Biomass, Conversion Routes and Products - An Overview

JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm 1 AT ER IA L Biomass, Conversion Routes and Products – An Overview K.K. Pant and Pravakar Mohanty Introduction GH TE D 1.1 M Department of Chemical Engineering, Indian Institute of Technology Delhi, India The world consumes nearly two barrels of oil for every barrel produced. The depletion of conventional resources and stricter environmental regulations, along with increasing demand for commercial fuels and chemicals, has led to the need to find the alternatives to conventional fuel and chemical sources. Renewable plant materials are considered as one of the most promising alternatives for the production of fuels and chemicals. The conventional sources for fuels and chemicals are fossil fuels, crude oil natural gas, coal and so on, which are dwindling rapidly. With the concept of green chemistry, there is every necessity to produce alternative sources of energy and fuels from renewable biomass. Biomass refers to all organic matter generated through photosynthesis and many other biological processes. The ultimate source of energy this renewable biomass is inexhaustible solar energy, which is captured by plants through photosynthesis. It includes both terrestrial as well as aquatic matter, such as wood, herbaceous plants, algae, aquatic plants; residues such as straw, husks, corncobs, cow dung, sawdust, wood shavings, sawn wood, wood based panels, pulp for paper, paper board, and other wastes like disposable garbage, night soil, sewage solids, industrial refuse and so on [1]. Biomass can provide approximately 25% of our current CO PY RI JWST448-c01 Transformation of Biomass: Theory to Practice, First Edition. Edited by Andreas Hornung. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. Companion website: http://booksupport.wiley.com JWST448-c01 JWST448-Hornung 2 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass Table 1.1 Forest resources, area (ha), year (2010). Name of country Africa South America North and Central America Asia Europe Oceania Caribbean World Land area (million ha) Forest area (million ha) % Forest area per 1000 people 2965 1756 2110 3094 2214 849 23 13010 674 864 705 593 1005 191 7 4033 23 49 33 19 45 23 30 31 683 2246 1315 145 1373 5478 166 597 energy demand, if properly utilized. Taking into account the production of biomass with respect to land and forest area, there are 4033 million ha of forests worldwide, as presented in Table 1.1. In India 55.2 million ha of waste land is available for a wide range of short period energy crop productions [2]. Tropical and subtropical forests comprise 55% of the world’s forests, while temperate and boreal forests account for rest 45% [3]. The average area of forest and wooded land per inhabitant varies regionally. Production and use of wood fuel, industrial round wood, sawn wood, wood-based panels, pulp for paper, paper board (m3 ) usage and its production are presented in Table 1.2. The total carbon stored in forest biomass is approximately 331 Giga tonnes (GT). About 27% of biomass is used directly as carbon feedstock, for example, sawn wood, wood based panels, pulp for paper, paper and paper board, mainly in developing countries. However, 33% is used as an industrial raw material and the remaining 40% is used as primary or secondary process residues, suitable only for energy production, for example, for production of upgraded biofuels [2, 3]. Approximately 70–77% of the global wood harvest is either used or is potentially available as a renewable energy source. The most efficient utilization of these resources comes when they are converted to liquid and gaseous products by appropriate technologies. Non-commercial biomass (biofuels) is the main source of energy available in the rural areas. An estimation by the Food and Agriculture Organization (FAO) shows that the global production of wood fuel and round wood reached 3410 million m3 during 2010 [2–4]. Just over half of this was wood fuel, where 90% of that is being produced and consumed in developing countries. On the other hand, industrial round wood production, totaling around 1542 million m3 in 2010, is produced and consumed both by North and Central America and Europe. 1.2 Features of the Different Generations of Biomass Broadly, biomass can be categorized as first, second, third, and fourth generation. First generation biomass refers to traditional plant biomass like sugar and starch crops. Second generation biofuels include bioethanol and biodiesel produced from the residual, non-food parts of crops, and from other forms of lignocellulosic biomass, such as wood, grasses, and municipal solid wastes [5]. Third and fourth generation biofuels include algae-derived fuels, JWST448-c01 JWST448-Hornung Use Prod. Use 616 201 131 755 150 16 5 1868 616 201 131 755 148 16 5 1867 72 185 480 245 507 52 1 1542 69 181 473 292 488 41 2 1545 Wood based panels (m3 ) (106 ) Pulp for paper (m3 ) (106 ) Paper and paper board (m3 ) (106 ) Prod. Use Prod. Use Prod. Use Prod. Use 8 38 120 88 137 10 0.5 400 12 32 120 108 111 8 1 391 3 15 49 121 77 4 0.1 269 3.5 10 54 116 76 3 0.5 264 3 20 73 44 50 3 0.001 193 3 9 64 59 54 2 0.01 191 4 14 101 153 113 3 0.2 389 7 16 99 161 102 4 1 388 Trim: 244mm × 170mm Prod. Sawn wood (m3 ) (106 ) Printer Name: Vivar Africa South America North and Central America Asia Europe Oceania Caribbean World Industrial round wood (m3 ) (106 ) 10:27 Wood fuel (m3 ) (106 ) June 25, 2014 Table 1.2 Production and utilization of wood fuel, industrial round wood, sawn wood, wood-based panels, pulp for paper, and paper and paper board, year 2010. JWST448-c01 JWST448-Hornung 4 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass such as biodiesel from microalgae oil, bioethanol from micro algae and seaweeds, the fine chemicals and H2 from green microalgae, and microbes by sub- and supercritical extraction processes. Further these extracted microalgae can be utilized as biomass in thermochemical or biochemical routes of conversion [6]. “Drop in” fuels like “green gasoline,” “green diesel,” and “green aviation fuel” produced from biomass are also considered as fourth generation biofuels [7]. Efforts are also underway to genetically engineer organisms to open the secrete of these fourth generation hydrocarbon fuels. In Figure 1.1, both food and non-food biomass have been integrated in the sequential downward stream for establishment Dedicated Energy crop Herbaceous Wood Short rotation woody Aquatic plant Seaweeds, algae, hyacinth Oilseed plant Palm, jatropha Crop (food) Grass Starch sugar crop Grain (rice, wheat), sugar cane, Potatoes, Corn Biomass Agricultural waste Straw (rice, barley, wheat), bagasses, corn stover Cellulosic resources Non-food Biomass Forest waste Sawdust, pulp waste, thinned wood Municipal waste Food waste, yard waste, container and product packaging Industrial waste Manure (cattle/fresh) Black liquor from paper industry, waste from food industry Animal manure, plant manure, compost Figure 1.1 Biomass feedstock distribution in term of food and non-food basis for bio-refinery. JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview Table 1.3 5 Generation-wise biomass distribution with its features. 1st generation 2nd generation 3rd generation 4th generation Feedstock Sugar, starch crops, vegetable oil, soya bean, animal fat, straw Micro algae biomass Genetically modified crop Product Biodiesel, sugar alcohol, corn ethanol Environmentally friendly, economical and socially secure Wood, agricultural waste, municipal solid waste, animal manure, landfills, pyrooils, pulp sludge, grass Hydro treating oil, bio-oil, FT-oil etc. Algae oil Biofuel Not competing with food Environmentally friendly advanced technology under process to reduce the cost of conversion Acidic, viscous, high oxygenates content in pyrooils Availability of high value protein and nutrients, residual algae for jet fuel animal feed Easily captures CO2 and conversion to a carbon neutral fuel Advantage Disadvantage Limited feedstock, blended partly with conventional fuel Slow growth of algae, extraction of algae oil is difficult and costly — of the biorefinery concept towards energy surplus. Generation-wise details of the biomass diversifications are presented in Table 1.3 [7, 8]. At present, biomass represents approximately 14–18% of the world’s total energy consumption [3, 4]. In order to utilize these resources properly, biomass should be converted to energy that can meet a sizeable percentage of demands for fuel and chemicals. Efficient utilization of biomass as a potential feedstock depends on general information about the composition of plant species, heating value, production yields and bulk density. Organic component analysis reports on the kinds and amounts of plant chemicals, including proteins, oils, sugars, starches, and lignocelluloses (fibers) required much attention about their behavior [1, 7]. 