4.2 Biorefinery Concept

According to the Biomass Research and Development Technical Advisory Committee of the US Departments of Energy and Agriculture (2002) Report published by the U. S. Department of Energy and U. S. Department of Agriculture, the biorefinery is defined as "A processing and conversion facility that efficiently separates its biomass raw materials into individual components and converts these components into marketplace products including biofuels, biopower, and conventional and new bioproducts." Several papers discussed the major products and integrated biorefinery concept for syngas fermentation.

Ethanol is by far the most economical bio-product that is generated during syngas fermen­tation. Ethanol is currently being sold as a fuel additive to blend with gasoline. The existing gasoline engines can take up to 10% ethanol (known as E10) without modifying the engine. Biomass-derived syngas fermentation also produces other important bio-product such as acetic acid, butanol, and butyric acid (Datar et al., 2004). Acetic acid has numerous applications in chemical industries including synthesis of vinyl acetate and acetic anhydride. Butanol is con­sidered as a better transportation fuel compared to ethanol due its high energy content and high vapor pressure. In addition, butanol is used in the production of butyl acetate and butyl acrylate which can be used as fuel additives to enhance the octane value of gasoline. Butyric acid is being used as a flavoring agent in the food processing industry.

Apart from the main products, organic acids and alcohols, the growth of anaerobic microbes also produces valuable biochemical such as polyester which serves as an energy storage unit for the organism (Brown, 2006). Most of the syngas-fermenting microorganisms produce these polyesters under stressed conditions such as nutrient imbalances. Polyhydrox — yalkanoate (PHA) is one of the most known polyesters produced in the cells, and it is stored as a discrete granule. Polyester content of cell is as high as 80% (dry weight).

In conventional biochemical-based ethanol plants, lignin fraction of the biomass is consid­ered as a low-value residue. Usually, 10 to 30% of biomass feedstock contains lignin which has a higher heating value of 9,111 Btu/lb. Therefore, the lignin recovered from the diverse feedstocks should be integrated into the process. Thermal cracking of lignin at high temperatures ranging from 250 to 600 °C showed the potential of producing low molecular weight gaseous feedstocks for further processing.

In an integrated biorefinery, the process is optimized to produce biofuel, along with other high-value products such as biopower and bio-based materials for a long-term sustainability.

A simple method for the rapid heating of biomass particles is to mix them with the moving sand particles of a high-temperature fluid bed. High heat transfer rates can be achieved, as the bed usually contains small sand particles, generally about 250 pm. The heat required is generated by combustion of the pyrolysis gases, and/or char, and eventually transferred to the fluid bed by heating coils. While the sand to biomass heat transfer is excellent (over 500 W/m2 K), the heat transfer from the heating coils to the fluid bed will be low, due to the resistance inside the coils (gas to coil wall heat transfer estimated 100-200 W/m2K), and the limiting driving force of around 300 °C as a maximum. In an optimistic case, at least 10-20 m2 surface area is required per ton/h of biomass fed.

The temperature of the culture media affects the syngas fermentation in two ways. Firstly, it affects the growth kinetics and secondly, it affects the solubility of the syngas in aqueous medium. In most cases, the temperature of the culture media is decided based on the specific microorganism. The most favorable growth temperature for mesophilic microorganisms is 37-40 °C, while for thermophilic microbes, it is between 55 and 80 °C.

4.5 Types of Microorganism

Strict anaerobe, C. Ijungdahlii is one of the most studied microorganisms in syngas fermen­tation. Apart from C. Ijungdahlii, several other strict anaerobic mesophilic microbes have been identified that are capable of fermenting syngas into biofuels. Some of these microbes include C. aceticum, A. woodii, C. autoethanogenum, and C. carboxydivorans (Klasson et al., 1992; Rajagopalan et al., 2002; Younesi, et al., 2005). Significant efforts are being made to genetically engineer syngas-fermenting microbes to improve the yield.

(i) Use of fossil fuel and raw materials to produce biofuels: The whole life cycle of the

production of biofuels involves the use of fossil fuel and raw materials to some extent. Whether the net gain balance out of the fossil input obtained in terms of low emissions is positive or not remains under discussion.

(ii) Availability of land for fuel (food vs. fuel issue): Current biofuel producers do not always have a secure access to raw materials due to limited grain reserves and the fact that the current costs of crude vegetable oil from "food crops" are variable. Bio-based energy industries are also currently in competition with food producers, and we perceive them as being a primary cause of the increase in food prices. In order to make biofuel production profitable and more sustainable, avoiding as much as possible competition with the food market, companies have to focus on second-generation biofuels made from alternative cheap feedstocks (e. g., (waste)-biomass, waste oils and fats, residues, etc.). (Ligno) cellulosic ethanol and biodiesel from waste oils, nonfood crops, or algae emerge as real alternatives to tackle this problem.

(iii) Environmental impact: Despite the fact that some studies carried out to date show first-generation biofuels may offer a low carbon balance, fossil fuel usage and GHG balance, further outputs and environmental indicators must be addressed. Water usage (in the growth of the crops), eutrophication (run off of lawn fertilizers into natural waters), and soil erosion are some of them. Second — and potential third-generation biofuels are more attractive in terms of crop economy.

