Circulating fluidized bed combustion is a new highly efficient, low emission and coal cleaned technology, its main feature is that the boiler furnace contains a lot of material, which is carried to the upper chamber in the combustion ,through the separator arranged in furnace outlet, the material and gas separated, and sent back to the bed material through the non-mechanical valve, combusting circulately for several times. For the high concentrations of the material ,it has great heat capacity and good mix of materials, usually per kilogram of gas can carry kilograms of materials, which bring high heat transfer coefficient, so that there is a wide range of heat load regulation for boiler and wide fuel adaptability. Circulating fluidized bed boiler uses higher velocity than the bubbling fluidized bed, there isn’t apparent bed as a bubbling bed ,for there is strong turbulence and bed material circulation in furnace, strengthen the mix of fuel and extend the residence time of the fuel in furnace, thus there is higher combustion efficiency than the bubbling bed, can operate stably under low load, without adding auxiliary fuel.
The running temperature of circulating fluidized bed boiler is usually between 850 ~ 900 ℃, which is an ideal desulfurization temperature range, using furnace desulfurization technology, adding desulfurization agent (such as limestone) to the bed, fuel and sorbent is recycled, conducted low-temperature combustion and desulfurization reaction, coupled with intense turbulence furnace, Ca / S molar ratio is about 2.2, the desulfurization efficiency can reach about 90%, and SO2 emissions is greatly reduced. While low-temperature and class air supply is used for combustion of circulating fluidized bed, making combustion in low excessive air, thus greatly reducing generation and emissions of NOX. Circulating fluidized bed boiler also has a high combustion efficiency, can be fueled with low-grade fuel, boiler load regulation is good, ash is easily used, etc., so it has been developed rapidly in the world. With strict requirements of environment, generally agreed that the circulating fluidized bed boiler is the most practical and feasible and efficient low-pollution coal-fired equipment.
As the specificity of composition of biomass fuels, compared with coal, its distribution character of composition is《three higher ,three lower》, that is volatile, oxygen and hydrogen content is high, the first two of which is much higher than coal; and carbon , sulfur and ash content is relatively low. And SiO2 and K2O of ash content is significantly higher. Thus, selection for design of biomass combustion equipment and operation mode should be on basis of combustion characteristics of different types of biomass, to ensure economic operation and reliability of biomass combustion equipment ,to improve the efficiency of biomass utilization.
The circulating fluidized bed boiler combustion technology is used by us, we have the experience of producing circulating fluidized bed boiler for many years, combing with some advanced operating data, while relying on clean-burning technology theory in Tsinghua University. Draw on the experience of biomass boiler’s running and made by optimizing design. We used advanced low-temperature combustion technology for boiler, put biomass fuels and mineral fuels pretreated into combustion, which not only has reduced operating costs, improved combustion efficiency, while reduced SOx, NOx, dioxins and other harmful gases concentrations, but also increased the use efficiency of biomass. Especially reduced the effect of alkali metal, sulfur and chlorine to the boiler heating surface, reduced corrosion and wear on equipment, smoke pollution to the environment. With high combustion efficiency and low pollution features which is adaptable for the relative concentration and large-scale use of biomass resources. The fluidized combustion technology achieved energy saving, environmental protection and comprehensive utilization of ash, also applied to other biomass waste recycle, which product can save energy efficiently and protect environment.
2 Introduction to the overall boiler
Our boiler is second high-temperature and second high-pressure boiler, with single drum transverse type, single-chamber, natural circulation, the whole suspension structure, all steel π-type arrangement. Boiler is equipped with a light roof, and we set the operation level elevation at 8m.
The boiler is mainly composed of furnace, insulating cyclone separator, self-balancing back valve and tail convecting flue duct. All-membrane water-wall structure is used for furnace, the middle of the boiler is the adiabatic cyclone, flue tail shaft arranged three groups of two stage convecting superheater, under which it is arranged three sets of tubes economizer and primary ,secondary air and three groups of air preheater.
