Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 179

The quality and composition of a biofuel depends on the source of the biomass/feedstock as well as the types of processing and conversion techniques utilized in its manufacture. Biomass feedstock composition ultimately decides the yield from the chemical or biochemical conversion processes, which in turn, affects the economics involved. There are many plant varieties which are used as biofuel sources - the geography, weather conditions, soil composition, and legislation of a location normally dictates what types are grown specifically for biofuel production. Ethanol, biodiesel, and butanol are the main types of commercially produced biofuels.

The soil organic matter content contributes greatly to the grain and stover, and hence carbohydrate content of maize plants. Lignin and cell-wall cross-linking also affect the ethanol production. Selection for reduced lignin and increased cellulose in stover can potentially be expected to increase mechanical strength as well as ethanol yield. Although pretreatment and enzyme hydrolysis constitute two of the more costly steps in cellulosic ethanol production, stover with reduced lignin may still need to be treated before being subjected to enzyme hydrolysis. It seems unlikely that the cost savings in pretreatment from reduced lignin can be fully realized because of an accompanying reduction in biomass. However, for ethanol production to be commercially viable, improvements must not only be made to the efficiency of ethanol production per unit dry mass, but also per unit land area.

Biomass feedstock composition ultimately decides the yield from the chemical or biochemical conversion processes, which in turn, affects the economics involved. There are many plant varieties which are used as biofuel sources – the geography, weather conditions, soil composition, and legislation of a location normally dictates what types are grown specifically for biofuel production. Ethanol, biodiesel, and butanol are the main types of commercially produced biofuels.

The soil organic matter content contributes greatly to the grain and stover, and hence carbohydrate content of maize plants. Lignin and cell-wall cross-linking also affects the ethanol production. Selection for reduced lignin content and increased cellulose content in stover can potentially be expected to increase mechanical strength as well as ethanol yield. Although pretreatment and enzyme hydrolysis constitute two of the more costly steps in cellulosic ethanol production, stover with reduced lignin may still need to be treated before being subjected to enzyme hydrolysis.

The production of biofuels from lingo-cellulosic feedstocks can be achieved through two very different processing routes which are (i) the biochemical route in which enzymes and other micro-organisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol and (ii) the thermochemical route in which pyrolysis and gasification technologies produce a synthesis gas (carbon monoxide, CO, and hydrogen, H₂) from which a wide range of long chain biofuels, such as synthetic diesel or aviation fuel, can be reformed. One key difference between the biochemical and thermochemical routes is that lignin component is a residue of the enzymatic hydrolysis process and can be used for heat and power generation.

Genetics as well as environmental factors affect the chemical composition of the various parts of the plant, and it was found that husk, followed by rind and pith, has the highest sugar (glucan + xylan) content. The term glucan represents diverse glucose polymers that differ in the position of glycosidic bonds, which can be short or long, branched or unbranched, alpha or beta isomers, and soluble or insoluble. On the other hand, the term represents a group of hemicellulose derivatives.

The variation in the structure of the glucan derivatives and the xylan derivatives (Figure B-2) is due to differences in the amounts of the main chemical constituents of biomass (cellulose, hemicelluloses, and lignin, all of which have different uses) being present in different proportions in the various parts of the plant.

Figure B-2 Structure of xylan from hardwood.

The production of fuels from ligno-cellulosic feedstocks can be achieved through two different processing routes. These are (i) biochemical, whereby enzymes and other microorganisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol and (ii) the thermochemical route, where pyrolysis and gasification technologies produce a synthesis gas (carbon monoxide and hydrogen) from which a wide range of long chain biofuels, such as synthetic diesel or aviation fuel, can be reformed. In terms of the biochemical route, much remains to be done related to (i) improving feedstock characteristics, (ii) improving the efficiency of enzymes, and (iii) improving overall process integration. One key difference between the biochemical and thermochemical routes is that lignin component is a residue of the enzymatic hydrolysis process and can be used for heat and power generation.

In terms of the production of biodiesel, transesterification optimization is desired, which would depend on the chemical composition of alkyl esters of vegetable oils, animal fats, and cooking oil. Most common feedstocks possess fatty acid profiles consisting mainly of five C16 and C18 fatty acids, namely, palmitic (hexadecanoic), stearic (octadecanoic), oleic (9(Z)-octadecenoic), linoleic (9(Z),12(Z)-octadecadienoic), and linolenic (9(Z),12(Z),15(Z)-octadecatrienoic) acids, with the exception of a few oils such as coconut oil, which contains high amounts of saturated acids in the C12 to C16 range. Changing the fatty acid profile can be achieved by physical means, genetic modification of the feedstock, or use of alternative feedstocks with different fatty acid profiles, such as wood-derived fatty-acids and related compounds.