1.3 Analysis of Biomass The main components of biomass are cellulose, hemicelluloses, and lignin: Cellulose or carbohydrate is the principal constituent of wood and other biomass and forms the structural framework of wood cells. It is a polymer of glucose with a repeating unit of C6 H10 O5 strung together by 𝛽-glycosidic linkages. The 𝛽-linkages in cellulose form linear chains that are highly stable and resistant to chemical attack because of the high degree of hydrogen bonding that can occur between chains of cellulose. Hydrogen bonding between cellulose chains makes the polymers more rigid, inhibiting the flexing of the molecules that must occur in the hydrolytic breaking of the glycosidic linkages. Hydrolysis can reduce cellulose to a cellobiose repeating unit, C12 H22 O11 , and ultimately JWST448-c01 JWST448-Hornung 6 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass Table 1.4 Organic components and composition of lignocelluloses biomass (dry basis). Feedstock Bagasse Bamboo Corn stover Corncob Herbaceous energy crops Rice straw Short rotation woody crops Wheat straw Wheat chaff Waste paper Cellulose (wt. %) Hemicelluloses (wt. %) Lignin (wt. %) Other (wt. %) 35 55 53 32 45 38 50 38 38 76 25 28 15 44 30 25 23 36 36 13 20 17 16 13 15 12 22 16 16 11 20 0 16 11 10 25 5 10 11 0 to glucose, C6 H12 O6 . Heating values for cellulose may be slightly different based upon the feedstock [8, 9]. Hemicellulose consists of short, highly branched chains of sugars. In contrast to cellulose, which is a polymer of only glucose, a hemicellulose is a polymer of five different sugars. It contains five-carbon sugars (usually D-xylose and L-arabinose), six-carbon sugars (D-galactose, D-glucose, and D-mannose) and uronic acid. The sugars are highly substituted with acetic acid. The branched nature of hemicellulose renders amorphous properties which is relatively easy to hydrolyze to its constituent sugars compared to cellulose. When it hydrolyzed, the hemicellulose from hardwoods releases products which high in xylose (a five-carbon sugar). The hemicellulose that contained in softwoods, by contrast, yields six more carbon sugars [7, 8]. Lignin is the major non-carbohydrate, polypenolic structural constituent of wood and other native plant materials that encrusts the cell walls and helps in cementing the cells all together. It is a highly polymeric substance, with a complex, crosslinked, highly aromatic structure and having the molecular weight of about 10 000 derived principally from coniferyl alcohol (C10 H12 O3 ) by extensive condensation and polymerization [1, 8, 9]. For the efficient utilization of biomass, feedstock engineers are particularly evaluating the hemicellulosic component and the distribution among cellulose, hemicelluloses, and lignin. Table 1.4 gives an idea of the organic components of some of the dedicated energy crops, common sugar, and starch crops, respectively. 1.3.1 Proximate and Ultimate Analysis of Biomass Analysis of biomass and its characteristics is generally accomplished by both proximate and ultimate analysis. Proximate analysis separates the products into four groups: (i) moisture, (ii) volatile matter, consisting of gases and vapors driven off during torrefaction or pyrolysis, (iii) fixed carbon, the non-volatile fraction of biomass, and (iv) ash, the inorganic residue that remains after combustion. The remaining fraction is a mixture of solid carbon (fixed carbon) and mineral matter (ash), which can be distinguished by further heating the sample in the presence of oxygen; the carbon is converted to CO2 and only leaving the ash [9]. Table 1.5 JWST448-c01 JWST448-Hornung Table 1.5 Thermochemical properties of the selected biomass (proximate and ultimate analysis). Ash FC C H O N S Cl Ash Ref. 18.7 17.65 17.2 19.45 — — 20.60 26.70 16.14 18.064 15.68 16.28 17.33 18.64 16.02 17.51 73.56 75.17 86.57 — — — — — 65.47 70.55 55.5 69.33 73.78 81.36 — 71.30 3.51 5.58 1.27 — — — — — 17.86 0.83 19.52 13.42 11.27 3.61 22.40 8.90 19.9 19.25 12.16 — — — — — 16.67 16.35 14.99 17.25 14.95 15.03 — 19.80 48.62 43.65 44.00 47.97 47.09 50.90 50.30 58.51 40.96 45.66 38.43 41.78 44.80 47.45 41.10 43.20 5.90 5.56 6.11 5.88 5.54 6.30 6.00 8.57 4.30 4.86 2.97 4.63 5.35 5.75 5.29 5.00 45.15 43.31 47.24 45.57 39.79 38.60 43.50 23.46 35.86 34.94 36.36 36.57 39.55 42.37 — 39.40 0.33 0.61 1.24 0.30 0.81 1.37 0.10 3.67 0.40 1.38 0.49 0.70 0.38 0.74 1.96 0.61 — 0.01 0.14 — 0.12 0.03 — — 0.02 0.06 0.07 0.08 0.01 0.08 0.41 0.11 — 0.6 — — — — — 0.12 — 6.26 1.27 0.50 5.77 2.80 0.20 5.78 18.34 0 0.34 0.12 0.03 — 0.28 21.68 15.90 9.79 3.50 — 11.40 [10] [9] [9] [11] [9] [12] [13] [15] [9] [9] [9] [9] [9] [9] [9] [9] Trim: 244mm × 170mm Volatile Printer Name: Vivar HHV (dry) MJ/kg 10:27 Bamboo Corn stover Corn grain Coconut shell Maize straw Olive husk Pine sawdust Rape seed Rice hull Sawdust Rice husk Rice straw Sugar cane bagasses Switch grass Water hyacinth Wheat straw Ultimate analysis (% wt., dry) June 25, 2014 Biomass Proximate analysis (% wt. dry) JWST448-c01 JWST448-Hornung 8 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass provides both the proximate and ultimate analysis (dry basis) for a wide range of biomass materials. Ultimate analysis deals with the determination of the carbon and hydrogen in the material, are found in the gaseous products after combustion. Using these analysis, the molecular weight analysis becomes simpler. For example, cellulose and starch having the generic molecular formula C1 H1.7 O0.83 , hemicelluloses can be represented by C1 H1.6 O0.8 and wood by C1 H1.7 O0.83 . Typical thermochemical properties of some selected biomass materials based on proximate and ultimate analysis are given below (Table 1.5) [9–15]. The calorific value of the char and the conversion efficiency based on calorific value are given in Table 1.5. The higher heating value (HHV) of the biomass is calculated by implementing the HHVs of lignocellulosic fuels, as the equation given below [16]: HHV(MJ/Kg) = 0.335(C) + 1.423(H) − 0.154(O) (1.1) Chaniwala and Parikh [17] have developed an empirical correlation based on elemental and proximate analysis to predict the HHV of raw biomass as stated below: HHV(MJ/Kg) = 0.3491(C) + 1.1783(H) − 0.10(S) − 0.0134(O) − 0.0151(N) − 0.0211(A) (1.2) Here C, H, S, O, N, and A refer to the weight percent of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash in biomass respectively. 1.3.2 Inorganic Minerals’ Ash Content and Properties Fuel contains various impurities in the form of incombustible components mainly known as ash. Ash itself is undesirable, since it requires purification of the flue gas for particles with subsequent ash and slag disposal as a result. The ash from wood comes primarily from soil and sand absorbed into the bark. Wood also contains salts thus having the importance to the combustion process. They are primarily potassium (K), and partly sodium (Na), based salts resulting in sticky ash, which may cause deposits in the boiler unit. The Na and K contents in wood are normally so low that they will not cause problems for traditional heating technologies. Typical mineral fractions in wood chips expressed as percentage of the dry matter (DM) of the wood are shown in Table 1.6. Apart from all these individual analysis processes, NREL researchers have developed a very interesting and rapid analysis method for biomass Table 1.6 Total inorganic components of plant biomass (dry basis). Elements Potassium (K) Sodium (Na) Phosphorus (P) Calcium (Ca) Magnesium (Mg) Chlorine (Cl) Silica (Si) % of dry basis 0.1 0.015 0.02 0.2 0.04 0.2 to 2.0 0.2 to 15 JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 9 composition using near-infrared (NIR) spectroscopy. By applying this technique, the light reflected off a biomass sample is analyzed to determine the sample’s composition [8, 18]. 