22 Types of Biofuels, Energy Conversion Routes From Biomass

Dr. Shiv Prasad Prasad

1. Introduction

The fuel (liquid, solid or gas) that has its origin from biomass is termed as ‘biofuel’. The chemical properties of biofuels are very different from those of petroleum due to its oxygen levels of 10–45%. The biofuel are more associated with energy security, the environment, trading, and socioeconomic issues related to the rural sector as biomass are more abundant and available in those region.

Biofuels include bioethanol, methanol, vegetable oils, biodiesel, biogas, bio-synthetic gas (bio-syngas), bio-oil, biochar (charcoal created by biomass pyrolysis), Fischer-Tropsch liquids and hydrogen. Biogas and bio-oil are primary products, the preliminary processing of which is almost reduced to collecting the raw material.

2. Types of Biofuels

Biofuels are broadly divided as primary and secondary biofuels (Fig. 1). The major biofuels that are used in an unprocessed form is known as primary biofuel. This is mainly used directly for heating, cooking or electricity production The fuel wood, wood chips, and pellets are some common examples of primary biofuel. The secondary biofuels are generated by processing of plants/crops biomass e.g. ethanol, biodiesel, DME, etc. that can be used in vehicles and various industrial processes (FAO, 2008).

The secondary biofuels are further classified into first, second, third and fourth generation biofuels based on raw material and technology used for their production.

The first generation biofuels refer to the fuels that have been originated from sources like starch, sugar, animal fats and vegetable oil. The second generation biofuels use biomass to liquid technology including cellulosic biofuels, biohydrogen, bioethanol, etc. from lignocellulosics. The feedstock for third generation of biodiesel is algae. The fourth generation biofuels are derived from the bioconversion of living organisms (microorganisms and plants) using biotechnological tools (Rutz and Janseen 2007).

Biofuels are also grouped according to their source and type. They may be derived from the forest, agricultural, fishery products, municipal wastes, by-products and wastes originated from agro-industry, food processing, and food industrial services.

Biofuels can be solid pellets i.e. fuelwood, charcoal, and wood pellets; or liquid such as ethanol, biodiesel and pyrolysis oils; or gaseous such as biogas.

2.1 First generation biofuel technology

The first generation biofuels are produced from cereal crops (e.g. wheat, maize), oil crops (e.g. rapeseed, palm oil) and sugar crops (e.g. sugar beet, sugar cane) using established technology. So the conversion of sugars (sugar cane) and starch (potato, cassava, maize) or oil (oil palm, rapeseed) accumulated in food crops into ethanol and biodiesel respectively accounts the first generation biofuels. However, the scientific evidence that plant traits can be genetically modified to satisfy the requirements for fuel without any trade-off on the desirability as a food crop is absent. Alternatively, different varieties may be exploited for food and fuel production.

2.1.1 Bio-alcohols

The alcohols such as bioethanol, propanol and butanol are produced by microbial fermentation of sugars or starches, derived from feedstocks of wheat, corn, sugar beet, sugarcane, molasses, potato, etc. In the first step complex sugars are hydrolysed and glucose released undergo second fermentation step carried out by yeasts such as Saccharomyces cerevisiae producing ethanol and carbon dioxide. Further diluted ethanol undergo distillation to obtain highly concentrated ethanol in the final step.

Ethanol can be used in petrol engines as a replacement for gasoline; it can be mixed with gasoline to any percentage (eg. E10 means 10 percent blending with gasoline). Butanol (C4H9OH) formed by ABE fermentation (acetone, butanol, and ethanol) is a better biofuel as it will produce more energy and allegedly can be burned “straight” in existing gasoline engines without modification to the engine or car and is less corrosive and less water soluble.

