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 970°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: • Flexibility to changes in biomass fuel properties, sizes and shapes; • Acceptance of biomass fuel moisture content up to 60%; • Can handle high-ash fuels and agricultural biomass residue (>50%); • Compact construction with high heat exchange and reaction rates; • Low NOx emissions; • 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 electrical 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: • Wood • Agricultural waste • 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 850ºC. 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 – 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 – 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.