关于生物质燃烧技术及气化技术概要英文.docx

1、关于生物质燃烧技术及气化技术概要英文Chapter 5 Major Technologies for the Exploitation of Renewable Energy Sources5.1 Technology characterisationAccording to the degree of maturity, biomass conversion technologies can be classified into three categories:- Traditional technologies- State-of-the-art technologies- Emergi

2、ng technologiesTraditional technologiesTraditional technologies are conventional technologies which are used for a long time without any technological barrier. The major traditional technologies are:Pile burners. Approximately fifty years ago, all common ways of burning wood waste involved some form

3、 of pile burning. The pile burning combustor was typically composed of number of cells. Each of the cells consisted of a lower, refractory lined combustion chamber with a grate floor to support fuel pile and an upper, second combustion chamber. The furnace and the boiler were separated with the furn

4、ace generally located above the secondary combustion chamber. The pile burning boiler designs were simple to design and inexpensive. In pile burning system, wood waste is normally piled as high as ten to twelve feet. Combustion air flow upward through the grates from underneath the pile and inward f

5、rom the cell walls providing oxygen for combustion, cooling air for the grates and promoting turbulence and fuel drying. Most of the fuel will be burned on the grates. Burning of the volatiles will be completed in the secondary combustion zone where overfire air is introduced. Operating temperatures

6、 typically ranged from 1100C to 1400C. Typically, pile burners had a slow response time to demand fluctuations.The combustion process is very difficult to accurately control and the cells had to be shutdown periodically for cleaning. The load and the amount of excess air are difficult to control. Du

7、e to high operating temperatures, the wood ash actually slagged and pooled on top of the grate. The ash is then manually broken up and removed from the furnace.Pile burners are capable of handling wood fuel with a high moisture content with large quantities of dirt and contaminants mixed in with the

8、 fuel. Fuel sizing is less critical with pile burners than with other combustion systems.Relative to other combustion technologies, the efficiency is low. Especially in older designs, boiler efficiency is low, generally 50% to 60%, due to large surface area of the furnace and the absence of radiant

9、air heating. The high operating temperatures will basically be a disadvantage with respect to thermal NOx creation.Combustion on grate. There are three kinds: stationary sloping grate system, travelling grate system, and vibrating grate system.Stationary sloping grate system. The concept of the stat

10、ionary sloping grate was already developed in the late 1920s. Fuel is introduced at the top of the grate and slides down the grate. The fuel burns as it proceeds the bottom. This system is also referred to as semi-pile burning. Characteristic problems with early designs included avalanching of the f

11、uel on the grate and difficulty in controlling both the steam load and the rate of combustion.Travelling grate system. In travelling grate spreader stoker designs, the entire bottoms of the furnace is a slow moving platform or conveyor forming the grate. The grate is cooled by air fed from under the

12、 grate. In this way, the grate mechanism and its cooling system defines the maximum acceptable undergrate air temperature which, correspondingly, defines the moisture content of the fuel that can be burned. Water cooled walls could be used to prevent slag formation adjacent to the stoker. Fuel is fe

13、d from a pneumatic spreader stoker system located on the front of the furnace. Smaller and dryer fuel particles are burned in suspension, while the larger particles fall in a thin layer on the moving grate. The fuel has to burn at a uniform rate and a sufficient speed.Vibrating grate system. The vib

14、rating grate system offers the benefit of spreading the fuel so that small piles that might form on the grate are levelled out. There are less moving parts than with the moving grates and therefore less maintenance is required. Fuel can be mechanically distributed by screw feeders located at the top

15、 of the grate. Recent boilers use water-cooled vibrating grates, allowing the use of high temperature undergrate air and a higher percentage of overfire air. This at its turn enables lower combustion temperatures and therefore better control of NOx formation. Another advantage of lower quantities of

16、 underfire air is lower unburned particle carry-over. Other advantages of the system are:- load control capabilities comparable to those of an oil burner, because of the fact that a large amount of the fuel is burned in suspension; and- possibility to switch to 100% firing of alternative fuels such

17、as oil or gas without any further protection of the grate.Usually, fuel switching capacity of grate systems is limited. Moisture content should typically be kept within about 10% of the design rate. Fluctuations in moisture content outside this range result in significant changes in flue gas flows a

