Oxygen enhanced combustion of biomass
a biomass and oxygen-enhancing technology, applied in the field of biomass combustion, can solve the problems of increased moisture content, limited flue gas, and low energy density, and achieve the effects of reducing the flow rate of combustion air, and increasing the fuel combustion ra
Inactive Publication Date: 2014-11-20
PRAXAIR TECH INC
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Benefits of technology
[0012]Another aspect of the invention is a method of increasing fuel combustion rate in a combustion chamber with a convective heat transfer zone in which fuel that contains biomass is combusted with combustion air in said combustion chamber to produce flue gas containing a specific oxygen concentration between 3 vol. % and 8 vol. % at a given maximum fuel feed rate limited by the capacity of an FD fan if present for feeding said combustion air, the capacity of an ID fan if present to evacuate flue gas from said combustion chamber, the flue gas velocity in said convective heat transfer zone, or the carbon monoxide concentration in said flue gas, feeding into said combustion chamber additional fuel containing biomass and additional oxidant containing at least 50 vol. % O2, reducing said combustion air flow rate by the amount that reduces said oxygen concentration in said flue gas by 0.1 to 5.0 vol. % and combusting said additional fuel without exceeding said FD fan capacity, said ID fan capacity, said flue gas velocity, nor said carbon monoxide concentration.
[0013]Yet another aspect of the invention is a method of increasing fuel combustion rate in a combustion chamber with a grate for combustion of fuel with a convective heat transfer zone in which fuel that contains biomass is combusted with combustion air in said combustion chamber to produce flue gas containing a specific oxygen concentration between 3 vol. % and 8 vol. % at a given maximum fuel feed rate limited by the carbon monoxide concentration in said flue gas, feeding into said combustion chamber additional fuel containing biomass and additional oxidant containing at least 50 vol. % O2 to one or more oxygen deficient areas on said grate to maintain or reduce said carbon monoxide.
Problems solved by technology
This increased moisture content, and its low energy density, are among the primary issues with firing biomass in boilers and especially boilers that were designed for other fuels such as coal.
Many boilers are ‘flue gas limited’ and can only handle up to a specific amount of flue gas.
This flue gas limitation may be due to the capacity of fans if present for impelling flow of flue gas, or may be based on design limits.
This increase in heat transfer in the convective section often requires the use of water sprays into the steam flow to maintain the desired steam temperature, which may decrease overall efficiency.
This shifting of heat transfer from the radiative furnace section to the convective section of the boiler further reduces or derates the boiler capacity.
Conversion of an existing boiler to biomass firing can also significantly degrade the combustion performance of the unit.
The reduction in combustion performance is due to both changes in the fuel characteristics and the firing system.
The high moisture content the fuel makes it more difficult to ignite and burn.
This problem is compounded by the fact that grate firing systems often suffer from uneven fuel distribution over the grate and non-uniform mixing between the air and the fuel—leading to incomplete combustion on parts of the grate and high CO emissions in flue gas.
The extra air further increases the flue gas volume and impacts both the thermal efficiency of the boiler, and the auxiliary power required for the boiler.
In the first case the extra air volume carries heat out the stack, increasing the sensible heat loss.
Therefore the overall effect of the excess air is to increase the specific flue gas volume, which is the gas volume per units of energy output (further limiting the amount of fuel that can be fired), reduce the thermal efficiency (allowing less of the fuel that is fired to be used to raise steam), and increase the auxiliary power (reducing the net power available
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[0015]The present invention is an improvement in the combustion of fuel comprising biomass in a combustion chamber. “Biomass,” for the purposes of the present invention, means any material not derived from fossil resources and comprising at least carbon, hydrogen, and oxygen. Biomass includes, for example, plant and plant-derived material, vegetation, agricultural waste, forestry waste, wood, wood waste, paper waste, animal-derived waste, poultry-derived waste, and municipal solid waste. Other exemplary feedstocks include cellulose, hydrocarbons, carbohydrates or derivates thereof, and charcoal. Typically biomass can include one or more materials selected from: timber harvesting residues, softwood chips, hardwood chips, tree branches, tree stumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover, wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus, animal manure, municipal garbage, municipal sewage, commercial waste, grape pumice, almond shells, pecan sh...
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The energy output of a power plant combustion chamber that combusts fuel comprising biomass as all or part of the fuel can be increased by feeding oxygen into the combustion chamber so that said fuel is in contact with gaseous oxidant whose oxygen content exceeds that of air by up to 5 vol. % above that of air.
Description
RELATED APPLICATION[0001]This application is a continuation application of and claims priority from U.S. patent application Ser. Number 13 / 285,654 filed Oct. 31, 2011, which claims priority from U.S. Provisional Application Ser. No. 61 / 412,119 filed Nov. 10, 2010.FIELD OF THE INVENTION[0002]The present invention relates to combustion of biomass, especially in power plants that generate steam.BACKGROUND OF THE INVENTION[0003]Growing demand for electrical power obtained from fuel-fired power plants, combined with growing interest in using biomass as fuel for such plants, has increased interest in finding efficient methods for combusting biomass in power plants. The moisture content of biomass is typically very high. For example green wood typically contains 40 to 60% moisture. This increased moisture content, and its low energy density, are among the primary issues with firing biomass in boilers and especially boilers that were designed for other fuels such as coal. For example, conve...
Claims
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IPC IPC(8): F23G5/44F23L7/00
CPCF23G5/44F23G2207/30F23L7/007Y02E20/12F23L2900/07007F23G2206/203F23G5/00F23G2209/26F22B1/18Y02E20/344F23G2900/7012F23G2900/7003F23G7/10Y02E20/34Y02E50/10F23G5/442
InventorKOBAYASHI, HISASHIBOOL, III, LAWRENCE E.
OwnerPRAXAIR TECH INC