2.1.2 Biodiesel

The biodiesel is produced mainly by transesterification of fatty acids of lipids (vegetable oils or aimal fat) with alchol to form a mix of fatty acid alkyl esters (FAAE). When methanol is used for transesterification biodiesel is fatty acid methyl esters (FAME), while the ethanol is used it is fatty acid ethyl esters (FAEE). The characteristics of the biodiesel concerned from ethanol or methanol are very similar, but methanol is the preferred alcohol despite its toxicity and fossil fuel origin because of its low cost and wide availability. The transesterification enhanced by homogeneous or heterogeneous catalysts. The preferred catalysts are the alkaline catalysts because of their low cost, versatiliy and less corrosivity than acid catalysts. Biodiesel can be used in any diesel engine either in pure form or mixed with mineral diesel (eg.B20 means blending of 20 percent biodiesel to diesel).

2.1.3 Biogas

Biogas consists of methane produced by process of anaerobic digestion of organic material by anaerobic microorganisms which is used as an energy source and solid by-product, digested, is used as an organic manure. The biogas can be produced from any waste with organic fraction in comparison to ethanol ad biodiesel production from crops. The net energy yield per hectare per year is laso comparatively higher. The biogas could be even produced from the by-products and waste released from the current bioethanol and biodiesel industries (Jegannathan et al., 2009).

2.1.4 Syngas

Syngas is a mixture of carbon monoxide, hydrogen and other hydrocarbons produced by partial combustion of biomass, that is, the burning with a volume of oxygen that is not sufficient to transform the biomass waste completely to carbon dioxide and water. The syngas is more efficient than direct combustion of the original biofuel; more of the energy contained in the fuel is extracted. Syngas may be burned directly in internal combustion engines, turbines or high-temperature fuel cells. Syngas can be utilized to produce methanol, DME, and hydrogen, or converted via the Fischer-Tropsch process to produce a diesel substitute or a mixture of alcohols that can be blended into gasoline.

2.1.5 Solid biofuels

When the raw material is already in a suitable form (i.e. firewood), it can burn instantly in a stove or furnace to produce heat or steam. When biomass raw materials is in an inconvenient form (i.e. sawdust, wood chips, grass, urban waste timber or crop residues), the standard method is to densify the materials. This process involves grinding the biomass raw materials to a suitable particulate size (known as hog fuel), which depending on the densification type can be from 1 to 3 cm (1 inch), which is then compressed into a fuel product. The popular types of processes are wood pellet, cube, or puck. The other kinds of densification are bigger in size compared to wood pellet and are compatible with a wide variety of input feedstocks.

The densified fuels are easier to transport and feed into thermal generation units i.e. boilers. One of the advantages of solid biomass fuel is that it is often a by-product, residue or waste-product of other processes, such as farming, animal husbandry and forestry and thereby no competition between fuel and food production. A problem with the combustion of biomass raw materials is that it releases large amounts of pollutants such as particulates and PAHs (polyaromatic hydrocarbons).

2.1.6 Biochar

Biochar is one of the prodcut of pyrolysis and is often used to pre-dry biomass feedstock or sold as charcoal briquettes. Its high stability against decay and ability to retain more plant nutrients as compared to other forms of organic matter made the biochar as a good soil amendment. Fig. 2 graphically depicts pyrolysis coupled with organic matter returned through biochar, addresses the problems of loss of carbon from the soil surface, as about half of the original carbon can be returned to the ground. the biochar process. Biochar produced from farm waste can substitute for wood charcoal.

3. Pros and cons of secondary biofuels

3. 1 Pros and cons of first generation biofuels

The reduction in energy demand can be made by first generation biofuels. It reduces atmospheric pollution and increases usage of traditional fuels, reducing import oil demands in countries with sufficient agricultural capacity. But the biofuel production require large land area for its cultivation and conversion from food crops to fuel make a big gap in the demand and supply chain. The pros and cons of first generation biofuels summarized (Larson, 2008) in Table 1.

Table 1: Pros and Cons of First-generation biofuels

3.2 Second generation biofuel technology

Second-generation biofuels are produced from sustainable feedstock. The second-generation biofuels are produced from cellulosic materials and also based on the use of dedicated energy crops like switchgrass grown with reasonable inputs and using conversion techniques that provide high net energy efficiency (output/input). Many second generation biofuels are under development i.e. cellulosic ethanol, biohydrogen, methanol, DMF, Fischer-Tropsch diesel, etc. A few examples of first generation and second generation biofuels is summarized (Larson, 2008) in Table 2.

