Method For Reducing Nitrogen Oxide Emissions of a Bubbling Fluidized Bed Boiler and an Air Distribution System of a Bubbling Fluidized Bed Boiler

Abstract

A method for reducing nitrogen oxide emissions of a bubbling fluidized bed boiler burning biofuel and an air distribution system for a bubbling fluidized bed boiler biofuel. A fluidized bed is arranged in a lower part of a furnace of the boiler. The bed is fluidized by fluidizing gas that includes primary air. Fuel is fed to the fluidized bed, which dries and pyrolizes into pyrolysis gas including volatile matter of the fuel. The gas rises upwards in the furnace and burns. Secondary air is supplied above the fluidized bed from secondary air nozzles, and tertiary air is supplied above the secondary air nozzles. A part of primary air is supplied in connection with fuel feeding such that the fuel is forced substantially on the surface of the fluidized bed, thus pyrolizing entirely, and at least a part of the pyrolysis gases formed in the pyrolysis is burned by primary air such that the air coefficient in relation to the volatile matter of fuel in the pyrolysis gases is in the substoichiometric area.

Description

FIELD OF THE INVENTION[0001]The invention relates to a method for reducing nitrogen oxide emissions of a bubbling fluidized bed boiler burning biofuel according to the preamble of the appended claim 1. The invention also relates to an air distribution system of a biofuel-burning bubbling fluidized bed boiler in accordance with the preamble of the appended claim 13.[0002]In this description biofuel refers to solid fuels, wherein the portion of volatile matter in ash-free dry solids is over 60%. This type of fuels are, for example, peat, bark, wood chips, sawdust, waste construction timber, sludge created in process industry, and municipal solid waste.BACKGROUND OF THE INVENTION[0003]Bubbling fluidized bed boilers are generally used in energy production, wherein the fuels include biofuels, such as, for example, peat and wood chips. In the lower part of the furnace of a bubbling fluidized bed boiler there is a fluidized bed, which is composed of a fine, incombustible material, typicall...

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Benefits of technology

[0011]The purpose of the present invention is to provide a method for reducing nitrogen oxide emissions of a bubbling fluidized bed boiler burning biofuel, by means of which the above-mentioned drawbacks can be avoided. In addition, the purpose of the invention is to create an air distribution system of a bubbling fluidized bed boiler.

[0015]The invention is based on the idea that the nitrogen oxide emissions of a bubbling fluidized bed boiler are reduced by using a staged air supply in such a manner that a part of primary air is supplied in connection with fuel supply, i.e. with the fuel or within the immediate vicinity of the fuel supply point in the same direction as the fuel itself. This part of primary air is in this application referred to as the combustion air of volatile matter. Thus, substantially all the fuel fed to the furnace is forced onto the surface of the fluidized bed for mixing it to the fluidized bed and for drying it quickly due to the effect of the hot fluidized bed material. The pyrolysis following the drying and the combustion of the volatile matter released from the fuel in the pyrolysis also takes place almost immediately after the fuel has mixed with the fluidized bed, because the fuel and the oxygen in the air supplied in connection with it are mixed quickly. Due to the quick mixing of the fuel and the oxygen, most of the volatile matter released from the fuel can be burnt in the upper part of the fluidized bed and on the fluidized bed, before the supply of secondary air. The combustion of volatile matter creates a high temperature, which maximizes the creation of hydrocarbon radicals formed of the fuel and promotes the reduction of the released nitrogen oxides.

[0018]Thus, in the method according to the invention the combustion air amounts supplied in different stages of the staged combustion are thus adjusted, i.e. the total air coefficient SRtot needed for combustion and further the air coefficient in relation to the volatile matter SRv. This is illustrated later in tables 1 and 2 of this description. Feeding the fuel with air to the fluidized bed in such a manner that substantially all the fuel particles are forced there enables controlling the combustion substantially better than at present. Further, the amount of unburnt fuel can be minimized, because the delay time of fuel in the furnace is longer than in the solutions according to prior art.

[0022]The fact that the fuel particles are forced by means of air to the fluidized bed also reduces the fouling of heat exchange surfaces of the furnace. That is, the fuel particles escaping from the fuel supply and burning in the upper parts of the furnace raise the temperature of the fuel gases before they reach the heat surfaces and increase the fouling caused by ash melting. Fouling of the heat exchange surfaces diminishes the efficiency of the boiler and causes economical losses through boiler shutdowns required by the possible cleaning. By means of the invention fouling is minimized and through that, economic advantage is gained.

Problems solved by technology

Still problematic are the fine and light fuels, because most of the fuel particles do not end up in the fluidized bed, but they are pulled with the fluidizing gas and secondary air to the upper parts of the furnace.

Thus, it is almost impossible for the fuels to create fuel combustion conditions in the furnace that are controlled and favourable to reducing nitrogen oxides.

However, this method is not efficient enough for getting the nitrogen oxide emissions of the boiler in accordance with the strict emission standards.

The problem with this solution is that it causes the amount of flue gases in the boiler to increase, in which case the size of the furnace must be increased, which in turn raises the price of the boiler.

With wet fuels the amount of heat needed for drying reduces the combustion temperature in the secondary stage too much, thus preventing the creation of conditions favourable for reducing the nitrogen oxides.

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