The life produced by direct combustion process is heat and steam. Despite its apparent simplicity, direct combustion is a complex process from a technological spot of view. High reaction rates and high heat release and many reactants and reaction schemes are involved. Sequential to analyze the combustion process a division is created between the location where the biomass fuel is burned the furnace and the location where the heat from the flue gas is exchanged for a process moderate or life carrier the heat exchanger. The simple process flow diagram for direct combustion is shown within the following picture Figure 3 Principal scheme of direct combustion system Real drafted non-residential biomass combustion facilities can burn all kind of above listed biomass fuel.
In combustion process, volatile hydrocarbons CxHy are formed and burned in a warm combustion zone. Combustion technologies convert biomass fuels into multiple forms of useful life for commercial and or or non-residential uses. In a furnace, the biomass fuel converted via combustion process into heat energy. The heat life is released in shape of warm gases to heat exchanger that switches thermal life from the warm gases to process moderate steam, warm h2o or warm air. The efficiency regarding the furnace is defined as follows: Depending on the wet Little Heating Price LHV of received biomass fuel, typical combustion efficiencies varies within the section of 65% in poorly drafted furnaces up to 99% in high sophisticated, well maintained and perfectly insulated combustion systems.
In lone statement, the combustion efficiency is mainly determined by the completeness regarding the combustion process i. the extent to which the combustible biomass particles are burned and the heat losses from the furnace. Direct combustion processes are of neither fixed bed or fluidized-bed systems. Fixed-bed processes are basically distinguished by variations of grates and the method 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 space enters from below the grate. Within 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 warm bed of granular, noncombustible material, for example sand, limestone, or other. Injection of space into the bed creates turbulence resembling a boiling liquid.
The turbulence distributes and suspends the fuel. This creation increases heat transfer and allows for operating temperatures below 970C, reducing NOx emissions. Depending on the space velocity, a bubbling fluidized bed or circulating fluidized bed is created. The highest many important advantages comparing to fixed bed processes 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; Little NOx emissions; Little excess space factor, below 1. 4, resulting in little heat losses from flue gas.
More factor that determines the system efficiency is the efficiency regarding the heat exchanger, that is defined as follows: Typical heat exchanger efficiencies based on biomass LHV section between 60% and 95%, mainly depending on creation and kind of procedure and maintenance. The first losses are within the warm flue gas exiting from the stack. Within the non-residential practice, the furnace and heat exchanger shape together biomass-fired boiler unit. The boiler is a more adaptable direct combustion technology due to the fact that 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 processes with direct combustion for electrical force generation and co-generation. Such system can achieve an overall efficiency between 30% force generation processes and 85% co-generation systems. 3 cycles are likely for combining electric force generation with process steam production. Steam shall be used in process first and then re-routed through a steam turbine to generate electric power. This arrangement is called a bottoming cycle.
Within the alternate cycle, steam from the boiler passes first through a steam turbine to make 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 shall be applied, delivering steam at compulsory process conditions. Another possibility is a combination of condensing steam turbine with controlled steam extraction facilities. This alternative offers maximum flexibility, i.
during little process steam demand period maximum electric force shall be generated. Up to present time, many biomass fired co-generation force plants have been built worldwide, replacing little efficient heat-only boilers. Biomass gasification is other thermo chemical conversion process utilizing the following primary 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 location at about 850C. Due to the fact that the injected space prevents the ash from melting, steam injection is not always required.
A biomass gasifier can operate below atmospheric compression or elevated pressure. If the fuel gas is generated for combustion within the gas turbine the compression of gasification is always super-atmospheric. The resulting gas product, the syngas, contains combustible gases hydrogen H2 and carbon monoxide CO as the first constituents. By-products are liquids and tars, charcoal and mineral reason ash or slag. Reducing atmosphere regarding the gasification stage means that only 20% to 40% of stochiometric no.
of oxygen O2 related to a done combustion enters the reaction. This is enough to close the heat life compulsory for a done gasification. Speak in other words, the system is autothermic. It creates sensible heat compulsory to done gasification from its own internal resources. Prevailing chemical reactions are listed in Table 2, where the following first 3 gasification stages are described.
Stage I Gasification process starts as autothermal heating regarding the reaction mixture. The compulsory heat for this process is covered by the initial oxidation exothermic reactions by combustion of a component regarding the fuel. Stage II Within the 2nd pyrolysis stage, combustion gases are pyrolyzed by being passed through a bed of fuel at high temperature. Heavier biomass molecules distillate into moderate mass 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 first combustible components of syngas. These reactions, not necessarily related to reduction, occurre at high temperature. Gasification reactions, most important for the final quality heating price of syngas, take location within the reduction zone regarding the gasifier. Heat consumption prevails in this stage and the gas heat shall that is why decrease. Tar is mainly gasified, while char, depending upon the technology used, shall be significantly burned through reactions and reducing the concentration of particulates within the product.
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