Table 2: Examples of first generation and second generation biofuels

Cellulosic biomass is abundant and renewable. It is seen as a promising alternative to produce ethanol from starch or sugar. When using such material, i.e., corn stover, straw, bagasse, forest waste, and another cellulosic biomass, a chemical or enzymatic hydrolysis pretreatment is required to degrade the lignin, and to produce enough quantity of sugars to produce ethanol. The efficient and economically affordable enzymatic hydrolysis is still a limitation. Cellulosic ethanol production does not divert foodgrain away from the animal or human food chain.

The triacylglycerols derived from the oilseeds or vegetable oils are another potential feedstock to produce biofuels, mainly biodiesel. However, most of the countries in the world are facing the scarcity of edible oils supply. There is a growing interest in wood/tree/plants borne non-edible oils to avoid competition with food crops. Non-edible oils obtained from plant species such as Jatropha curcas (Ratanjyot), Pongamia pinnata (Karanj), Calophyllum inophyllum (Nag-champa), Hevea brasiliensis (Rubber) and other non-edible oils based resources also can be efficiently utilized for production of biodiesel. For example, Jatropha curcas is a drought resistant, perennial oil plant (ca. 40% oil content) with favorable traits to produce biodiesel in unfavorable regions of India, Sub-Saharan Africa and Latin America (Kumar & Sharma, 2008).

The Fischer–Tropsch process is a chemical technique that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. It was invented in the early 1900’s by Franz Fischer and Hans Tropsch studying the conversion of coal-derived syngas. The polymerization of H2 with CO and CO2 in presence of Iron, cobalt or ruthenium as catalysts yield linear hydrocarbons (Huber et al., 2006). The process at high-temperature at 330–350ºC yield mostly short-chain hydrocarbons (gasoline) and light olefins in a fluidized bed reactor and at low-temperature, 220– 250ºC in a slurry bubble column reactor, waxes and long-chain hydrocarbons are obtained (Bludowsky and Agar, 2009).

3.3 Third generation biofuel technology

Microalgae has been considered as third generation potential feedstock for producing sustainable transport fuels (biodiesel). Microalgae are sunlight-driven cell factories that convert carbon dioxide to potential biofuels. Depending on species, microalgae produce kinds of lipids, hydrocarbons, and other complex oils. Certain algae and cyanobacteria have high lipid contents. This lipid contents derived from microalgae can be used for biodiesel production. Under proper conditions, these micro-organisms can produce lipids for biodiesel with yields per unit area that are 50-l00% higher than those with any plant system. Microalgae can also provide various types of renewable biofuels which include methane by anaerobic digestion of the algal biomass and photo-biologically produced biohydrogen (Chisti, 2008).

3.4 Fourth generation biofuel technology

The 4th generation biofuels are based on photo-biological solar fuels, and electro fuel is expected to bring significant breakthroughs in the domain of biofuels. The solar biofuel relies on the direct conversion of solar energy into fuel using raw materials that are inexhaustible, inexpensive and widely available. This is expected to happen via advanced progress of synthetic biology as an enabling technology for such a change. These biofuels are obtained from the conversion of living organisms (microorganisms and plants) using biotechnological tools. The mean conversion efficiency for the total solar spectrum amounts to ca. 20%, which is on average about ten times higher than for seasonal crops. This excellent efficiency should be considered a potential level. Currently, this technology is still expensive and not yet ready for commercial exploitation.

4. Energy conversion routes from biomass

Conversion technologies for the production of energy from biomass can be categorized as thermo-chemical (direct combustion, pyrolysis, gasification) or biochemical (fermentation or anaerobic digestion). Biofuel technologies provide opportunities for conversion of biomass into liquid and gaseous fuels as well as electricity. Biomass can be used for direct combustion to produce steam turns a turbine, and the turbine drives a generator, producing electricity.

