Plant breeding is the traditional way of improving plants by selecting for desirable phenotypes. In the simplest form, this can involve changing the ploidy of a plant to enhance the biomass production. Plant breeding is a laborious and time-consuming process that requires significant investment of resources as well, and perhaps it is for this reason there has been very little effort focused on plants used for biofuels compared to the crop species such as rice and wheat. Most of the biofuel crops are polyploid and display self-incompatibility [47]. It is a well-established fact that intensive plant breeding efforts during the early 1960s led to the production of high yielding dwarf and semi-dwarf hybrids of wheat, corn and rice, which formed the basis of the Green Revolution. Some of the key features modified were plant height, tillering habit and grain yield relative to straw yield. One of the more recent success stories of marker-assisted breeding is the submergence tolerant rice where the SUB1 locus was introgressed into several commercial cultivars of rice [48]. Hence, with the relatively high degree of synteny among grass species, opportu­nities exist for adaptation of observations from model species to the biofuel species by marker-assisted breeding. It is evident from these examples that grasses are amenable for considerable increases in yield and alterations to overall plant architecture. If concerted breeding efforts are applied for the biofuel crops, we can realize remarkable enhancements of these species as with the cereal crops during the Green Revolution.

Ethanol is also inhibitory to the microorganisms producing it. It has three inhib­itory effects: inhibition of cell multiplication, inhibition of fermentation, and a lethal effect on cells (Table 9.11). It is toxic to yeasts and high bioethanol toler­ances capacity of yeast is a pre-requisite for production of bioethanol. It has been

shown that the inhibitory effect of ethanol is generally negligible at low concen­trations (less than 20 g/l) but increases rapidly at higher concentrations [13]. For most strains, ethanol production and cell growth are stopped completely at above l00 g ethanol/l although some very slow fermenting yeasts (Saccharomyces sake) can tolerate higher ethanol concentrations at low temperatures [23, 70]. Ethanol inhibition is directly related to the inhibition and denaturation of important gly­colytic enzymes as well as to the modification of the cell membrane [123, 153]. Various factors, viz., temperature, aeration, medium composition, etc. influence bioethanol sensitivity directly or indirectly, modify the properties of cell mem­brane, and membrane lipids.

Prasad Kaparaju et al. [39] investigated the production of biofuels—bioethanol (from cellulose), biohydrogen (from hemicelluloses), and biogas (from effluents from bioethanol and biohydrogen production) from wheat straw in an effort to establish an energy-efficient and economical process within a biorefinery network. Part of the wheat straw was used as such without pretreatment and part of it was pretreated using hydrothermal pretreatment techniques. The pretreated wheat straw resulted in a liquid fraction hydrolysate which contained mainly hemicel — luloses and a solid fraction which was rich in cellulose. Six different scenarios were studied: (1) untreated wheat straw was incinerated as such and energy was generated, (2) untreated wheat straw was anaerobically digested to generate bio­gas, (3) pre-treated wheat straw was used for conversion into biogas, (4) pretreated wheat straw was converted to bioethanol alone, via fermentation, (5) pretreated wheat straw was converted to bioethanol and biogas, and lastly, (6) pretreated wheat straw was converted to bioethanol, biohydrogen, and biogas. From among all six cases, they showed that the use of wheat straw for production of biogas alone, or for production of multiple biofuels, were the most energy-efficient pro­cesses as compared to production of monofuel such as bioethanol by fermenting hexose sugars alone. Thus, in other words, the biorefinery concept was more energy efficient rather than using biomass conversion technologies for generation of any one fuel.

There are two conventional operational temperature levels for anaerobic digesters, which are determined by the species of methanogens in the digesters:

• Mesophilic which takes place optimally around 30-38°C or at ambient tem­peratures between 20 and 45 °C where mesophiles are the primary microor­ganism present.

• Thermophilic which takes place optimally around 49-57°C at elevated tem­peratures up to 70°C where thermophiles are the primary microorganisms present.

There are a greater number of species of mesophiles than thermophiles. These bacteria are also more tolerant to changes in environmental conditions than ther — mophiles. Mesophilic systems are therefore considered to be more stable than thermophilic digestion systems.

