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Analysis of Bio Mass Energy System Using Direct Combustion

The energy produced by direct combustion process is heat and steam. Despite its apparent simplicity, direct combustion is a complex process from a technological point of view. High reaction rates and high heat release and many reactants and reaction schemes are involved. In order to analyze the combustion process a division is made between the place where the biomass fuel is burned (the furnace) and the place where the heat from the flue gas is exchanged for a process medium or energy carrier (the heat exchanger). The basic process flow diagram for direct combustion is shown in the following picture Figure 2 Principal scheme of direct combustion system Proper designed industrial biomass combustion facilities can burn all type of above listed biomass fuel. In combustion process, volatile hydrocarbons (CxHy) are formed and burned in a hot combustion zone. Combustion technologies convert biomass fuels into several forms of useful energy for commercial and/or industrial uses. In a furnace, the biomass fuel converted via combustion process into heat energy. The heat energy is released in form of hot gases to heat exchanger that switches thermal energy from the hot gases to the process medium (steam, hot water or hot air). The efficiency of the furnace is defined as follows: Depending on the wet Low Heating Value (LHV) of received biomass fuel, typical combustion efficiencies varies in the range of 65% in poorly designed furnaces up to 99% in high sophisticated, well maintained and perfectly insulated combustion systems. In single statement, the combustion efficiency is mainly determined by the completeness of the combustion process (i.e. the extent to which the combustible biomass particles are burned) and the heat losses from the furnace. Direct combustion systems are of either fixed bed or fluidized-bed systems. Fixed-bed systems are basically distinguished by types of grates and the way the biomass fuel is supplied to or transported through the furnace. In stationary or travelling grate combustor, a manual or automatic feeder distributes the fuel onto a grate, where the fuel burns. Combustion air enters from below the grate. In the stationary grate design, ashes fall into a pit for collection. In contrast, a travelling grate system has a moving grate that drops the ash into a hopper. Fluidized-Bed Combustors (FBC) burn biomass fuel in a hot bed of granular, noncombustible material, such as sand, limestone, or other. Injection of air into the bed creates turbulence resembling a boiling liquid. The turbulence distributes and suspends the fuel. This design increases heat transfer and allows for operating temperatures below 970Т°C, reducing NOx emissions. Depending on the air velocity, a bubbling fluidized bed or circulating fluidized bed is created. The most important advantages (comparing to fixed bed systems) of fluidized-bed combustion system are: тЂў Flexibility to changes in biomass fuel properties, sizes and shapes; тЂў Acceptance of biomass fuel moisture content up to 60%; тЂў Can handle high-ash fuels and agricultural biomass residue (>50%); тЂў Compact construction with high heat exchange and reaction rates; тЂў Low NOx emissions; тЂў Low excess air factor, below 1.2 to 1.4, resulting in low heat losses from flue gas. Additional factor that determines the system efficiency is the efficiency of the heat exchanger, which is defined as follows: Typical heat exchanger efficiencies based on biomass LHV range between 60% and 95%, mainly depending on design and kind of operation and maintenance. The main losses are in the hot flue gas exiting from the stack. In the industrial practice, the furnace and heat exchanger form together biomass-fired boiler unit. The boiler is a more adaptable direct combustion technology because the boiler transfers the heat of combustion directly into the process medium. Overall boiler efficiency is defined as follows: ?BOILER = ?COMBUSTION x ?HEAT EXCHANGER Overall boiler efficiency varies between 50% and 95%. Very common and most efficient are biomass systems with direct combustion for electr

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