18、nd in heat transfer rates. Fuels with low melting ashes, like many agricultural wastes, are typically kept below approximately 15% (heat input) of the total boiler fuel. The simplicity and flexibility of the grate system makes this design one of the most adaptable units to co-fire solid fuels. The d

19、ifference in bulk density of fuels may create difficulties in using the same spreader or distributor for both fuels.Efficiencies of recent designs range up to about 84% (LHV) for travelling grates and 96% (LHV) for vibrating grates. In the 1980s many of these systems adopted a staged combustion proc

20、ess in order to meet with NOx emissions standards.Examples of combustion on grate systems are presented in Table 21.Table 21. Combustion on grate - examplesParameterUnitValueValueValuePlant sizeMWe2.55034Technology type-inclined movinggratetravelling gratewater cooled vibrating gratePlant-Chia Meng

21、McNeil Plant Mbjergvket CHP Plant4Country-ThailandUSADenmarkStart up year-199719841993TechnologyFuel used (moisture)%rice husk (10)wood (47)straw (16)wood (40)msw1 (23)natural gas (0)Steam data:- temperature- pressure- flowCbarkg/s4203417510926152010035Efficiencies:- boiler (LHV)- turbine (gross)- n

22、et (LHV)%-833930893630CostInvestment costs (1992 US$)$/kWe1550218002900EmissionsEmissions:- NOx - CO - particulatesmg/Mjinmg/Mjinmg/Mjin1503333-741774108130-1 municipal solid waste2 In 1997 US $3 Average emission in ppm unit4 For CHP plants, a theoretical estimation has been made of how much electri

23、city could be generated when there was no heat supplySource: Broek et al., 1995, COGENState-of-the-art technologiesState-of-the-art technologies are the technologies, that can be used at the present time with minimal developmental barriers. The major technologies are:Suspension burning. Suspension f

24、ired boilers resemble pulverised coal boilers in that the combustion occurs while the fuel particles are pneumatically suspended in an air stream. An important attraction of the suspension fired boilers is the reduced furnace size due to drier fuel.Two basic types of suspension burners are available

25、 for use on steam generators, namely cyclonic burners and solid-fuel burners. Cyclonic burners are designed to mix fuel and air in the correct proportion and to complete combustion before swirling particles of fuel reach the end of the refractory chamber. Solid-fuel burners mix the air and fuel toge

26、ther in the correct proportion and ignite the combustible mixture. Burnout of fuel particles is completed in a vertical cylindrical furnace.Elaborate fuel preparation and feeding system are required for suspension firing system. For proper combustion, biomass fuels are required to have a moisture co

27、ntent of less than 15% and a fuel particle size of less than 6mm. If fuels like wood chips or straw are used, it has to be dried and processed through a hammer-mill to reduce the particle size. The presence of dry fine fuel particles creates a potential explosion hazard. Thus, suspension burning fue

28、l handling systems require more careful design than conventional biomass fuel handling systems.The efficiency of a suspension fired boiler is, as high as 80% (HHV). This is partly caused by low excess combustion air required, which results in a better heat transfer because of lower flue gas velocity

29、. NOx emission control can be undertaken in a similar way as with pulverised coal firing. Burners should be adjusted in such a way that temperature peaks within the combustion area are prevented.Atmospheric fluidised bed combustion. Of the different fluidised bed boilers which are used at the moment

30、, most of them are either bubbling (BFB) or circulating fluidised bed (CFB) boilers.In fluidised bed combustion, the primary combustion air from the bottom of the furnace is injected with such high velocity that the material inside the furnace becomes a seething mass of particles and bubbles. This s

31、eething mass consists of both the fuel and of granular inert material. When starting up boiler, this inert material is heated to ignition point of the fuel at which point the fuel is fed from above the bubbling bed. A steady combustion takes place, in which the fuel, because of the fluidity of the s

32、ystem, is rapidly mixed throughout the bed and in which there is a high heat transfer because of the intimate contact of fuel and inert material. This permits combustion to take place with a minimum of excess air and at a low combustion temperature, typically 800-900C, as compared with stoker fired boilers. Another advantage of the fast heat transfer is that the installations have rela