As mentioned above, thermophilic digestion systems are considered to be less stable, the energy input is higher and more energy is removed from the organic matter. However, the increased temperatures facilitate faster reaction rates and hence faster gas yields. Operation at higher temperatures facilitates greater ster­ilization of the end digestate.

As-produced commercial ILs can contain halides, water, organics, and unreacted salts from their synthesis [60]. The presence of impurities could explain differ­ences in performance between identical ILs from different manufacturers [67]. The presence of residual chloride salts can dramatically increase the viscosity of the IL and decrease its density. 1H NMR studies suggested that the viscosity increase may be due to the increase in hydrogen bonding between the chloride anion and the protons of the imidazolium cation [68]. Halides in a few ILs, such as 1-butyl — 3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), can be removed by water washing, but the water excess would have to be removed under vacuum or by distillation [60].

Impurities, such as methyl imidazole (source of imidazolium-based ILs), can affect the pH of the solution and reduce ion concentrations [69]. The mixture of water with commercially available [EMIM] [Cl] (equal weight) has a pH around 7, whereas the mixture of purified [EMIM][Cl] and water (equal weight) has a pH of 5.12. The addition of methyl imidazole to the purified IL brought the pH back to around 7. The lower pH obtained with the purified was attributed to the enhanced water dissociation and the resulting higher ion concentrations. Ab initio simulations predicted enhanced water dissociation at a high ionic strength (high

IL content) or at a high dielectric constant (high water content). The ions from the enhanced water dissociation in purified ILs could effectively catalyze the con­version of cellulose to sugars without the addition of acid catalysts [69].

To evaluate usefulness of the process it is crucial to determine whether the reaction can occur with sufficient rates using feedstocks available on the market in large quantities. Therefore, one good example of cheap raw material is waste cooking oil (WCO) obtained from different sources [22]. The results show complete deoxygenation of the WCO with high selectivity to hydrocarbons (>90%) using sulfided Ni-Mo, NiW, Co-Mo on Al2O3 ,and Ni-Mo/B2O3-Al2O3 at 250-350°C under 7 MPa of hydrogen. The experiments were performed in the fixed bed reactor to evaluate deactivation of the catalysts. Apart from the initial deactivation Ni-Mo/Al2O3, NiW/Al2O3, and Ni-Mo/B2O3-Al2O3 show no deactivation during 80 h time-on-stream at 350°C and 5 MPa of hydrogen pressure, in contrast to Co — Mo/Al2O3 which deactivated significantly due to extensive catalyst desulfuriza­tion. It is worth to mention that desulfurization of Co-Mo/Al2O3 catalyst in the reaction with stearic, oleic, and linoleic acids increases with increasing unsatu­ration of the fatty acids.

Improvement of cetane number for diesel fuels could be achieved by blending it with deoxygenated bio-oil. It was proven that deoxygenation of rapeseed oil [23] and cottonseed oil [24] blends in diesel fuel, in the quantity of 10-20% and 10%, respectively, was successful. In case of rapeseed oil some hydrocracking occurs, at temperature of 350-380°C and 5 MPa hydrogen pressure over Ni-Mo/Al203 cat­alyst, which leads to a decrease of the flash point of obtained diesel. For 10% cottonseed oil blend in diesel, at temperature of 305-345°C and 3 MPa hydrogen pressure, using Co-Mo/Al2O3 catalyst an increase of cetane number by 3 was achieved [24]. The transformation of cottonseed oil did not affect cloud point or density of the diesel blend compared to pure diesel fuel.

Combination of HDO and hydrodesulfurization processes was proposed recently [25]. The idea of the process is to deoxygenate triglycerides in the same unit, in which HDS of atmospheric gas oil occurs (Fig. 6.3). It is beneficial

HYDROGEN MAKE-UP

COMPRESSOR

Fig. 6.3 Flow scheme proposal for co-processing hydrodesulfurization of atmospheric gas oil with hydrodeoxygenation of renewable feedstocks. Taken from Ref. [25] because the same catalyst is used in both processes (Ni-Mo, Co-Mo, etc.) and the problem with desulfurization of the catalyst did not appear.

The substrates mainly used in alcoholic fermentation are sugars with ethanol as the main product. The ability is widely distributed among the microorganisms.

