1.3 GASIFICATION THEORY
1.3.1 Bioenergy and the Role of Biomass Gasification
The demand for energy sources to satisfy human energy consumption continues to increase. Prior to the use of fossil fuels, biomass was the primary source of energy for heat via combustion. With the introduction of fossil fuels in the forms of coal, petroleum and natural gas, the world increasingly became dependent on these fossil fuel sources. Currently, the main energy source in the world is fossil fuels. The use of plastics and other chemicals which are derived from these fossil fuels also have increased. These tremendous increases have led to many concerns. Although it is not known how much fossil fuel is still available,…show more content… Other consequences associated with fossil fuel use include the release of the trapped carbon in the fossil fuels to the atmosphere in the form of carbon dioxide which has led to increased concerns about global warming. Also, fossil fuel resources are not distributed evenly around the globe which makes many countries heavily dependent on imports[2].
Biomass combines solar energy and carbon dioxide into chemical energy in the form of carbohydrates via photosynthesis. The use of biomass as a fuel is a carbon neutral process since the carbon dioxide captured during photosynthesis is released during its combustion. Biomass includes agricultural and forestry residues, wood, byproducts from processing of biological materials, and organic parts of municipal and sludge wastes. Photosynthesis by plants captures around 4,000 EJ/year in the form of energy in biomass and food. The estimates of potential global biomass energy vary widely in literature[3]. The variability arises from the different sources of biomass and the different methods of determining estimates for those biomasses. Fischer and Schrattenholzer estimated the global biomass potential to be 91 to 675…show more content… While combustion of biomass is the most direct and technically easiest process, the overall efficiency of generating heat from biomass energy is low. Gasification has many advantages over combustion. It can use low-value feed-stocks and convert them not only into electricity, but also into transportation fuels. In the upcoming years, it will serve as a major technology for complementing the energy needs of the world [4]. Use of advanced technologies such as gas turbines and fuel cells with the syngas generated from gasification results in increased efficiency. For complete combustion of solid fuels, excess air is needed, and high combustion temperatures generate more Knox and other emissions, as compared with the combustion of products by gasification. During combined cycles for combined heat and power generation, contaminants in the syngas such as sulfur and nitrogen species and trace elements are removed efficiently resulting in much lower emissions. Moreover, liquid and gaseous fuels are more of interest because of their ease of handling and operations, and their applications as transportation fuels. High oxygen content in biomass reduces the energy density of the biomass. The production of hydrocarbons, similar to petroleum transportation fuels, requires the removal of oxygen from the carbohydrate structure. The oxygen may be removed in the forms of 〖CO〗_2 and H_2 O. Thermochemical