Analysis of Bio Mass Energy System Using Direct Combustion
ical power generation and co-generation. Such system can achieve an overall efficiency between 30% (power generation systems) and 85% (co-generation systems). Two cycles are possible for combining electric power generation with process steam production. Steam can be used in process first and then re-routed through a steam turbine to generate electric power. This arrangement is called a bottoming cycle. In the alternate cycle, steam from the boiler passes first through a steam turbine to produce electric power. More efficient co-generation system based on above shown steam cycle is very easy to design. Instead of condensing steam turbine a backpressure steam turbine can be applied, delivering steam at required process conditions. Another possibility is a combination of condensing steam turbine with controlled steam extraction facilities. This alternative offers maximum flexibility, i.e. during low process steam demand period maximum electric power can be generated. Up to the present time, many biomass fired co-generation power plants have been built worldwide, replacing low efficient heat-only boilers. Biomass gasification is other thermo chemical conversion process utilizing the following major feedstock: ò€¢ Wood ò€¢ Agricultural waste ò€¢ Municipal solid waste Chemical process of gasification means the thermal decomposition of hydrocarbons from biomass in a reducing (oxygen-deficient) atmosphere. The process usually takes place at about 850ÒºC. Because the injected air prevents the ash from melting, steam injection is not always required. A biomass gasifier can operate under atmospheric pressure or elevated pressure. If the fuel gas is generated for combustion in the gas turbine the pressure of gasification is always super-atmospheric. The resulting gas product, the syngas, contains combustible gases ò€“ hydrogen (H2) and carbon monoxide (CO) as the main constituents. By-products are liquids and tars, charcoal and mineral matter (ash or slag). Reducing atmosphere of the gasification stage means that only 20% to 40% of stochiometric amount of oxygen (O2) related to a complete combustion enters the reaction. This is enough to cover the heat energy necessary for a complete gasification. Say in other words, the system is autothermic. It creates sensible heat necessary to complete gasification from its own internal resources. Prevailing chemical reactions are listed in Table 2, where the following main three gasification stages are described. Stage I Gasification process starts as autothermal heating of the reaction mixture. The necessary heat for this process is covered by the initial oxidation exothermic reactions by combustion of a part of the fuel . Stage II In the second ò€“ pyrolysis stage, combustion gases are pyrolyzed by being passed through a bed of fuel at high temperature. Heavier biomass molecules distillate into medium weight organic molecules and CO2, through reactions In this stage, tar and char are also produced. Stage III Initial products of combustion, carbon dioxide (CO2) and (H2O) are reconverted by reduction reaction to carbon monoxide (CO), hydrogen (H2) and methane (CH4). These are the main combustible components of syngas. These reactions, not necessarily related to reduction, occurre at high temperature. Gasification reactions , most important for the final quality (heating value) of syngas, take place in the reduction zone of the gasifier. Heat consumption prevails in this stage and the gas temperature will therefore decrease. Tar is mainly gasified, while char, depending upon the technology used, can be significantly "burned" through reactions and reducing the concentration of particulates in the product.