The production of biogas is essentially achieved by wastewater treatment facilities which use anaerobic digestion to reduce the organic content of sewage sludge and animal wastes. The variation in the composition of biogas will depend on the organic content of the biomass as well as the pretreatment and processing. In addition to the main components (methane and carbon dioxide), biogas can also contain a variety of contaminants and impurities, such as sulfur compounds (hydrogen sulfide, mercaptans), halogens, ammonia, and dust particles.

Concerning biodiesel production, transesterification optimization is desired, which would depend on the chemical composition of alkyl esters of vegetable oils, animal fats, and cooking oil. Most common feedstocks possess fatty acid profiles consisting mainly of five C₁₆ and C₁₈ fatty acids, namely, palmitic (hexadecanoic), stearic (octadecanoic), oleic (9(Z)-octadecenoic), linoleic (9(Z),12(Z)-octadecadienoic), and linolenic (9(Z),12(Z),15(Z)-octadecatrienoic) acids, with the exception of a few oils such as coconut oil, which contains high amounts of saturated acids in the C₁₂ to C₁₆ range or others. Changing the fatty acid profile can be achieved by physical means, genetic modification of the feedstock, or use of alternative feedstocks with different fatty acid profiles, such as wood-derived fatty acids and related compounds.

Bio-oil can be used can be used directly as fuel or can be fractionated to obtain purified hydrocarbon derivatives boiling in the range of gasoline and diesel fuel – analysis of waste fish oil has shown that the main composition of fatty acids is: C_16:0 (15.9% w/w), C_18:2 (20.9% w/w), C_18:1 (17.3% w/w), C_20:5 (5.1% w/w), C_20:1 (7.6% w/w), C_22:6 (4.3% w/w), and C_22:1 (10.4% w/w) – the first number in the carbon-related subscript is the chin length and the second number is the position of the double bond in the carbon chain. Other compounds were classified as paraffin derivatives (4.5% w/w), iso-paraffin derivatives (8.3% w/w), olefin derivatives (26.6% w/w), naphthene derivatives (6.1% w/w), and aromatic derivatives (16.9% w/w).

Tall oil is a dark, viscous, and odorous liquid that phase-separates from the used pulping liquor (alkaline black liquor) that remains after the pulping of wood chips, and contains sodium soaps of rosin and fatty acids. Fish oil is also a potentially good source of fatty acids for the production of bio-oil. In a particular study, waste fish oil was converted into bio-oil by a fast pyrolysis process at 525°C (975°F) in a continuous pilot plant reactor with a yield on the order of 72% yield. Many different chemical groups can be produced during the pyrolysis reaction, and the liquid product (bio-oil) obtained from triglyceride pyrolysis has a complex composition. This bio-oil can be used directly as fuel or can be fractionated to obtain purified hydrocarbon derivatives in the range of gasoline and diesel.

Green algae (aquatic and unicellular) biomass is also used in biodiesel production, and is an attractive source of triglycerides, as under good conditions, green algae can double its biomass in less than 24 hours. Factors such as carbon dioxide availability, sunlight, water, and space affect algal density, and the annual productivity and oil content of algae are far greater than seed crops.

One of the largest issues seems to be overall greenhouse gas emissions from the various biofuels when compared with crude oil fuels. To estimate the impacts of increases in renewable and alternative fuels on greenhouse gas emissions, the entire fuel lifecycle including fossil fuel extraction or feedstock growth, fuel production, distribution, and combustion should be taken into consideration to provide a comparison of the carbon dioxide emissions from different fuels.

It is generally accepted that biofuels have the potential to drastically lower carbon-dioxide emissions than fuels derived from crude oil, but in many instances, this is not the case. For example, ethanol made from corn requires a substantial amount of energy in terms of fertilization, irrigation, harvesting and fermentation processes and most of this energy comes from fossil fuels. As a result, some ethanol production scenarios emit more lifecycle carbon-dioxide emissions than gasoline. Cellulose-based ethanol, however, allows for more efficient and cost-effective fuel production, and the carbon footprint is decreased. Conversely, biodiesel blended into diesel at low levels reduces emissions of volatile organic compounds, carbon monoxide, particulate matter, and sulfur oxides during combustion. Further, over the full life cycle, biodiesel blends reduce carbon monoxide, particulate matter, and sulfur oxides compared with diesel.