1.4 Biomass Conversion Routes By a number of processes, biomass can be converted into solid, liquid, and gaseous fuels. The technologies include thermal, thermochemical, and biochemical conversions. Reactions involved during conversion are hydrolysis, dehydration, isomerization, oxidation, de-hydrogenation, and hydrogenation. The actual processes included these technologies are combustion, pyrolysis, gasification, alcoholic fermentation, liquefaction, and so on [8]. A schematic flow diagram for biomass conversion is shown in Figure 1.2. The main products of conversion technologies are energy (thermal, steam, electricity), solid fuels (charcoal, combustibles), and synthetic fuels (methanol, methane, hydrogen gas, etc.). These can be used for different purposes such as cooking, lighting, heating, water pumping, electricity generation, and as industrial and transport fuels. Biomass fuels and residues can be converted to energy via thermal, biological, chemical, and physical processes. In a commercial process, biodiesel is produced by the reaction of vegetable oil or animal fat with methanol in the presence of base or acid catalysts. Concerns over the downstream processing of the homogeneous transesterification processes have motivated intense research on the heterogeneously catalyzed transesterification process [18, 19]. In general, heterogeneous biodiesel production processes have few numbers of unit operations, with simpler separation and purification steps for products as no neutralization process is required. There are three types of solid catalysts: acid, base, and enzyme. Solid base catalysts, such as alkaline–earth metal hydroxide, oxides, and alkoxides such as Ca(OH)2 , CaO, and Ca(CH3 O)2 function as effective catalysts for the transesterification of triglycerides [18, 20]. The main advantage of solid acid catalysts is their ability to carry out the esterification of free fatty acids and transesterification of triglycerides simultaneously [20–23]. Moreover, these are reactive on esterification and transesterification reactions at relatively low temperatures (i.e., 80 ◦ C), as shown in Figure 1.3 [8]. Lipase has been shown to have a high catalytic reactivity to produce high quality biodiesel [18, 20–23]. As lipases break down natural lipids and oils into free fatty acids and glycerol, therefore this group of enzymes is widely used in the leather and detergent industries. Recent findings show that an alternative acyl acceptor, such as methyl acetate is used to replace methanol, and it can obtain methyl ester yield up to 92%. In addition, the byproduct (glycerol) has a more expansive market, which can further be used for H2 production, acrolein, or several other chemicals [20]. In thermal conversion, combustion is already practiced widely, where as; gasification attracts high level of interest as it offers higher efficiencies compared to combustion. Pyrolysis is interesting as it results into liquid product that offers advantages in storage, easy transport and versatility in applications, although it is still at a stage of early development [8, 23]. 1.4.1 Pyrolysis There are different types of pyrolysis carried out under various operating conditions, among which fast, intermediate, flash, and slow having the substantial importance in the conversion JWST448-c01 JWST448-Hornung 10 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass Direct Liquefaction Heavy oil Indirect Thermo chemical conversion Fast Pyrolysis catalytic/noncatalytic Vacuum Slow Bio oil, biogas, char, tar Flash Intermediate liquefaction Gasification (partial air) Torrefaction Direct Combustion (excess air) FT oil, syngas, solvents, acids Bio-hydrogen, Conditioned gas Anaerobic Biomass Feedstock Batch Fed batch Fermentation Biochemical conversion Continuous Ethanol, Gobar gas Semi arranged continuous flow arrangement Partly microbial Various anaerobes Enzyme Cyano bacteria Photosynthesis bacteria Ethanol, amino acid, bio hydrogen and protein based chemicals Facultative group Klebsiella and clostridium Acid hydrolysis Hydrolysis Cellulose, hemicelluloses, lignin, sugar Enzymatic hydrolysis Chemical conversion Solvent extraction Supercritical conversion Leaching Liquid liquid extraction Primary and secondary metabolites Cellulose, hemicelluloses, lignin Mechanical extraction Physical conversion Briquetting Distillation Figure 1.2 Different conversion routes to get end products (liquid and gases). (Adopted from Mohanty et al., 2014 [3]) JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview O O H2C H3C C O OH C R R O C OH C OH H2C O CH Catalyst HC C O + H3C R OH O H2C R OH C O R Triglyceride OH O H3C C + C O C O H2C 11 Methanol R Methyl Ester Glycerol Figure 1.3 Reaction scheme of transesterification reaction. of biomass to different liquid and gaseous products Cn Hm Ok → (1 − n) CO + (m/2) H2 + C 180 (kJ/gmol) Cn Hm Ok → (1 − n) CO + ((m − 4)/2) H2 + CH4 300 (kJ/gmol) 1.4.1.1 (1.3) (1.4) Fast Pyrolysis Currently, targeting the liquids production through fast pyrolysis is capturing the interest. The main features of fast pyrolysis are high heating rates and short vapor residence time. It generally requires a feedstock prepared with smaller particle sizes and a design that removes the vapors quickly from the presence of the hot solids. There are a number of different reactor configurations that can achieve this, including ablative systems, fluidized beds, stirred or moving beds, and vacuum pyrolysis systems. Fast pyrolysis occurs in few seconds or less. Therefore, not only chemical reaction kinetics but also heat and mass transfer processes, as well as phase transition phenomena, play important roles. The critical issue is to bring the reacting biomass particle to an optimum process temperature and to minimize its exposure to the intermediate (lower) temperatures that favor formation of charcoal. This can be achieved by using smaller particles in fast pyrolysis as biomass decomposes to generate vapors, aerosols, and charcoal. After cooling and condensation, a dark brown liquid bio-oil is formed having the heating value of about half that of conventional fuel oil. Fast pyrolysis is an advanced process, with carefully controlled parameters to give higher yields of liquid. The essential features of the fast pyrolysis process for producing liquids are: (i) very high heating and heat transfer rates at the reaction interface, (ii) which usually requires a finely ground biomass feed, a carefully controlled pyrolysis reaction temperature of around 450–600 ◦ C and a vapor phase temperature of 400–450 ◦ C, short vapor residence times of typically less than 2 s, and rapid cooling of the pyrolysis vapor to produce the bio-oil product. The main product (bio-oil) is obtained in yields of up to 75% wt on a dry feed basis (in case of wood), together with byproduct char and gases which are used within the process so there are no waste streams other than flue gas and ash. During pyrolysis, how different variants within the main operating parameters affect the yield and product distribution is tabulated in Table 1.7 JWST448-c01 JWST448-Hornung 12 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass Table 1.7 Range of the main variants with main operating parameters and characterization for pyrolysis methods. (Adopted from Mohanty et al., 2014 [3]) Different pyrolysis process Slow Feed Intermediate Fast Flash Scores of feed reported Temperature (◦ C) Range Typical 250–750 350–400 320–500 350–450 450–1050 550–750 550–1300 1050–1150 Time Range Typical min 2–30 min 15 min 4 min 0.5–10 0.5–5 <1 s <1 s Heating rate (◦ C/s) Particle size (mm) Yields (% wt) on dry basis 1–50 5–50 10–100 5–50 100–500 <1.0 >1000 <0.2 Char Range Typical 2–60 25–35 19–73 30–40 15–35 20–25 10–35 10–20 Liquid Range Typical 0–60 20–50 25–60 35–45 20–75 46–53 20–65 46–71 Gas Range Typical 0–60 20–50 20–40 20–30 10–25 11–15 11–28 15–22 [24]. Some researchers have defined this process as thermolysis, in which a material, like biomass, is rapidly heated to high temperatures in the absence of air (specifically oxygen). 