The thermochemical processes and biochemical processes are mainly used for conversion of biomass to biofuel.

4.1 Thermochemical processes

4.1.1 Direct Combustion

It is the widely used commercial technology. In the direct combustion, biomass is burned in a boiler to produce high-pressure steam. This steam is injected into a steam turbine, where it flows over a series of turbine blades, causing the turbine to rotate. The turbine is connected to the electric generator. The electric generator turns, producing electricity.

4.1.2 Gasification

Gasification is the conversion of carbonaceous feedstock into synthetic gas. The high temperatures and a controlled environment lead to practically all the raw material being converted to gas. Gasification takes place in two stages, in the first step, the raw material is partly combusted to form producer gas and char. In the second phase, the CO2 and H2O produced in the first stage are chemically reduced by the charcoal, creating CO and H2. The composition of the producer gas is 18 to 20% hydrogen, an equal portion of carbon monoxide, 2 to 3% methane, 8 to 10% carbon dioxide, and the residue nitrogen (Makunda, 1992).

This process is achieved by reacting the material at high temperatures (>700 °C), and at or greater atmospheric pressure. The energy density of the gas is less than 5.6 MJ/m3, which is low in contrast to natural gas at 38 MJ/m3, giving only 60% the energy rating of diesel when practiced in a modified diesel engine. The use of BIG/STIG (Biomass Integrated Gasifier Steam Injected Gas Turbine) initially and BIG/GTCC (Biomass Integrated Gasifier Gas Turbine Combined Cycle) as the technology matures, is predicted to allow energy conversion efficiencies of 40% to 55%.

Combined Heat and Power systems could ultimately provide energy at efficiencies of 50% to 80%. The use of low-grade feedstocks coupled with high conversion efficiencies makes these systems economically competitive with cheap coal-based plants and energetically competitive with natural gas-based power stations. Gasifier technology has penetrated the applications such as village electrification, captive power generation and process heat generation in industries producing biomass waste. Over 1600 gasifier systems, having 16 MW total capacity, have generated 42 million Kilo Watt hour (KWh) of electricity, replacing 8.8 million liters of oil annually (CMIE, 1996).

4.1.3 Pyrolysis

Pyrolysis is the thermic degradation of organic material occurring in the absence of oxygen, thus inhibiting complete combustion. In this process, biomass is degraded into a gaseous mixture called producer gas (CH4 and CO and H2). Carbon dioxide may also be produced as well, but under the pyrolytic conditions of the reactor, it is reduced back to CO and H2O. This water further aids the reaction. Liquid phase products result from temperatures which are too small to crack all the long chain carbon molecules so resulting in oils, methanol, acetone, and tars, etc. Once all the volatiles component have been driven off, the residual biomass is in the form of char which is virtually pure carbon. Pyrolysis has received attention recently for the production of liquid fuels from cellulosic feedstocks by “fast” and “flash” pyrolysis in which the biomass has a short residence time in the reactor.

Fast pyrolysis is a thermal degradation process that rapidly heats biomass to an entirely controlled temperature (~500°C), then very quickly cools the volatile products (<2 sec) formed in the reactor. It offers the unique advantage of producing a liquid that can be stored and transported. Currently, there are many types of Fast Pyrolysis Reactors used, for example, bubbling fluidized bed, circulating fluidized beds/transport reactor, rotating cone pyrolyzer, ablative pyrolyzer, etc.

Bio-oil is water miscible and is comprised of many oxygenated organic chemicals which are acidic, dark brown mobile liquid, combustible, not miscible with hydrocarbons. The heating value is ~ 17 MJ/kg, and density is ~ 1.2 kg/l. However, the complexity and nature of the liquid result in some unusual properties due to physical-chemical processes such as polymerization/condensation, esterification, and etherification, agglomeration of oligomeric molecules. With aging its viscosity increases, volatility decreases phase separation, deposits, gums. Physical Methods like filtration for char removal, emulsification with hydrocarbons, solvent addition; Chemical Methods such as reaction with alcohols, catalytic deoxygenation: hydrotreating, catalytic (zeolite) vapor cracking,etc. are used to upgrade bio-oils.