(iv) Socioeconomic impact: Some sectors of the industry estimated that a robust global biofuel market will be fully established around 2012. The implementation of biofuels will also be highly dependent on the feasibility of the technologies employed for their production and the economics of the processes play a fundamental role in this regard.

At the moment, there is not yet a widely accepted definition of "sustainable biofuels," or a scheme for certification and labeling (Mol, 2007). Nevertheless, some agreement can be observed on four ecological issues that should be included in sustainability schemes such as GHG emissions, energy balance, biodiversity loss, specific environmental effects (i. e., soil condition and water use).The problem is that each feedstock is different and many crops produce their best yields in specific regions of the world or require certain soil or water conditions. These local differences demand specific attention and are not easily generalized. Furthermore, there is wide disagreement on the implementation of international conventions, while inclusion of social criteria is even more difficult (Oosterveer and Mol, 2010).

A holistic approach to valorization of lignocellulosic biomass needs to take into account sustainability of chosen options. If concepts are too heavily orientated toward energy produc­tion or industrial use, this can even be at the expense of environmental protection. If crop residues such as straw are no longer left on field, this will result in depletion of soil organic matter. While anaerobic digestion results in a digestate, which can be brought back to field to supply not only nutrients but also organic matter, thermal valorization and production of second-generation biofuels result in complete consumption of the biomass and consequently a lack of nutrients and organic matter. Soil requirements vary within a wide range and need to be assessed locally. Only lignocellulosic biomass which is in surplus of soil demand for organic matter should be considered for treatment options with complete consumption of the substrate. In regions with concern about declining organic content of soils, anaerobic diges­tion should be given special attention even if the net energy recovery is lower compared to that of alternative technologies with total consumption of the biomass (Sigrid and Morar, 2009).

Ethanol is one of the major desirable products of syngas fermentation. Ethanol is com­monly used as a direct additive to gasoline. It has an octane value of 129 and the energy con­tent is about 70% of that of gasoline. Most of the syngas-fermenting microbes use acetyl-CoA pathway to produce ethanol. During that process, CO and H2 are oxidized and produce electrons and H+ ions necessary for the reactions, while CO2 gets reduced to Co-CH3 by accepting the electrons and H+ ions. Toward the end of the pathway, Co-CH3 and Co-A react with CO and produce acetyl-CoA under the influence of CODH and acetyl-CoA synthase enzymes. Acetyl-CoA acts as a building block for the production of a variety of biofuels including ethanol (Figure 1).

C. Ijungdahlii is one of the most frequently used microorganisms in syngas fermentation to eth­anol. Younesi et al. (2005) achieved an ethanol concentration of 0.6 g/L maintaining a syngas pressure of 1.8 atm in their bioreactor. The authors further reported that the high syngas pressure did not have a significant impact on acetic acid production, though it enhanced the ethanol yield. Klasson et al. (1990) reported a higher ethanol yield (3.0 g/L) by adding 0.02% yeast extract followed by cellobiose. The study further showed improvement in molar ratio of ethanol to ace­tate (>1.1) with the addition of 30 mg/L benzyl-viologen. Klasson et al. (1993) reported the highest ethanol concentration ever recorded (^48 g/L) with C. Ijungdahlii at a pH of 4.0-4.5 in a completely stirred tank reactor under nutrient-limited condition during 560 h of fermentation.

CFB reactor has been widely used for the pyrolysis of lignocellulosic biomass into high yield of liquid products (Rapid Thermal Process, RTP; UOP).The CFB reactor has many advantages, for example, the simple structures, high production capacity, favorable conditions of heat and mass transfer, and the convenience of operation, etc., the CFB was used as the main reactor in this study. To reduce the operation cost, part of the pyrolysis gas was used as the carrier gas, while the rest and the pyrolysis char were recycled as heat.

The CFB could be divided into two zones corresponding to the main chemical processes.

(i) pyrolysis zone: In this zone, feedstock was loaded into the bed and pyrolyzed very quickly. Since the feedstock particles were small and the heat exchanged rapidly, the heating rate was very high. For example, a small particle at 0.1-0.2 mm diameter could be heated at the rate of about 103 °C/s in an atmosphere at 1000 °C. In this zone, the main chemical process could be described as

Biomass! char + tar + H2O + gas (CO2, CO, CH4, CnHm, H2).

Temperature was another essential factor affecting the pyrolysis besides heating rate. Because the relatively high temperature was favorable to form more noncondensable gas and decrease the tar yield, moderate and carefully controlled temperature was needed.

(ii) Reduction and cracking zone: Before the pyrolysis vapors were quenched by the condenser, further reactions had taken place; for example, the tar cracked and the char was reduced. These processes produced more noncondensable gas such as CO and H2. Some CnHm also cracked at the same time. The main reactions could be expressed as

C + H2O! CO + H2,

C + CO2 ! 2CO,

CH4 + H2O! CO + 3H2,

CO + H2O! CO2 + H2,

Tar! CH4 + H2O + CnHm + H2.