1.4.1.2 Intermediate Pyrolysis During intermediate pyrolysis the reactor is operated at temperatures ranging between 400 and 550 ◦ C and the reactor consists of two coaxial conveyor screws, an inner screw and a covering screw widely known as a pyrolyzer. When the outer screw transports the biochar from one end to the other end of pyrolyzer, the chars act as a heat carrier with bed formation. The intermediate pyrolysis of biomass is carried out in a very reasonable way, resulting in bio-oil with low tar yields and viscosity, which is distinctive in intermediate pyrolysis in comparison to fast pyrolysis. Typically, this reactor has the flexibility to provide a moderate residence time [25]. This is only the case for woody biomass, when it leads towards a herbaceous stream it fluctuates to larger extent leading to a liquid phase high in water, acids, and tars. In terms of other feedstock like straws, grasses, or industrial residues from agricultural products like husks the picture is very different. The intermediate pyrolysis prevents the formation of high molecular tars with dry and brittle chars suitable for other applications like biofertilization and gasification. The advantage of such pyrolysis is that the non-milling character endures with the pellet charged to the pyroformer. The ease of access for larger sized feedstock offers the opportunity to separate it easily as a char; and to enrich a tied gasifier with the low ash content of biochar from the pyroformer [25]. The haloclean process was primarily developed for the thermal treatment of halogenated polymeric wastes. Any contaminated biomass can be handled inside a kiln heated from outside, with single- or double-screw rotation either clockwise or anticlockwise or both as per the equipment design and flexibility. This pyrolysis facilitates operating conditions for JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 13 preventing the formation of high molecular tar and enhancing quality, that is, the dryness and brittleness of the char which can be further utilized for the purpose of fertilization and carbon sequestration. In this case, mechanical briquetting is not required for the processing of feedstock. 1.4.1.3 Slow Pyrolysis Slow pyrolysis is also termed as carbonization due to similarities in its process conditions, like low temperature and more residence time. It can be divided into traditional charcoal making and more modern processes that are characterized by slower heating rates, relatively long solid and vapor residence times, and usually a lower temperature than fast pyrolysis, typically 400 ± 10 ◦ C. The target product is often the char, but this is accompanied by liquid and gas products, although these are not always recovered. Traditional processes, using pits, mounds, or kilns, generally involve some direct combustion of the biomass, usually wood, as a heat source in the kiln. Liquid and gas products are often not collected but escape as smoke, with consequent environmental issues [1, 25]. It can be characterized by slow biomass heating rates, low temperatures, and lengthy gas and solid residence times. Depending on the system, heating rates are about 0.1 to 2 ◦ C per second and prevailing temperatures are around 500 ◦ C. Gas residence time may be greater than 5 s. During conventional pyrolysis, the biomass is slowly devolatillized; hence tar and char are the main products. This process yields a different range of products whose form and characteristics are dependent on the temperature, oxygen level, and process time used. 1.4.1.4 Torrefaction This is a thermochemical treatment of biomass in the temperature range of about 200 to 320 ◦ C, a kind of mild pyrolysis process that improves the fuel properties of biomass. It is carried out under atmospheric conditions and in the absence of oxygen. During this process, the water contained in the biomass, as well as superfluous volatiles, are removed, while the biopolymers (cellulose, hemicelluloses, and lignin) partly decompose by giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as “torrefied biomass” or “biocoal” [26, 27]. Torrefied products and volatiles are formed, resulting in a hardened, dried, and more volatile-free solid product. The product is at much higher energy density than the raw biomass, increasing the distance over which the biomass can be transported to plants for use or further processing, because of its relative lower weight and volume. Torrefied biomass is also hydrophobic, meaning it can be stored in the open space for long periods without taking up water, similar to the infrastructures used for coal. Torrefied biomass requires less energy to crush, grind, or pulverize and the same tools as for crushing coal can be used. Therefore, a well-developed biomass refinement method must interact and be integrated to obtain a biomass to liquid (BTL) process with high well-to-wheel efficiency. Other developments have led to slow/intermediate pyrolysis technologies to create that are of much attention for 3-different pyrolysis product distribution in wide ranges. These are generally based on a horizontal tubular kiln where the biomass is moved at a controlled rate through the kiln; these include agitated drum kilns, rotary kilns, and the screw pyrolyzer [28]. In several cases these have been adapted for biomass pyrolysis from their original uses, such as the coking of coal with production of “towns-gas” or the extraction of hydrocarbons JWST448-c01 JWST448-Hornung 14 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass Extractives (organic matters) Cellulose 30–45% (Plant biomass) Low molecular, macromole cular and Polysaccharide substances Hemicellulose 20–25% Lignin 14–32% Ash CO, CO2, CH4, H2, C2 - C5 Non condensable gases Methanol, acetic acid and acetone Aqueous phase Organic (Liquid oil) phase Cannot mix with hydrocarbon, cannot be distilled, substitute for fuel oil and diesel Char As heating continues there is an 80% loss of weight and remaining cellulose is converted to char. Prolonged heating or exposure to higher temperature (900 K) reduces char formation to 9% Ash (inorganic matter) Inherent organic material contains S and Cl contains alkali material Figure 1.4 Biomass component pyrolytic conversion for biorefineries. (Adopted from Mohanty et al., 2014 [3]) from oil shale (e.g., the Lurgi twin-screw pyrolyzer). Although some of these technologies have well-established for commercial applications, yet and yet considerable numbers of commercial applications are still under development to acquire potential market value with biomass to biochar production. The liquid fraction of the pyrolysis products consists of two phases: an aqueous phase containing a wide range of organo-oxygenate compounds with low molecular weight, and a non-aqueous phase containing insoluble organics (mainly aromatics), phenolic compounds of higher molecular weight. This non-aqueous phase is called bio-oil, which is a product of current interest. The ratios of acetic acid, methanol, and acetone of the aqueous phase were higher than those of the non-aqueous phase. For char production, one has to focus on low temperature and low heating rate; however, for maximum flue gas production a high temperature, low heating rate, and long residence time process would be preferable [29]. Distinct involvement of components’ during the pyrolysis process is summarized in Figure 1.4. 1.4.1.5 Gasification This is an alternative thermochemical conversion technology suitable for the treatment of biomass or other organic matter, including municipal solid wastes or hydrocarbons such as coal. It involves partial combustion of biomass under a gas flow containing a controlled level of oxygen at relatively high temperatures (500–800 ◦ C) yielding a main product of combustible producer gas/syngas with some char with low carbon percent. The main reaction involved during the gasification process is given below. Partial oxidation can be represented by these reaction schemes: Cn Hm Ok + (1/2) O2 → nCO + (m/2) H2 71 (kJ/gmol) Cn Hm Ok + O2 → (1 − n) CO + CO2 + (m/2) H2 − 213 (kJ/gmol) Cn Hm Ok + 2O2 → (n/2) CO + (n/2) CO2 + (m/2) H2 − 778 (kJ/gmol) (1.5) (1.6) (1.7) JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 15 Although designed for produce gas, under some conditions gasifiers can produce reasonable yields of producer gas, syngas, and char for an effective energy decentralization process [28]. The syngas production from biomass gasification can be reformed into a variety of chemicals like methanol, olefins, green diesel, gasoline, and wax through Fischer Tropsch routes [30]. 1.4.1.6 Hydrothermal Carbonization This is a completely different process involving the conversion of carbohydrate components of biomass (from cellulose) into carbon-rich solids in water at elevated temperatures and pressures [31]. Under acidic conditions with catalysis by iron salts the reaction temperature may be as low as 200 ◦ C. The process may be suitable to concentrate the carbon (%) and to handle the high moisture content in the waste streams that would otherwise require drying before pyrolysis, making it complementary to pyrolysis and a potential alternative to anaerobic digestion. 