4.2 Chemical process

4.2.1 Transesterification

Biodiesel can be produced from different feedstocks, such as oil feedstock (e.g., rapeseed, soybean oils, jatropha, palm oil, hemp, algae, canola, flax, and mustard), animal fats, and/or waste vegetable oilby process of transesterification. The Transesterification process is the reaction of a triglyceride (fat/oil) with alcohol to form esters and glycerol. A triglyceride has a glycerine molecule as its base with three long chain fatty acids attached. The characteristics of the fat are determined by the nature of the fatty acids attached to the glycerine. Thecharacter of the fatty acids can, in turn, affect the characteristics of the biodiesel.

The first conventional way of producing biodiesel is using a base catalyst. In the alkali process sodium hydroxide (NaOH) or potassium hydroxide (KOH) is used as a catalyst along with methanol or ethanol. Initially, during the process, alcoxy is formed by reaction of the catalyst with alcohol and the alcoxy is then reacted with any vegetable oil to form biodiesel and glycerol. The alcoxy reaction is as follows:

R-CH2OH + NaOH    H2O + R-CH2ONa

Glycerol being denser settles at the bottom and biodiesel can be decanted. This process is the most efficient and least corrosive of all the processes, and the reaction rate is reasonably high even at a low temperature of 60 ºC. There may be a risk of the free acid or water contamination, and soap formation is likely to take place which makes the separation process difficult.

4.3 Biochemical Process

4.3.1 Anaerobic Digestion

Anaerobic digestion is a series of anaerobic processes in which microorganisms break down biodegradable material in the absence of oxygen into biogas and manure. The digestion process starts with bacterial hydrolysis of the input materials to break down into soluble organic polymers, i.e. simple sugars and make them available for acidogenic bacteria. These bacteria then convert this sugars and amino acids into carbon dioxide, hydrogen, ammonia, and other organic acids. Then the acetogenic bacteria convert these organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide. It is a type of fermentation that converts organic material into biogas, which mainly consists of methane (approximately 60%) and carbon dioxide (about 40%) under anaerobic conditions by microbes. Biomass can be converted to biogas it including animal and human wastes, sewage sludge, crop residues, industrial processing byproducts, etc.

4.3.2 Fermentation

Microbial fermentation is an efficient and extensively used method for biofuels production. It includes bioethanol, biobutanol, biohydrogen, etc. Ethanol, butanol, and methanol are produced principally from energy crops such as sugarcane, maize, beets, yam, or sweet sorghum. A variety of microorganisms ferment sugars into ethanol i.e. Saccharomyces cerevisiae, Pichia stipitis, Candida shehatae and Pachysolan tannophilus, etc. The ethanol recovery is done by distillation and concentrated in a rectifying column to a 95 %. Anhydrous ethanol (99.0 %), can be mixed with gasoline and can be used as fuel.

Raw materials containing sugars, or materials which can be transformed into sugars, can be used as fermentation substrates. The fermentable raw materials can be grouped as directly fermentable sugar materials, starchy, lignocellulosic materials and other biomass wastes. Direct fermentation of sugarcane, sugar beet, and sweet sorghum to produce ethanol has been reported. Commercially, cane and beet molasses, sugarcane, sugar beets and in small quantities sweet sorghum, Jerusalem artichoke fruit, and fruit juices are used directly as fermentable material (Prasad et al., 2007b).

Conclusion

Recent advancements in bioengineering, synthetic biofuel processing, and energy conversion technologies are offering new opportunities for the transition to traditional biomass energy uses to modern bio-energy services. This change is made viable by improving technological, organizational and institutional capabilities in developing countries and ever increasing global linkages. As an energy resource, biomass is an essential part of climate change policies. Sustainable biomass production can deliver benefits vis-à-vis food and energy goals while contributing to climate change objectives. Biofuels from Biomass can provide not only environments benefits but also many co-benefits like land restoration, local employment and enhance mitigating and adaptive capacities for dealing with air pollution.