of carbon monoxide, carbon dioxide, hydrogen, and other compounds by applying heat and pressure in the presence of steam and a small amount of oxygen, typical of gasification of any organic material. Biomass produces hydrocarbon compounds in the gas, especially in the absence of oxygen, requiring an extra step to remove them with a catalyst downstream of the gasifier. The water gas shift reaction is then used to convert some of the carbon monoxide with water to form more carbon dioxide and hydrogen:
Generally, biomass does not gasify as easily as coal but most of the main type of gasifiers can be designed to handle biomass or a combination of biomass and coal, but each has positives and negatives and the selection depends on the specific feedstock and the desired product or products.
Raw materials that can be used to produce biomass derived fuels are widely available; they come from a large number of different sources and in numerous forms (Rajvanshi, 1986). The main basic sources of biomass include: (i) wood, including bark, logs, sawdust, wood chips, wood pellets and briquettes, (ii) high-yield energy crops, such as wheat, grown specifically for energy applications, (iii) agricultural crops and residues (e.g., straw), and (iv) industrial waste, such as wood pulp or paper pulp. For processing, a simple form of biomass such as untreated and unfinished wood may be converted into a number of physical forms, including pellets and wood chips, for use in biomass boilers and stoves.
Biomass includes a wide range of materials that produce a variety of products which are dependent upon the feedstock (Balat, 2011; Demirbaş, 2011; Ramroop Singh, 2011; Speight, 2011a). In addition, the heat content of the different types of biomass widely varies and have to be taken into consideration when designing any conversion process (Jenkins and Ebeling, 1985). Thermal conversion processes use heat as the dominant mechanism to convert biomass into another chemical form. The basic alternatives of combustion, torrefaction, pyrolysis, and gasification are separated principally by the extent to which the chemical reactions involved are allowed to proceed (mainly controlled by the availability of oxygen and conversion temperature) (Speight, 2011a).
Energy created by burning biomass (fuel wood), also known as dendrothermal energy, is particularly suited for countries where fuel wood grow more rapidly, e.g., tropical countries. There is a number of other less common, more experimental or proprietary thermal processes that may offer benefits including hydrothermal upgrading (HTU) and hydroprocessing. Some have been developed to be compatible with high moisture content biomass (e.g., aqueous slurries) and allow them to be converted into more convenient forms. Some of the applications of thermal conversion are combined heat and power (CHP) and co-firing. In a typical dedicated biomass power plant, efficiencies range from 7 to 27%. In contrast, biomass co-firing with coal, typically occurs at efficiencies close to those of coal combustors (30 to 40%) (Baxter, 2005; Liu et al., 2011).
Many forms of biomass contain a high percentage of moisture (along with carbohydrates and sugars) and mineral constituents – both of which can influence the economics and viability of a gasification process. The presence of high levels of moisture in biomass reduces the temperature inside the gasifier, which then reduces the efficiency of the gasifier. Many biomass gasification technologies therefore require dried biomass to reduce the moisture content prior to feeding into the gasifier. In addition, biomass can come in a range of sizes. In many biomass gasification systems, biomass must be processed to a uniform size or shape to be fed into the gasifier at a consistent rate as well as to maximize gasification efficiency.
Biomass such as wood pellets, yard and crop waste and “energy crops” including switch grass and waste from pulp and paper mills can also be employed to produce bioethanol and synthetic diesel. Biomass is first gasified to produce synthesis gas and then subsequently converted via catalytic processes to the aforementioned downstream products. Biomass can also be used to produce electricity – either blended with traditional feedstocks, such as coal, or as a single feedstock. Also, most biomass gasification systems use air instead of oxygen for gasification reactions (which is typically used in large-scale industrial and power gasification plants). Gasifiers that use oxygen require an air separation unit to provide the gaseous/liquid oxygen; this is usually not cost-effective at the smaller scales used in biomass gasification plants. Air-blown gasifiers utilize oxygen from air for gasification processes.
The use of biomass to offset use of fossil fuels can lower greenhouse gas emissions. Burning biomass for energy does create carbon dioxide; however, the same amount of carbon dioxide is recaptured in future biomass growth. So as long as there is no carbon dioxide released into the atmosphere during production of the biomass (growth, harvest, and processing), the process sums to zero net carbon dioxide to the atmosphere. Also, if biomass is left to decompose naturally, or openly burned, a large portion of the carbon in the biomass is converted to methane (CH4). The percentage of carbon in the biomass which is converted to methane is 50% in the case of rotting, and 5 to 10% in the case of open burning. These processes are actually worse in regard to greenhouse gas emissions than is biomass gasification, which converts nearly all of the carbon contained in the biomass to carbon dioxide, since methane is a much stronger greenhouse gas than carbon dioxide.
In general, biomass gasification plants are comparatively smaller to those of typical coal or crude oil coke plants used in the power, chemical, fertilizer and refining industries. As such, they are less expensive to build and have a smaller environmental footprint. While a large industrial gasification plant may take up 150 acres of land and process 2,500 to 15,000 tons per day of feedstock (e.g., coal or crude oil coke), smaller biomass plants typically process 25 to 200 tons of feedstock per day and take up less than 10 acres.
Finally, while biomass may seem to some observers to be the answer to the global climate change issue, advantages and disadvantages of biomass as feedstock must be considered carefully (Table 2.5). In addition, while taking the issues of global climate change into account, it must not be ignored that the Earth is in an interglacial period when warming will take place. The extent of this warming is not known – no one was around to measure the temperature change in the last interglacial period – and by the same token the contribution of anthropological sources to global climate change cannot be measured accurately.
The potential variability of biomass feedstocks, longer-term changes in refuse and the size limitation of a power plant using only waste and/or biomass can be overcome combining biomass, refuse, and coal. It also allows benefit from a premium electricity price for electricity from biomass and the gate fee associated with waste. If the power plant is gasification-based, rather than direct combustion, further benefits may be available. These include a premium price for the electricity from waste, the range of technologies available for the gas to electricity part of the process, gas cleaning prior to the main combustion stage instead of after combustion and public image, which is currently generally better for gasification as compared to combustion. These considerations lead to current studies of co-gasification of wastes/biomass with coal (Speight, 2008).
Table 2.5 Advantages and disadvantages of using biomass as a gasification feedstock.
| Advantages Disadvantages | Theoretically inexhaustible fuel source. |
| Minimal environmental impact when direct combustion of plant mass is not used to generate energy (i.e., fermentation, pyrolysis, etc., are used instead), (iii) alcohol derivatives and other fuels produced by biomass are efficient, viable, and relatively clean-burning. | |
| Available on a worldwide basis. | |
| Possible adverse contribution to global climate change and particulate pollution if combusted directly. | |
| Not always a cheap source of energy, both in terms of producing biomass and the conversion to alcohols or other fuels. | |
|
Life cycle assessments (LCA) should be taken into account to address energy inputs and outputs but there is most likely a net loss of energy when operated on a small scale which requires that energy to grow the plant mass must be taken into account.
|