1.4.1.7 Combustion Combustion is the rapid oxidation of fuel to obtain energy in the form of heat. For combustion, biomass is used as the main feedstock and is primarily composed of carbon, hydrogen, and oxygen. Further it can produce H2 , CO, CO2 , and water by partial combustion [9]. Combustion takes place in the presence of excess air; therefore carbon dioxide and water are the pivotal components of gasification. At lower temperatures, formation of hydrocarbons takes place during gasification in the fluidized bed reactors. The flame temperature can go beyond 2000 ◦ C, depending on various factors like the heating value and the moisture content of the fuel, the amount of air used to burn the fuel, and construction of the furnace. For combustion, mainly a combustor is used as the device to convert the chemical energy of fuels into high temperature exhaust gases [15, 16]. 1.5 Bio-Oil Characteristics and Biochar Bio-oil is typically a dark brown liquid with a smoky acrid smell. It tends to have relatively high water content – typically in the range of 20 to 25% [9]. The water comes from the pyrolysis conversion process, as well as from the initial water in the biomass feedstock. When the water content of the bio-oil is in the 20 to 25% range, it is entirely miscible in bio-oil (i.e., it does not separate). At higher moisture levels, the water can tend to separate from the bio-oil. To prevent this from happening, it is desirable to have the incoming biomass feedstock dried to 10% moisture content, or less, before it is fed into the pyrolysis conversion process [1, 8, 23]. Bio-oil characteristics vary somewhat, depending on the production technology and the type of biomass feedstock from which the bio-oil is produced. This means that bio-oil fuel specifications are likely to be fairly important. Bio-oil’s energy content is in the range of 18–23 MJ/kg. (At the higher end of this range, there will typically be greater amounts of suspended char in the bio-oil.) Conventional heating oil has an energy content of about 42 ± 1 MJ/kg (lower heating value), thus bio-oil has about 52 to 58% as much energy as heating oil per gallon. However, it is interesting to note out that bio-oil weighs about 40% more per gallon than heating oil [9]. Bio-oil is a free JWST448-c01 JWST448-Hornung 16 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass flowing liquid. Its viscosity tends to be slightly higher than conventional no. 2 fuel oil. As the water content in bio-oil increases, its viscosity decreases (as does its energy content). Bio-oil is moderately acidic, having a pH in the range of 2.5 to 3.5 (similar to the acidity of vinegar). This means that bio-oil fuel storage tanks will need to be made of a material that will not corrode due to acidic character of the fuel (i.e., they will need to be made of materials such as stainless steel, plastic, fiberglass, etc.). Bio-oils are multicomponent mixtures comprised of different size molecules derived primarily from the depolymerization and fragmentation reactions of three key biomass building blocks: cellulose, hemicellulose, and lignin. Therefore, the elemental composition of bio-oil resembles that of biomass rather than that of petroleum oils [32,33]. This raises a significant issue regarding the use of bio-oil in existing residential or commercial installations, since most of the existing fuel storage tanks used for heating oil are likely to be made of plain mild steel or stainless steel that is vulnerable to corrosion from bio-oil. As a result, it will generally be necessary to install a new fuel storage tank if bio-oil is to be used for an existing heating oil installation [9, 32]. Bio-oil is a complex mixture of oxygenated compounds, which carries potential drawbacks as well as potential benefits: from a fuel storage perspective, bio-oil is not as stable as petroleum fuel. However, bio-oil developers (such as Dyna-Motive, part of Dynamotive Energy Systems Corporation) have found that bio-oil samples stored for over a year have remained stable [34]. Producing bio-oil with a lower ash (char) content and/or a lower water content helps in prolong stability of bio-oil during storage [29]. Growing concerns about climate change have brought biochar into the limelight. Combustion and decomposition of woody biomass and agricultural residues results in the emission of a large amount of carbon dioxide. Biochar can store this CO2 in the soil, leading to a reduction in GHGs emission and enhancement of soil fertility. In addition to its potential for carbon sequestration, biochar has many other advantages [23]. It can increase the available nutrients for plant growth, increase water retention, and reduce the amount of fertilizer used by preventing the leaching of nutrients out of the soil. It can reduce methane and nitrous oxide emissions from soil, thus further reducing GHGs emissions, and can be utilized in many applications as a replacement for other biomass energy systems. Biochar can be used as a soil amendment to increase plant growth yield. Further, the char can be used as a solid fuel in boilers and can be converted into briquettes alone or mixed with powdered biomass for high efficiency fuel. The char could be used for the production of activated carbon. Furthermore, the possibility of using this carbon feedstock for making carbon nano tubes can be explored. However it can also be used further for the gasification process to obtain hydrogen rich gas by thermal cracking [8, 9, 23]. 1.6 Scope of Pyrolysis Process Control and Yield Ranges The primary products of lignocellulose (hemicellulose and cellulose) decomposition are condensable vapors (yields to liquid products) and gases. Lignin decomposes to liquid, gas, and solid char products. Extractives contribute to liquid and gas products either through simple volatilization or decomposition. Minerals in general remain in the char, finally converted into ash. This distribution of components into products is shown schematically in Figure 1.4. JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 17 Vapors formed by primary decomposition of biomass components can be involved in secondary reactions in the gas phase, forming soot, or at hot surfaces – especially hot char surfaces where a secondary char is formed [35]. This is particularly important in understanding the differences between slow, intermediate, and fast pyrolysis and the factors affecting oil, gas, and char yields. Minerals in biomass, particularly the alkali metals, can have a catalytic effect on pyrolysis reactions leading to increased char yields in some circumstances, in addition to that the effect of ash also contributing directly to char yield [35–38]. After synthesis of bio-oil, the physicochemical properties of the bio-oil can be tested by using the standard method, making a comparison with conventional diesel as tabulated in Table 1.8 [9, 23, 24]. Table 1.8 bio-oil. Summary of typical properties and characteristics of biomass derived crude Property ASTM D975 (diesel) pH Flash point Moisture content Elemental analysis 52 ◦ C min < 0.05 max vol.% — Odor Water and sediment Kinematic viscosity ((mm2 /S) 40 ◦ C) Sulfated ash Ash Sulfur Density (kg/m3 ) Iodine number Aging — 0.01 max wt.% 0.05 max wt.% 820–845 — — Miscibility Miscible in ethanol Acid value KOH.mg.g−1 Boiling temperature Appearance 260–315 ◦ C Cetane number Aromaticity (%Vol, max) Carbon residue Distillation temperature (90% volume recycle) 0.05 max vol.% 1.3–4.1 mm2 /s 40 min — 0.35 max mass% 275–611 ◦ C max Pyrolysis oil 2.5–3.5 — 15–25% C = 56.6% H = 6.2% N = 0.1% O = 37.2% Smoky smell 0.01–0.04 25–1000 — 0.05–0.01 wt.% 0.001–0.02 wt.% ∼900–1200 90–125 As viscosity increases, volatility decreases, phase separation, slow decomposition and deposition of a tarry layer happens over time. Miscible with polar solvents like methanol, acetone, etc., but totally miscible with petroleum-derived fuels 0.16 Black or dark red-brown to dark green 48–65 — 0.001–0.02 wt.% — JWST448-c01 JWST448-Hornung 18 1.6.1 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass Moisture Content This can have different effects on pyrolysis product yields depending on the conditions [35]. Fast pyrolysis processes in general require fairly dry feed, around 10% moisture, so that the rate of temperature rise can not restricted by evaporation of water. Slow pyrolysis processes are more tolerant of moisture, the main issue being the effect on process energy requirement. For charcoal making, wood moisture contents of 15–20% are typical [36, 37]. In all pyrolysis processes, water is also a product which collects together along with other condensable vapors in the liquid product. Moisture in the reaction affects char properties, which in turn helps to produce activated carbons through pyrolysis of biomass. 