentation product metabolism for survival. In each case, this has resulted in substantial increases in the yield of hydrogen per glucose consumed (Kumar et al., 2001; Ito et al., 2004). In addition, overexpression of a native ferredoxin-dependent hydrogenase in Clostridium paraputrificum also resulted in a near doubling of fermentative hydrogen yield (Morimoto et al., 2005). However, progress in the field of metabolic pathway engineering needs to be made in order to develop optimized microorganisms producing high yields of hydrogen, at competitive rates and being able to utilize broader substrate ranges. In this respect, lab-scale hydrogen production will be soon scaled up and applied in real systems converting rich in carbohydrates feedstocks to hydrogen.