The species of Saccharomyces are the main alcohol producers amongst the yeast Z. mobilis can also produce ethanol from glucose, which otherwise only utilize hexoses [150]. Alcohol is not a predominant end product in other bacteria. Certain yeasts including S. cerevisiae can also ferment pentose sugar, xylose to ethanol though the yield is lower compared to the fermentation of hexoses. For industrial alcohol production, yeast strains are generally chosen from S. cerevisiae, Sac­charomyces ellipsoideus, Saccharomyces carlsbergensis, Saccharomyces fragilis, and S. pombe. For whey fermentation, Torula cremoris or Candida pseudotropi — calis is used. Yeasts are carefully selected for high growth and fermentation rate, high ethanol yield, ethanol and glucose tolerance, osmo-tolerance, low pH fer­mentation optimum, high temperature fermentation and general hardiness under physical and chemical stress. Ethanol and glucose tolerance allows the conversion of concentrated feeds into concentrated products reducing the energy requirements for distillation and stillage handling. The osmo-tolerance property allows the handling of relatively concentrated raw materials such as blackstrap molasses with its high salt content. The osmo-tolerance capacity it also allows the recycle of large protein of stillage liquids, thus reducing stillage handling costs. Low pH fermentation combats contamination by competing organisms by preventing their growth. High temperature tolerance simplifies fermenter cooling. General hardi­ness allows yeast to survive both the ordinary stress of handling as well as the stresses arising from plant upset. The years of careful selection by industrial use have led to the selection of yeast strains with these desirable characteristics. Many of the best strains of yeast are proprietary but others are available from the culture collections [33].

Possibilities of the utilization of rice husk and subsequent chemical conversion of hemicellulose into xylose, followed by furfural, xylitol, xylonic acid, and ulti­mately the food yeast is explored [55]. Similarly, hydrolysis of cellulose to glucose which, then, can be converted into ethanol, sorbitol, hydroxy methyl furfural, levulinic acid, etc. is outlined. However, ethyl alcohol production would be eco­nomical only if all the by-products are recovered and processed. Mucilagenos material from cocoa waste is another source of alcohol [129]. The waste from tapioca spent pulp after concentration by centrifugation to 20% solids after hydrolysis holds promise for production of alcohol.

9.7.9 Barley

The waste from a novel, vacuum distillation procedure (30-45°C) called Mugi (Barley) contained a large number of viable yeast (7 x 106 cells/ml), with glu — coamylase (19.7 units/ml), acid protease (940 units/ml), and neutral protease (420 units/ml). The waste was mixed with mash composed of glucose as the sole source of carbon. After distillation of fermentation broth, the non-volatile residues were again used in the next ethanol fermentation and the cycle was repeated successfully ten times. The system is developed for the distillery waste which is treated as per the conventional waste water [186].

The process of combustion can be considered as an interaction between fuel, energy and environment. Fuel is burnt in excess air to produce heat. The excess air

serves as a source of oxygen which initiates a chemical reaction between the fuel and oxygen, as a result of which, energy is liberated. Volatilization of combustible vapors from the biomass occurs which then burns as flames. This volatile degra­dation product consists of three fractions: gaseous fraction containing CO, CO2, some hydrocarbons, and H2; a condensable fraction consisting of water and low molecular weight organic compounds such as aldehydes, ketones, and alcohols; and tar fraction containing higher molecular weight sugar residues, furan deriva­tives, and phenolic compounds. The proportion of these volatiles and residue is determined by thermal analysis methods. The residual material which remains is the carbon char which is subsequently burnt when more air is added. Demirbas [1] gives some important combustion properties of selected biomass samples. The combustion process can result in production of heat, or by using secondary con­version processes, in generation of electricity (Fig. 1.6).

The open fire at home or the small domestic stove is the simplest example of the use of the combustion process to generate energy/heat. However, this process has an efficiency of only 10-15% as most of the volatile oils released go into the environment along with most heat. More sophisticated combustion technologies have been developed to give increased efficiencies. The use of more efficient wood stove designs results in greatly increased efficiencies of up to 60%. The com­bustion technologies were originally designed for production of energy from coal or fossil fuel. However, the rapid depletion of fossil fuel and the search for renewable source of energy have directed all efforts toward adapting these tech­nologies to the use of biomass in place of fossil fuels for the generation of energy.