1.6.2 Feed Particle Size This can significantly affect the balance between char and liquid yields. Larger particle sizes tend to give more char by restricting the rate of disengagement of primary vapor products from the hot char particles, so increasing the scope for secondary char-forming reactions [35]. 1.6.3 Effect of Temperature on Product Distribution The temperature profile is the most important aspect of operational control for pyrolysis processes. Material flow rates, both solid and gas phases, together with the reactor temperature control, are the key parameters of heating rate, peak temperature, residence time of solids, and contact time between solid and gas phases. These factors affect the product distribution and the product properties. For fast pyrolysis, a rapid heating rate and a rapid rate for of cooling for vapors are required to minimize the extent of secondary reactions. These reactions not only reduce the liquid yield but also tend to reduce its quality, yielding a more complex acidic mixture, an increased degree of polymerization and higher viscosity [8, 37]. Conversely, in slow pyrolysis there is some evidence that slow heating leads to higher char yields, but this is not consistent [35]. Higher temperatures lead to lower char yield in all pyrolysis reactions [9, 35]. 1.6.4 Solid Residence Time This is also important but to a lesser degree than peak temperature, longer time at temperature leading to lower char yield [35]. The effect of temperature on liquid and gas yields is more complex. Liquid yields are higher with increased pyrolysis temperatures up to a maximum value, usually at 400–550 ◦ C but dependent on equipment and other conditions. Above this temperature, secondary reactions causing vapor decomposition become more dominant and the condensed liquid yields are reduced [25, 39]. 1.6.5 Gas Environment Gas environment conditions in the gas phase during pyrolysis have a profound influence on product distributions and on the thermodynamics of the reaction. Most of the effects can be understood by considering the secondary char-forming reactions between primary vapor products and hot-char [35]. The gas flow rate through the reactor affects the contact JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 19 time between primary vapors and hot char and also affects the degree of secondary char formation. Low flows favor char yield and are preferred for slow pyrolysis; high gas flows are used in fast pyrolysis, effectively stripping off the vapors as soon as they are formed. 1.6.6 Effect of Pressure on Product Distribution Pressure has a similar effect. Higher pressure increases the activity of vapors within and at the surfaces of char particles, so increasing secondary char formation. The effect is most marked at pressures up to 0.5 MPa. Conversely, pyrolysis under vacuum gives little char, favoring liquid products. For pyrolysis under pressure, moisture in the vapor phase can systematically increase the yield of char, believed to be due to an autocatalytic effect of water, reducing the activation energy for pyrolysis reactions. The thermodynamics of pyrolysis are also influenced by the gas environment. The reaction is more exothermic at higher pressures and low flow rates. This is rationalized as being due to the greater degree of secondary char-forming reaction occurring. Hence, higher char yields are associated with conditions where pyrolysis is exothermic; such conditions will favor the overall energy balance of processes targeting char as product [8, 23]. 1.7 Catalytic Bio-Oil Upgradation Steam reforming, partial oxidation, and autothermal reforming (ATR) can be an attractive processes for the upgradation of bio-oils. ATR is a combination of steam reforming and partial oxidation of the hydrocarbons to produce CO, CO2 , and H2 . Bio-oil obtained from the pyrolysis consists of a complex mixture of aliphatic and aromatic oxygenates and particulates. It is a very viscous, acidic, and unstable liquid with relatively low-energy density compared to conventional fossil oil. Such poor quality bio-oil requires costly posttreatment and makes the complete process less economically attractive. The presence of proper catalysts during the pyrolysis process can affect the network of reactions and upgrade the bio-oil. Providing good contact between the solid catalyst and solid biomass/waste is essential to improve the efficiency of the pyrolysis process [29, 34, 39,40, 41]. The presence of proper catalysts during the pyrolysis process can affect the network of reactions (e.g., deoxygenation) and allows in situ upgrading of the bio-oil. This catalytic upgradation of different bio-oils produced from pyrolysis can be further deoxygenated with energy content improvement through different reactor configurations and reaction parameters, which is tabulated in Table 1.9 [35, 36, 42–45]. Providing a good contact between the solid catalyst and solid biomass is essential to improve the efficiency of the pyrolysis process [39–41]. Further, a lower pyrolysis temperature is crucial for maximizing bio-oil yield and quality [42–44]. From an elemental analysis perspective, bio-oil produced from wood contains about 56% carbon, 6% hydrogen, 37% oxygen, 0.1% nitrogen, 0.1% ash, and negligible sulfur, which could translate into a number of benefits, (1) the high oxygen content of biooil could help to improve its combustion characteristics in comparison to petroleum-based fuels, (2) it could help to reduce the amount of carbon dioxide emissions/pollution produced when bio-oil is burned as a fuel. The low nitrogen content of bio-oil could help to reduce NOx emissions. For example, tests of a combustion turbine showed that NOx emissions JWST448-c01 JWST448-Hornung Table 1.9 Catalytic upgradation of bio-oil with different reactor configuration and reaction parameters. 900 Oil palm shell 𝛾-Al2 O3 900 Rice straw Sawdust Commercial wood Sawdust Oak wood Cr2 O3 Cr2 O3 Cu/ MCM-41 850 850 — Ni Ni/Fe nitrate 700 700 Criterion 424/BP3198 MCM-41 700 500 Ni-ZSM-5 Quartz sand ZSM-5 600 450 600 Pistacia khinjuk seed Commercial wood biomass Aspen wood Pine chips Pine wood sawdust Gas yield Counter-current fixed bed Counter-current fixed bed Pyrolysis reactor Pyrolysis reactor Ref. 2.60 38.45 vol.% [42, 45] 2.62 34.63 vol.% 22.87 25.7 0.87 5.3 49.5 wt.% 51.4 wt.% 9 vol.% [45] Horizontal tubular reactor Fixed bed reactor 44.5 mass% 33.1 mass% [42] [45] 66.5%/69.2% 21%/25wt% [42] Bench scale fixed bed reactor 55.68 wt.% 9.87 wt% 16 wt.% 27.3 wt.% 52 wt.% Semi-batch pyro probe reactor — [43] 0.02 wt% [45, 36] [43] [45, 36] [45, 36] Trim: 244mm × 170mm La/Al2 O3 Bio-oil Printer Name: Vivar Oil palm shell Reactor Composition of hydrogen in gas 10:27 Catalyst Yields of H2 wt% (kg H2 /kg biomass) June 25, 2014 Biomass Reaction temp. (◦ C) JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview Table 1.10 21 Density and volumetric energy content of various solid and liquid fuels. Fuel Ethanol Methanol Biodiesel Bio-oil Gasoline Diesel Agricultural residue Hard wood Softwood Baled straw Bagasse Rice hulls Nut shells Coal Density (kg/m3 ) Volumetric energy content (GJ/m3 ) 790 790 900 1280 740 850 50–200 280–480 200–340 160–300 160 130 64 600–900 23.5 17.6 35.6 10.6 35.7 39.1 0.8–3.6 5.3–9.1 4.0–6.8 2.6–4.9 2.8 2.1 1.3 11–33 using bio-oil were about half as much as when using diesel fuel [35–37, 45]. The low sulfur content of bio-oil could also result in reduced SOx emissions compared to the use of petroleum-based fuel oil or diesel fuel. Bio-oil does not naturally blend with conventional petroleum fuel [28]. It may be possible to add a solvent or to emulsify mixtures of bio-oil and fuel oil in order to get homogeneous blends. Bio-oil manufacturers indicate that they are working on techniques that will allow blending of bio-oil and fuel oil. They are optimistic that workable approaches will be available in the future. But a necessary invention, which has been developed through the efforts of the Canadian government (Natural Resources Canada) that produces a stable bio-oil/diesel fuel mixture with properties similar to those of no. 2 fuel oil [36–38]. A broad comparison of both density and volumetric energy content of various solid and liquid fuels is tabulated in Table 1.10. The biomass