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.

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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