Indeed, the efforts required are enormous as the nature of biomass is radically different from that of fossil fuels. Also, the composition of biomass varies widely depending on its source. In case of biomass, the biomass, directly fed into the combustion furnace, is first converted into a mixture of volatiles and a carbona­ceous char which burn with entirely different combustion characteristics as com­pared to fossil fuels. The heat of combustion AH for any combustion process is calculated on the basis of the standard equation:

AG = AH — T AS

where G is the free energy, H is enthalpy, T is the absolute temperature, and S is the entropy. While using this equation for biomass, the change in entropy or the energy lost in converting the solid fuel into gaseous combustion products must be included [6]. This correction factor may vary greatly depending on the charac­teristics of the biomass used. When biomass is used as the fuel for the com­bustion process, there are a number of factors which are responsible for lowering the efficiency of the process and the net usable energy that could be obtained from the process. Some of the important factors are, the variable nature of the biomass, the variable moisture content and ash content present in the biomass, the dissipation of some of the heat of combustion by the combustion products of the biomass, and the incomplete combustion of biomass. The moisture content in biomass varies from an equilibrium moisture content of 10-12% in agricultural residue such as straw to as high as about 50% in biomass such as wood residue and bagasse. This moisture content acts as a heat sink and has to be dried up before it can be used for direct combustion. The extra energy required for this will reduce the net energy output of the process. Therefore the combustion process is best suited for biomass with a moisture content lower than 50%. Biomass containing moisture contents higher than this is better suited for bio — chemical/biological conversion processes. The proportion of volatile matter and fixed carbon present in the biomass also differs depending on the source [7]. Softwoods contain about 76.6% of volatile matter, whereas hardwood contains 80.2% of volatile matter. As compared to these values, bituminous black coal contains only 37.4% of volatile matter. As most of the combustion process is characterized by the volatile fraction, this difference is of great significance. The mineral content in biomass also varies from 0.5% in woody biomass to 18% in cereal straws. The wood ash mainly consists of alkali and alkali earth cations present as carbonates, carboxylic acids, and some silica crystals. The silica and insoluble organic compounds act as a heat sink, whereas the soluble organic compounds may have a catalytic effect in gasification and combustion of bio­mass. Complete combustion of biomass releases CO2 and water which are harmless. However, incomplete combustion leaves carbonaceous residue (fly ash), smoke, and other odorous and noxious gases (containing carbonyl deriva­tives, unsaturated compounds and CO) which are detrimental to the environment. In addition to this, a considerable amount of biomass is wasted. Figure 1.7 shows a typical combustion plant using municipal solid waste as biomass feed.

A. K. Kurchania

2.1 Introduction

Biomass energy or ‘‘bioenergy’’ includes any solid, liquid or gaseous fuel, or any electric power or useful chemical product derived from organic matter, whether directly from plants or indirectly from plant-derived industrial, commercial or urban wastes, or agricultural and forestry residues. Thus bio­energy can be derived from a wide range of raw materials and produced in a variety of ways. Because of the wide range of potential feedstocks and the variety of technologies to produce them and process them, bioenergy is usually considered as a series of many different feedstock/technology combinations. In practice, we tend to use different terms for different end uses—e. g., electric power or transportation.

The term ‘‘biopower’’ describe biomass power systems that use biomass feedstocks instead of the usual fossil fuels (natural gas or coal) to produce elec­tricity, and the term ‘‘biofuel’’ is used mostly for liquid transportation fuels which substitute for petroleum products such as gasoline or diesel. ‘‘Biofuel’’ is short for biomass fuel.

The term ‘‘biomass’’ generally refers to renewable organic matter generated by plants through photosynthesis. During photosynthesis, plants combine carbon dioxide from the air and water from the ground to form carbohydrates, which form the biochemical ‘‘building blocks’’ of biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the carbohydrates and other molecules contained in the biomass. If biomass is cultivated and harvested in a way that allows further growth without depleting nutrient and water resources, it is

A. K. Kurchania (H)

Renewable Energy Sources Department, College of Technology and Engineering, Maharana Pratap University of Agriculture and Technology, Udaipur, India e-mail: kurchania@rediffmail. com

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_2, © Springer-Verlag Berlin Heidelberg 2012 a renewable resource that can be used to generate energy on demand, with little net additional contributions to global greenhouse gas emissions [1].