feed, the char and liquid products have energy values roughly related to their carbon contents. Release of this energy by combustion can again be considered as renewable and is largely carbon neutral (some emissions are associated with feedstock production and transport); the carbon returned to the atmosphere as carbon dioxide in the same way would otherwise have resulted from biomass decomposition. If the char product is not burnt, but retained in such a way that the carbon in it is stable, then that carbon can be equated to carbon dioxide removed from the atmosphere and sequestered [28]. The gas product is typically a mixture of carbon dioxide (9–55% by volume), carbon monoxide (16–51%), hydrogen (2–43%), methane (4–11%), and small amounts of higher hydrocarbons [9]. The gases are usually present with nitrogen introduced to make the working space inert; thus this can be treated as a diluent and ignored for material balancing but will affect the heating value of the syngas. The carbon dioxide and nitrogen provide no energy value in combustion; the other gases are flammable and provide energy value in proportion to their individual properties. Again, use of the energy in the gas can be considered renewable and largely carbon neutral. No special consideration of the carbon JWST448-c01 JWST448-Hornung 22 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass dioxide in the pyrolysis gas is required as it is not additional to what would result from biomass decomposition [1, 8]. 1.8 Bio-Oil Reforming The deleterious properties of high viscosity, thermal instability and corrosiveness present many obstacles to the substitution of fossil derived fuels by bio-oils. Steam reforming of bio-oil or its model compounds is a simplified way to remove the oxygenated organic compound (Cn Hm Ok ) by the following reactions [20, 29], where the enthalpy (kJ/gmol) of each step is given at reference temperature 27 ◦ C and n = 6: Cn Hm Ok + H2 O = nCO + mH2 310 kJ/gmol Cn Hm Ok + nH2 O → xCO + (n − x) CO2 + mH2 230 kJ/gmol (1.8) (1.9) The above reaction is followed by the water–gas shift reaction: CO + H2 O → CO2 + H2 (1.10) The overall process can be represented as follows: Cn Hm Ok + (2n − k) H2 O → nCO2 + (2n + m/2 − k) H2 64 kJ/gmol (1.11) Upgrading bio-oil to the quality of transport liquid fuel still poses several technical challenges and difficulties and is not currently economically attractive. Some chemicals, especially those produced from the whole bio-oil (such as fertilizers) or its major fractions (such as liquid smoke or for wood resins) offer more interesting commercial opportunities. There are still many challenges to overcome before bio-oil finds large-scale acceptance as a fuel, including: (i) the cost of bio-oil, this is 10 to 100% more than fossil fuel in energy terms; (ii) the availability of bio-oil for applications development remains a problem and there are limited supplies for testing; (iii) the lack of standards for use and distribution of bio-oil in consistent quality inhibits wider usage; (iv) considerable work is required to characterize and standardize these liquids and develop a wider range of energy applications; (v) the compatibility of bio-oil with conventional fuels and, therefore, the need for dedicated fuel handling systems; (vi) users are unfamiliar with bio-oil; (vii) environmental health and safety issues need to be completely resolved;(viii) pyrolysis as a technology does not enjoy a good image; (ix) more research and development is needed in the fields of fast pyrolysis and bio-oil testing to develop large-scale applications. Figure 1.5 depicts the possible routes for the upgradation and conversion of bio-oil into various fine chemicals and hydrogen fuel and so on [8, 9, 21, 23]. The most important issues that need to be addressed are: (i) scale-up; (ii) cost reduction; (iii) better oil quality; (iv) norms and standards for producers and users; (v) environment health and safety issues in handling, transport, and usage; (vi) encouragement to implement processes and applications; (vii) information dissemination [46]. JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 23 Bio-oil upgradation Hydrodeoxygenation Hydro cracking Catalytic cracking of pyro-vapours Catalytic hydro treatment Thermo catalytic effect Light organic, heavy organic, coke, tar Hydrocarbon Emulsification Sub-and supercritical extraction Hydro treating Gasification Steam reforming Transportation fuel Light fraction of bio-oil and no. 2 diesel Fine chemicals, furfurals and hydrocarbon Fermentation Hydrogen Ethanol Syngas, producer gas hydrocarbons Figure 1.5 Integrated approach for bio-oil upgradation. 1.9 Sub and Supercritical Water Hydrolysis and Gasification Water is an ecologically safe and abundantly available solvent in nature. Water has a relatively high critical point (374 ◦ C and 22.1 MPa) because of the strong interaction between the molecules due to strong hydrogen bonds. Liquid water below the critical point is referred to as subcritical water whereas water above the critical point is called supercritical water. The density and dielectric constant of the water medium play major roles in solubilizing different compounds. Water at ambient conditions (25 ◦ C and 0.1 MPa) is a good solvent for electrolytes because of its high dielectric constant, whereas most organic matters are poorly soluble at this condition [38, 47, 48]. As water is heated up, the H-bonding begins to weaken, allowing dissociation of water into acidic hydronium ions, (H3 O + ) and basic hydroxide ions (OH− ). It is important to mention that the dielectric behavior of 200 ◦ C water is similar to that of ambient methanol, 300 ◦ C water is similar to ambient acetone, 370 ◦ C water is similar to methylene chloride, and 500 ◦ C water is similar to ambient hexane [46, 49, 50]. Sub and supercritical water offers several advantages over other biofuel production methods. Some of the major benefits are: (i) high energy and separation efficiency (since water remains in liquid phase and the phase change is avoided); (ii) high throughputs; (iii) versatility of chemistry and its mechanism (solid, liquid, and gaseous fuels); (iv) reduced mass transfer resistance in hydrothermal conditions; (v) improved selectivity for the desired energy products (methane, hydrogen, liquid fuel) or biochemical (sugars, furfural, organic acids, etc.); (vi) ability to use mixed feedstock as well as wet waste biomass to produce biodiesel, which is considered a “carbon neutral” JWST448-c01 JWST448-Hornung 24 June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Transformation of Biomass fuel. After upgradation to generate high energy content bio-fuel it may become easy to reduce GHG emissions, with little or no toxicity [9, 48, 51]. The pyrolysis-derived bio-oil from different biomass origins was upgraded in sub and supercritical ethanol using an appropriate catalyst. It is under intense research for the supercritical upgrading process, as it performs better than the subcritical upgrading process. Mainly, acidic HZSM-5 facilitates esterification in supercritical ethanol to convert acids contained in crude bio-oil into various kinds of esters [20, 23]. Stronger acidic HZSM-5 (different Si/Al ratio) with a bimetallic catalyst effect can facilitate cracking of heavy components of crude bio-oil more effectively in the supercritical upgrading process. Studies on promoter effects of alkali and other metals on cobalt are rare. Although the effects of K, Zn, Cu, Mn, Ca, Al, and Zr have been used and can be studied extensively, the effects of these promoters on bimetallic catalysts are still not clear because these studies were conducted under different conditions or over different catalyst systems [38, 47, 48]. 