In the combustion system, the coal stoker drops coal through coal pipe into the furnace, air required by combustion is provided by primary and secondary fans seperately. The air sent by primary fan through the primary air preheater into the water rooms from right and left duct, through the hood on water-cooled plate of air ditribution into the combustion chamber; The air sent by secondary fan through the secondary air preheater and the coal dropping tube distributed on the front wall as well as the secondary air nozzle on the back wall is injected into the furnace, add air to enhance disturbance and mixing. The mixture of fuel and air in the furnace combusts in fluidized state and exchanges heat with the heating surface. Furnace flue gas (carry large amounts of unburned carbon particles) in the upper chamber for further combustion heat release. Afte a large number of materials entrained by the flue gas through the furnace exit into the insulating cyclone, the vast majority of the material was separated through refeeder return into the furnace, to achieve the cycle combustion. After the flue gas separated through the turning chamber, high temperature superheater, low temperature superheater, economizer, the primary and secondary air preheater,exhausted from tail flue duct.
Boiler feed water through three groups of tube economizer arranged horizontally,heated ,then through four tubes of φ89 and twelve tubes ofφ57 into the drum. Drum water from the concentrate down pipe, distribution pipe into the low headerof water wall, water-wall tube, the upper header, and then by the guiding tube into the steam drum. Drum is equipped with steam-water separator unit.
The saturated steam qualified which is leaded from the steam connecting pipe at the top of drum into header imports of low-temperature superheater, passing the two stage low-temperature superheater, primary spray desuperheater, high-temperature superheater, steam collection box, Finally, the superheated steam is out.
As the specificity of biomass fuels composition, it is generally quite different from coal-fired boilers. Due to characteristics of fuel for Biomass boiler, use the following special measures to meet needs of the biomass combustion and safe operation of the boiler.
1)、Use the tall structure and lower flue gas flow rate of chamber furnace. There is more space and adequate furnace height in funace, so that there is sufficient burning time for fuel in the furnace, while arrange hierarchically sufficient secondary air within a certain height in the furnace to make a lot of volatile combustion focus on the release of combustion, to ensure fuel and air mixture combust adequately, to make the inflammable matter burn off . Greatly improved the utilization of biomass fuels.
2)、 Due to biomass fuels contain alkali metals, sulfur and chlorine and other elements, the combustion-generated derivatives together with the alkali metal salts caused high temperature corrosion to the superheater. Therefore, when design, to take steps to try to enhance the effect of heat transfer within the furnace, so that imports of high-temperature superheater flue gas temperature is at a low level, and make flying ash harden ahead of time; while employ the higher flue gas flow rate in high temperature superheater area with a reasonable blowing device, essentially to avoid the high temperature superheater fouling, eliminating one of the incentives of high temperature corrosion .
3)、while on the use of materials ,high-temperature superheater with niobium-containing austenitic stainless steel TP347-type heat pipe of good creep resistant performence and oxidation resistance of, to ensure the safe operation of high-temperature superheater.
3 basic characteristics of the boiler
Boiler is suitable for indoor or semi-open layout. Employ a fixed way that a combination for hanging in the front and supporting at the back , the boiler operating floor elevation is 8m. Boiler uses single drum, natural circulation, concentrated down tube, balanced ventilation, adiabatic gas-solid cyclone separator, circulating fluidized bed combustion, convecting heating surface is arranged in convecting shaft flue duct.
3.1. the main working parameter
the rated evaporating capacity 110 t/h
the rated steam preesure 6.5 MPa
the rated steam temperature 485 ℃
feed water temperature 130 ℃
fume exhaust temperature for boiler 135 ℃
blow down ratio ≤2 %
cool air temperature 30 ℃
hot air temperature 144 ℃
the design thermal efficiency for boiler 88.08 %
(Ca/S摩尔比约为2.2)desulphurizing efficiency ≥85%
the mixed fuel consumption 23136.1 kg/h
the coal consumption 16195.27 kg/h
the rice husk consumption 6940.83 kg/h
3.2. Design fuel
3.2.1 Design coal quality is mixture coal for the user, namely: 70% coal (Coal) 30% rice husk (Rice Husk)
3.2.2 The use of fuel: low calorific value is greater than 14.65MJ/Kg coal fuel or coal mixed with biomass fuel.