Process for reducing nitric oxide emission during the combustion

Details Process for reducing nitric oxide emission during the combustion Process for reducing nitric oxide emission during the combustion

1991-08-26

1992-07-21

Favors, Edward G.

Furnaces

Process

Treating fuel constituent or combustion product

110212, 110214, 422182, 422183, 431 5, F23J 1100, F23J 1500

Patent

active

051313359

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a process for reducing the nitric oxide emission during the combustion of solid fuels, particularly of hard coals, whereby reducing fuel is added to flue gases leaving a main combustion zone and is burned.
Nitric oxide emission of combustion plants operated with solid fuels may be influenced both by suitable control of the combustion process, so-called primary measures, and by the treatment of the flue gases leaving the combustor, so-called secondary measures.
While the primary measures are intended to reduce the generation of nitric oxides in the combustor, the secondary measures have the goal of removing the generated nitric oxides from the flue gases leaving the combustor.
Known secondary measures are e.g. the catalytic processes for the selective separation of nitric oxides. But these processes are complex and expensive. The disposal of the used or loaded catalysts also causes problems.
Another, comparatively simple, possibility for reducing nitric oxide emission consists of mixing an additional reducing fuel to the flue gases leaving the main combustion zone of a combustor and to burn it in a so-called reduction zone. But this measure by itself is often not sufficient for keeping within the limits of legally prescribed emission limits for nitric oxides so that secondary measures such as e.g. catalytic denitrogenation as a rule cannot be foregone.


SUMMARY OF THE INVENTION

It is therefore the task of this invention to improve this process of the initially described type in such a way that high denitrogenation degrees may be realized in a simple and economical manner.
According to the invention this task is solved in that the flue gases pass consecutively through two reaction zones; that the first reaction zone is operated hypostoichiometrically at temperatures above 1,000.degree. C., and while adding a reducing fuel; and that the second reduction zone is operated hyperstoichiometrically in the presence of nitric oxide-reducing substances at temperatures from 950.degree. C. to 1,000.degree. C. The used nitric oxide-reducing substances are primarily ammonia, ammonia water, carbamide solutions, etc.
The overall denitrogenation degree is improved perceptively by the invented combination of nitric oxide reduction by means of a reducing fuel in a hypostoichiometric reduction zone and nitric oxide-reducing substances in a hyperstoichiometric reduction zone.
The nitric oxide-reducing substances preferably are added, at least in part, together with the reduction fuel to the first reduction zone. This already further increases the nitrix oxide reduction in the first reduction zone, since in the hypostoichiometric atmosphere present there the nitric oxide-reducing substances have an additional reducing effect even at the high temperatures above 1,000.degree. C. The remaining time of the fuel gases in this first reduction zone should preferably be at least 0.1 s.
The additional nitric oxide reduction then takes place in the second reduction zone through the nitric oxide-reducing substances, whereby the hyperstoichiometric parameters however require a temperature range from 950.degree. C. to 1,000.degree. C. The adjustment of the hyperstoichiometric conditions in the second reduction zone is preferably accomplished through addition of an excess volume of combustion air above the requirement necessary for the complete combustion of the reduction volume.
According to another characteristic of the invention, the narrow temperature range which must be maintained for hyperstoichiometric conditions may be kept simply and accurately by controlling the volume and temperature of added combustion air. Since the nitric oxide-reducing substances are already added in the first reduction zone, it is ensured that they pass evenly distributed in the flue gas through the temperature range which must be maintained for nitric oxide reduction.
The invented process is further described using a melting chamber combustor shown as an example in the drawing.


BRIEF DESCRIPTION OF THE

REFERENCES:
patent: 4779545 (1988-10-01), Breen et al.
patent: 5078064 (1992-01-01), Breen et al.

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