1.9.1 Biochemical Conversion Routes Another promising approach that can be used for the production of chemicals is the biochemical route, where bioethanol and biobutanol can be produced through hydrolysis in the presence of enzymes. In many countries like India, US ethanol plays a very critical role as a gasoline substituent and also as a feedstock for various chemicals. One ton of cane can produce approximately 100 liters of ethanol. Ethanol can be used for the production of acetaldehyde, acetic anhydride, ethyl acetate, monoethylene glycol, and so on [24]. During biochemical conversion, the aim is to extract cellulose, out of which one can easily extract ethanol as a final product for its wide acceptance. Cellulose is protected by a sheath of lignin and hemicellulose that widely found in plant biomass. Researchers with leading roles are developing pretreatment technologies to hydrolyze hemicellulosic sugars and open up the structure of sugars to allow further enzyme hydrolysis of the cellulose to glucose [23, 29]. Likewise many biomass researchers internationally have focused on a process involving dilute acid hydrolysis of hemicellulose to a xylose and other sugars compounds [52]. Taking advantage of conditioning and enzymatic hydrolysis, the material must be made less acidic for enzymes and organisms to function optimally in the hydrolyzate environment. During the process of pH adjustment or conditioning, the aim is to minimize sugar losses and to promote low hydrolyzate toxicity by removing toxic byproducts that inhibit enzyme and fermentation microorganism activity. The effectiveness of enzymatic hydrolysis depends on a variety of processing conditions, including different enzyme and solid loadings, mixing and conditioning methods, and pretreatment conditions. A new generation of enzymes and enzyme production technology is needed to cost-effectively hydrolyze cellulose and hemicellulose to free the sugars needed for fermentation. To get a high yield one has to focus on decreasing the cost of the enzyme unit operation in the biomass saccharification process, which is a key factor for developing cost-competitive cellulosic ethanol. Starchy materials are first cooked at 100 and 130 ◦ C, and then hydrolyzed to glucose by using 𝛼-amylase and gluco-amylase. After the development of enzymatic hydrolysis, research work is in progress in close association with major industrial enzyme producers to apply recombinant DNA technology to bacteria and fungi to develop improved cellulose and hemicellulose JWST448-c01 JWST448-Hornung June 25, 2014 10:27 Printer Name: Vivar Trim: 244mm × 170mm Biomass, Conversion Routes and Products – An Overview 25 enzymes and to determine the most efficient method for producing these enzymes [9, 51, 53, 54]. 1.9.2 Microorganisms for Fermentation The fermentation process has been developed both at lab and industrial scale to evaluate and scale-up its use through biochemical plants for ethanol production. Researchers are applying sophisticated metabolic engineering techniques to develop microorganisms that can more effectively ferment the variety of sugars derived from biomass. Lignocellulosic biomass contains five-carbon sugars such as xylose (from the hemicellulose) as well as the more common six-carbon sugars, such as glucose found in grains. These make fermentation and other bioprocessing processes far more challenging. Researchers are developing microorganisms that can co-ferment all the sugars in biomass to improve ethanol production economics. Sophisticated metabolic engineering techniques, like the application of Zymomonas mobilis can co-ferment both xylose and arabinose along with glucose. With industrial partners, researchers are working to develop designer strains for specific feedstocks, feed streams, and processes and to validate the performance of these strains [46, 52, 55–57]. 1.9.3 Integrating the Bioprocess After integrating all the unit operations of biomass conversion through biochemical routes with extensive knowledge of the individual unit operations, one can select different lignocellulosic biomasses for industrial application, and on demonstrating integrated processes at the mini-pilot and pilot scales one can attain the production of bioethanol for further use [24, 58]. C6 H12 O6 → 2C2 H5 OH + 2CO2 (1.12) Ethanol fermentation can be carried out at room temperature and atmospheric pressure where Saccharomyces cerevisiae is used as the yeast for its excellent ethanol ability and ethanol tolerance. The yeast strain produces approximately 51 g of ethanol from 100 g of glucose according to the Equation 1.12. In this reaction, around 50% of carbon is consumed in terms of CO2 production, in fact 91% of energy contained in glucose (2.87 MJ/mol) is retained in ethanol. Whereas S. cerevisiae has the ability to ferment many sugar molecules like glucose, fructose, galactose, mannose, sucrose, and maltose, still many researchers are trying to identify the potential of S. cerevisiae to explore and break pentoses like xylose and arabinose [59, 60]. Table 1.11 summarizes the various possible biobased products with their broad classification based on their market demand and industrial use [55]. Questions 1. What are the differences between torrefaction, pyrolysis, and gasification? 2. What are the main building blocks of biomass? 3. What are first, second, third, and fourth generation biofuels? JWST448-c01 Biobased product with classifications and market strategy. Market opportunity Market impact Polytrimethylene, terephthalate (PTT) Polyster plastics, Polymers, Resins For nylon, PET, poly-butylene and terephthalate Acrylic acid Adhesives, polymers Acrylonitrile Polymers Acrylamide Resin Medium energy syngas Gaseous fuel Refinable hydrocarbon chemicals including: cyclohexane, methyl ethyl benzene, toluene, cyclopropane Solvent Fatty acid oils Pharmaceuticals, detergents, surfactants, personal care products PTT surpass nylon and polyethylene terephthalate (PET) in fiber applications and polybutylene terephthalate. PTT is a high performance polyester polymer with remarkable “stretch recovery” properties, and is used in apparel, upholstery, and carpet. Acrylates (e.g., coatings, adhesives), co-monomer, super absorbent polymers, detergent polymers. Acrylic fibers (carpets, clothing), acrylonitrile-butadiene-styrene and styrene-acrylonitrile (pipes and fittings, automobiles, furniture, packaging), nitrile rubber copolymers, adiponitrile, acrylamide. Polyacrylamide, comonomers (styrene butadiene latex, acrylic resins etc. and others). As auxiliary fuel in boilers, or as a fuel for electricity and steam generation via gas turbine combined cycle plants or fuel cells. Cyclohexane is used as a paint remover, solvent for lacquers and in making nylon. Methyl ethyl benzene is typically used in producing rubber, waxes, and resins and is often blended with gasoline and other fuels. Toluene is primarily utilized as a solvent in the manufacture of explosives, dyestuffs, and is added to aviation fuel to improve its octane. Fatty acids are used in a range of applications: detergents, soaps, cleaners, stabilizers, industrial surfactants and pharmaceuticals, personal care products, lubricants and rubber products. The fatty acids produced by Changing World Technology process are primarily palmitic, stearic and oleic acids. For acrylic acid derivatives 10:27 Classifications June 25, 2014 Biobased product JWST448-Hornung Table 1.11 For acrylonitrile derivatives Printer Name: Vivar For acrylamide derivatives Large Large Trim: 244mm × 170mm Large JWST448-c01 Ethyl lactate Pharmaceuticals, solvent, fine chemical Solvent Acrylic acid Adhesives, polymer Propylene glycol Resins, antifreeze, solvents, hydraulic fluid Resins Various extracted chemicals Levoglucosan Polymers, pharmaceuticals, pesticides, surfactants Friendly solvent, for Lactate esters synthesis, coatings, inks, cleaners and straight use cleaning. Acrylic acid is an attractive target for new biobased products; Conversion of lactic acid to acrylic acid would require either the enzymatic or thermochemical dehydration of lactic acid. Propylene glycol is a commodity chemical. Petroleum derived phenol-formaldehyde resin is used in plywood, oriented strand board, and other wood composites. Resin from pyrolysis bio-oils could replace up to 50% of the phenol-formaldehyde. Levoglucosan is considered a potential building block for synthesis of polymers, pharmaceuticals, pesticides, and surfactants. Microorganisms have been identified that can ferment levoglucosan to citric acid and itaconic acid. Potential to displace 80% of the 8 to 10 billion pounds of solvents per year. Large Large Large Large Trim: 244mm × 170mm Glycerin 364 million pounds of activated carbon are sold annually in USA. Large Printer Name: Vivar Filtration agent For butanol and derivatives 10:27 Activated carbon Butanol and derivatives are used in plasticizers, amino resins, and butyl amines. It is also used as a solvent in lacquers, lacquer thinners, liquid printing inks, disinfectants and fungicides. Also as a fruity food flavoring. Activated carbon is heavily used in both liquid and gas filtration. Conversion of char to activated carbon can be performed with steam or acid. 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