The bulk density (related to energy density) of most solid biomass feedstocks is generally low, even after compression, and is approximately between 10 and 40% of the bulk density of most fossil fuels; however, liquid biofuels have comparable values. Since biomass materials are more reactive, with higher ignition stability, they are generally easier to gasify and thermochemically process into methanol and hydrogen (higher-value fuels). Coal ash may contain toxic metals and small amounts of other trace contaminants, while biomass ash may be used as a soil amendment to assist in replacing nutrients removed by harvest. Bioethanol possesses approximately 70% of the heating value of crude oil distillates, but its sulfur and ash contents are appreciably lower.

Ethanol has a lower energy density than gasoline, so for a given volume of gasoline, a larger volume of ethanol is needed to produce an equivalent amount of energy. Unfortunately, when ethanol is used to power vehicles, it leads to lower gas mileage, since energy density is correlated with gas mileage. Ethanol, which has approximately a 30% lower energy density than gasoline, contributes to a reduction in fuel mileage when it is mixed with gasoline, as now commonly practiced in the United States. Another problem with ethanol as an auto fuel is it is extremely hygroscopic. Ethanol is difficult to separate from water and readily absorbs water from its surrounding. This is a problem in the transport and storage of ethanol, as well as the purification of ethanol from water. Ethanol-containing fuels can easily absorb water, and decontamination (i.e., removal of water from fuel) is difficult. The difference between the properties of gasoline and ethanol also causes compatibility problems in engines. Biodiesel is the only alternative fuel to have fully completed the health effects testing requirements of the United States Clean Air Act.

The use of biodiesel in a conventional diesel engine results in substantial reduction of unburned hydrocarbon derivatives, carbon monoxide, and particulate matter compared to emissions from diesel fuel. In addition, the exhaust emissions of sulfur oxides and sulfates (major components of acid rain) from biodiesel are essentially eliminated compared to diesel. Of the major exhaust pollutants, both unburned hydrocarbon derivatives and nitrogen oxides are ozone or smog-forming precursors. The use of biodiesel results in a substantial reduction of unburned hydrocarbon derivatives. Emissions of nitrogen oxides are either slightly reduced or slightly increased depending on the duty cycle of the engine and testing methods used.

Based on engine testing, using the most stringent emissions testing protocols required by EPA for certification of fuels or fuel additives in the US, the overall ozone-forming potential of the speciated hydrocarbon emissions from biodiesel was nearly 50% less than that measured for diesel fuel. In a review published in 2009,⁴³ the environmental indicators related to biofuel production were examined.

The postulated superior properties of agrofuels when compared with fossil fuels, as we have seen, must be weighed very carefully among the various factors of cost, environmental impact, energy density, chemical composition and availability, and life cycle of food crops. No doubt, increasing advances in technology will tip the scales in favor of biofuels.

In terms of fuel properties, one of the largest issues seems to be overall greenhouse gas emissions from the various biofuels when compared with crude oil fuels. To estimate the impacts of increases in renewable and alternative fuels on greenhouse gas emissions, it is necessary to account for the entire fuel lifecycle including fossil fuel extraction or feedstock growth, fuel production, distribution, and combustion.

The fuels are compared on an energy equivalent or Btu basis. Thus, for instance, for every Btu of gasoline which is replaced by corn ethanol, the total lifecycle greenhouse gas emissions that would have been produced from that Btu of gasoline would be reduced by 21.8%. These emissions account not only for CO₂, but also methane and nitrous oxide.

It is generally accepted that biofuels have the potential to drastically lower carbon-dioxide emissions than fuels derived from crude oil, but in many instances, this is not the case. For example, ethanol made from corn requires a substantial amount of energy in fertilization, irrigation, harvesting, and fermentation processes and most of this energy comes from fossil fuels. As a result, some ethanol production scenarios emit more lifecycle carbon-dioxide emissions than gasoline. Cellulose-based ethanol, however, allows for more efficient and cost-effective fuel production, and the carbon footprint is decreased.

However, the use of biodiesel in a conventional diesel engine results in substantial reduction of unburned hydrocarbons, carbon monoxide, and particulate matter compared to emissions from diesel fuel. In addition, the exhaust emissions of sulfur oxides and sulfates (major components of acid rain) from biodiesel are essentially eliminated compared to diesel. Of the major exhaust pollutants, both unburned hydrocarbons and nitrogen oxides are ozone or smog-forming precursors. The use of biodiesel results in a substantial reduction of unburned hydrocarbons. Emissions of nitrogen oxides are either slightly reduced or slightly increased depending on the duty cycle of the engine and testing methods used.

The postulated superior properties of agrofuels when compared with fossil fuels, as we have seen, must be considered and compared very carefully among the various factors of cost, environmental impact, energy density, chemical composition, and availability and life cycle of food crops.