Materials having organic combustible matter are also referred under biomass. Biomass can be directly utilized as fuel or can be converted through different routes into useful forms of fuel. Biomass is a scientific term for living matter, but the word biomass is also used to denote products derived from living organisms— wood from trees, harvested grasses, plant parts and residues such as twigs, stems and leaves, as well as aquatic plants and animal wastes.

Burning biomass efficiently results in little or no net emission of carbon dioxide to the atmosphere, since the bioenergy crop plants actually took up an equal amount of carbon dioxide from the air when they grew. However, burning conventional fossil fuels such as gasoline, oil, coal or natural gas results in an increase in carbon dioxide in the atmosphere, the major greenhouse gas which is thought to be responsible for global climate change. Some nitrogen oxides inev­itably result from biomass burning (as with all combustion processes) but these are comparable to emissions from natural wildfires, and generally lower than those from burning fossil fuels. Other greenhouse gas emissions are associated with the use of fossil fuels by farm equipment, and with the application of inorganic fer­tilizers to the bioenergy crop. However, these may be offset by the increase in carbon storage in soil organic matter compared with conventional crops. Utiliza­tion of biomass residues which would otherwise have been dumped in landfills (e. g. urban and industrial residues) greatly reduces greenhouse gas emissions by preventing the formation of methane.

All the Earth’s biomass exists in a thin surface layer called the biosphere. This represents only a tiny fraction of the total mass of the Earth, but in human terms it is an enormous store of energy—as fuel and as food. More importantly, it is a store which is being replenished continually. The source which supplies the energy is of course the Sun, and although only a tiny fraction of the solar energy reaching the Earth each year is converted into biomass, it is nevertheless equivalent to over five times the total world. The annual world of biomass is estimated at 146 billion metric tons, mostly from uncontrolled plant growth. The current world demand for oil and gas can be met with about 6% of the global production of biomass. Biomass is significant as heating fuel, and in some parts of the world the fuel is most widely used for cooking [2]. An advantage of this source of energy is that use of biomass for fuel would not add any net carbon dioxide to the atmosphere.

The Earth’s land-based production which is used by the human population worldwide ranges from a low figure of about 5% to a high of over 30% (including food, animal fodder, timber and other products, as well as bioenergy). The higher estimates include a lot of wasted material and inefficient activities such as forest clearance, as well as losses of productivity due to human activity. Globally bio­mass energy use has been independently estimated at about 55 exajoules per year, or about 2% of annual biomass production on land.

Biomass has the following advantages:

• It is widely available.

• Its technology for production and conversion is well understood.

• It is suitable for small or large applications.

• Its production and utilization requires only low light intensity and low tem­perature (535°C).

• It incorporates advantage of storage and transportation.

• Comparatively, it is associated with low or negligible pollution.

Biomass can be classified as:

• Agricultural and forestry residues. They include silvicultural crops.

• Herbaceous crops. Include weeds, Napier grass.

• Aquatic and marine biomass. This category include algae, water hyacinth, aquatic weeds, plants, sea grass beds, kelp and coral reap, etc.

• Wastes. Various wastes such as municipal solid waste, municipal sewage sludge, animal waste and industrial waste, etc.

Worldwide, biomass is the fourth largest energy resource after coal, oil and natural gas—estimated at about 14% of global primary energy (and much higher in many developing countries). In the US, biomass today provides about 3-4% of primary energy (depending on the method of calculation). Biomass is used for heating (such as wood stoves in homes and for process heat in bioprocess industries), cooking (especially in many parts of the developing world), trans­portation (fuels such as ethanol) and, increasingly, for electric power production. The installed capacity of biomass power generation worldwide is about 35,000 MW, with about 7,000 MW in the US derived from forest-product-industry and agricultural residues (plus an additional 2,500 MW of municipal solid waste-fired capacity, which is often not counted as part of biomass power, and 500 MW of landfill gas-fired and other capacity). Much of this 7,000 MW capacity is presently found in the pulp and paper industry, in combined heat and power (cogeneration) systems.