Encyclopedia of Renewable Energy. James G. Speight
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produces a dark liquid called pyrolysis oil, a synthetic gas called syngas, and a solid residue called biochar. All of these components can be used for energy. Pyrolysis oil, sometimes called bio-oil or biocrude, is a type of tar. It can be combusted to generate electricity and is also used as a component in other fuels and plastics as an alternative to crude oil.
Biochar is a type of charcoal. Biochar is a carbon-rich solid that is particularly useful in agriculture. Biochar enriches soil and prevents it from leaching pesticides and other nutrients into runoff. Biochar is also an excellent carbon sink. Carbon sinks are reservoirs for carbon-containing chemicals, including greenhouse gases.
Gasification
Biomass can also be directly converted to energy through gasification. During the gasification process, a biomass feedstock (usually MSW) is heated to more than 700°C (1,300°F) with a controlled amount of oxygen. The molecules break down, and produce syngas and slag.
Synthesis gas is a combination of hydrogen and carbon monoxide. During gasification, syngas is cleaned of sulfur, particulates, mercury, and other pollutants. The clean syngas can be combusted for heat or electricity, or processed into transportation biofuels, chemicals, and fertilizers. Synthesis gas can also be converted into fuel (such as synthetic natural gas). It can also be converted into methane and used as a replacement for natural gas.
Slag forms as a glassy, molten liquid and can be used to make shingles, cement, or asphalt.
Anaerobic Decomposition
Anaerobic decomposition is the process where microorganisms, usually bacteria, break down material in the absence of oxygen. Anaerobic decomposition is an important process in landfills, where biomass is crushed and compressed, creating an anaerobic (or oxygen-poor) environment.
In an anaerobic environment, biomass decays and produces methane, which is a valuable energy source. This methane can replace fossil fuels. In addition to landfills, anaerobic decomposition can also be implemented on ranches and livestock farms. Manure and other animal waste can be converted to sustainably meet the energy needs of the farm.
See also: Alternate Fuels.
Biomass Energy Systems
Biomass has to be produced either by the cultivation of dedicated crops (such as wood by means of short rotation forestry, perennial grasses), by harvesting forest and other residues (thinnings, straw, etc.), or by collecting biomass waste (such as sludge, organic industrial waste, and organic domestic waste). Next, the biomass has to be harvested or collected, then transported, and if necessary, it may have to stored and transferred.
Biomass can be converted by means of numerous processes. The actual choice of a process will depend on the type and quantity of available biomass feedstock, the desired energy carrier(s) (end-use), environmental standards, economic conditions, and other factors.
Most biomass energy conversion processes can be divided into (i) thermochemical conversion and (ii) biochemical conversion routes, and (iii) mechanical processes. With respect to thermochemical conversion options, a distinction can be made between combustion, gasification and pyrolysis. Biochemical conversion options can be divided into digestion (production of biogas, a mixture of mainly methane and carbon dioxide) and fermentation (such as the production of ethanol). Extraction is another, mainly mechanical, process for producing an energy carrier from biomass (e.g., rapeseed oil from rapeseed). With regard to the energy carriers produced from biomass, a distinction can be made between the production of heat, electricity, and fuels.
See also: Bioconversion Platform, Thermoconversion Platform.
Biomass – Extraction
Extraction is a mechanical conversion process and two well-established technologies for oil extraction are (i) the simple screw press, which is a device for physically extracting the oil from the plant – this technology is well suited to small-scale production of oil as fuel or as foodstuff in rural areas, and (ii) solvent extraction, a chemical process which requires large, sophisticated equipment – this method is more efficient and extracts a greater percentage of the oil from the plant.
The process can be used to derive rapeseed oil from rapeseed. In this case, the process produces not only oil but also rapeseed cake, which is suitable for fodder. Approximately 3 tons of rapeseed is required per ton of oil.
Rapeseed oil can be esterified to obtain rapeseed methyl ester (RME) or biodiesel. This process is being used commercially on a substantial scale, especially in Europe.
See also: Biomass Extraction, Rapeseed.
Biomass Feedstocks
Consideration of biomass feedstocks for processes must go beyond such a simple representation as the van Krevelen diagram since the composition varies with the nature of the feedstock. For most agricultural residues, the heating values are even more uniform – approximately 6,450 to 7,300 Btu/lb, and the values for most woody materials are 7,750 to 8,200 Btu/lb. The moisture content is extremely important in determining the heating value of biomass as well as its suitability as a process feedstock. Air-dried biomass typically has approximately 15 to 20% moisture. The moisture content is also an important characteristic of coals, varying in the range of 2 to 30%. However, the bulk density (and hence energy density) of most biomass feedstocks is generally low, even after densification, approximately 10 and 40% of the bulk density of most fossil fuels. Liquid biofuels have comparable bulk densities to fossil fuels.
Most biomass materials are easier to gasify than coal because they are more reactive with higher ignition stability. This characteristic also makes them easier to process thermochemically into higher-value fuels such as methanol or hydrogen. Ash content is typically lower than for most coals, and sulfur content is much lower than for many fossil fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants, biomass ash may be used as a soil amendment to help replenish nutrients removed by harvest.
Some biomass feedstocks stand out for their peculiar properties, such as high silicon or alkali metal contents – these may require special precautions for harvesting, processing, and combustion equipment. The mineral content can also vary as a function of soil type and the timing of feedstock harvest. In contrast to their fairly uniform physical properties, biomass fuels are rather heterogeneous with respect to their chemical elemental composition.
Among the liquid biomass fuels, biodiesel (vegetable oil ester) is noteworthy for its similarity to crude oil-derived diesel fuel, apart from its negligible sulfur and ash content. Bioethanol has only approximately 70% the heating value of crude oil distillates such as gasoline, but the sulfur content and the ash content are also low. Both of these liquid fuels have lower vapor pressure and flammability than their crude oil-based competitors – an advantage in some cases (e.g., use in confined spaces such as mines) but a disadvantage in others (e.g., engine starting at cold temperatures).
Plants offer a unique and diverse feedstock for chemicals. Plant biomass can be gasified to produce synthesis gas, a basic chemical feedstock and also a source of hydrogen for a future hydrogen economy. In addition, the specific components of plants such as carbohydrates, vegetable oils, plant fiber, and complex organic molecules known as primary and secondary metabolites can be utilized to produce a range of valuable monomers, chemical intermediates, pharmaceuticals, and materials:
Carbohydrates (starch, cellulose, sugars) are readily obtained from wheat and potato, while cellulose is obtained from wood pulp. The structures of these polysaccharides can be readily manipulated to produce a range of biodegradable polymers with properties similar to those of conventional plastics such as polystyrene foams and polyethylene film. In addition, these polysaccharides can be hydrolyzed, catalytically or enzymatically, to produce sugars, a valuable fermentation feedstock for the production of ethanol, citric acid, lactic acid, and dibasic acids such as succinic acid.
Vegetable