Pyrolysis char contributed to secondary cracking by catalyzing secondary cracking in the vapor phase; rapid object of gasification is to get high-quality gas product. Thus, the high temperature of up to 900 ° C is wanted to increase the gas product and decrease the tar, while the relatively long residence time contributes to the secondary reactions including char reduction, tar cracking, shift reaction, etc. So the amount of CO2, CO, CH4, and H2 is far more, and the amount of CnHm is less in the gas product of gasification. By contrast, the objective of fast pyrolysis is to obtain more liquid product; it determines the operation conditions of moderate temperature and short residence time to increase the liquid pro­duction rate. Such operation conditions lead to the higher amount of CnHm and less amount of CO, CH4, and H2, which indicate that the degree of pyrolysis is not excessive.

Growth media provides the microbes with all essential nutrients such as minerals, trace elements, vitamins, and reducing agents for their maximal growth. The selection of the growth media depends on the selected species and the targeted end products. Reducing agents (e. g., sodium thioglycolate, ascorbic acid, and benzyl-viologen) result in shift in the electron flow, thereby diverting the carbon flow from acid to alcohol production (Klasson et al., 1992). Researchers have developed their own protocols for media preparation, while some specific media are provided by American Type Culture Collection (ATCC) (e. g., ATCC culture 1754 for C. Ijungdahlii, acetobacterium medium ATCC 1019).

Several potential scenarios for biofuels can be foreseen in the future. The big hopes for the transport sector are second — and future third-generation biofuels, including biodiesel from microbial oil, the production of biobutanol (from nonedible feedstocks) as a more petrol-like fuel, and the preparation of biofuels from cellulosic and biomass nonedible feedstocks.

One of the critical factors that will influence the future prospects of biofuels is diversifica­tion. The future of biofuels as a sustainable (economic, social, and environmental definitions) technology is directly linked to the maximum use of byproducts that will make its production more cost effective.

Biology and synthetic biology would have the opportunity to design plants with special properties through genetic engineering to produce biomass feedstocks with requisite ratio and functionalities of lignin/cellulose (Luque et al., 2008). Such discoveries can significantly change the future of biofuels, as a major contribution to the GHG reduction through the regulation of the CO2 fixed by plants in crops.

Acknowledgments

The authors thank The Director, Indian Institute of Petroleum, Dehradun, for his constant encouragement and support. RS thanks Council of Scientific and Industrial Research (CSIR), New Delhi, India, for providing Junior Research Fellowship (JRF).

B. methylotrophicum has the ability to convert syngas into acetic acid, butyric acid, and buta­nol. Shen et al. (1999) compared the physiological differences between the wild-type and the CO-adapted strains of B. methylotrophicum, and the production of both butyrate and butanol from CO. The authors found that the activity of the wild-type B. methylotrophicum was completely inhibited by the presence of CO. The study further reported that the CO-adapted strain produced significant amount of butyrate, while the wild-type produced only trace amounts of butyrate. The CO-adapted strain produced 0.33 g/L of butanol and 0.5 g/L etha­nol at pH 6.0 from the microbes grown at 100% CO.

In a different study, Worden et al. (1989) studied the possibilities of ethanol and butanol production via syngas fermentation. The authors found an increase electron flow of 6-70% from CO into butyrate when the pH was lowered from 6.8 to 6.0. The high level of butyrate essentially increased the butanol yield in a two-stage fermentation process (Worden et al.,

1991) . During the two-stage process including acidogenic and solventogenic bioconversions, Worden et al. (1991) used two different biocatalysts, B. methylotrophicum and Clostridium acetobutylicum, in a two-stage process. The authors reported high butyrate (4 g/L) and acetate (8 g/L) concentrations while the biomass recirculation was maintained. The authors further reported a butanol concentration of 2.7 g/L from the continuous operation. Eqs (6) and (7) show the change in Gibbs free energy (AG°) for the reactions of CO bioconversion to butyric acid (C3H7COOH) and butanol (C4H9OH) (Worden et al., 1991).

10CO + 4H2O! C3H7COOH + 6CO2 AG° = -40.61kJ/gmole CO (6)

12CO + 5H2O! C4H9OH + 8CO2 AG° = —37.68kJ/gmole CO (7)

The auger type of pyrolyzer has been identified as especially appealing for its potential to reduce operating costs associated with bio-oil production. This design may also be well suited for small, portable pyrolysis systems in a highly distributed or decentralized biomass processing scheme. The operating principle of this design is that biomass is continuously pyrolyzed by being brought into direct contact with a bulk solid heat transfer medium referred to as a "heat carrier." The heat carrier material, such as sand or steel shot, is heated independently before being metered into the reactor. On a gravimetric basis, thermodynamic calculations suggest a heat carrier feed rate 20 times the biomass feed rate. Two intermeshing, co-rotating linch augers quickly combine biomass and heat carrier in a shallow bed to effec­tively carry out the pyrolysis reactions. This mechanical mixing process, though not well understood, appears to be the essence of this alternative pyrolyzer design. Volatile vapors and aerosols exit at various ports, while char is transported axially through the 20 inch long reactor section and stored in a canister with the heat carrier (Brown, 2009).