GB2228070A

GB2228070A - Corrosion reduction in fossil fuel furnaces

Info

Publication number: GB2228070A
Application number: GB8830373A
Authority: GB
Inventors: Peter Hadden Stein
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-12-30
Filing date: 1988-12-30
Publication date: 1990-08-15
Anticipated expiration: 2008-12-30
Also published as: GB8830373D0; GB2228070B

Abstract

The ratio of air to fuel in the burners of a front fired furnace burning pulverised coal is reduced, and the remainder of the air required is introduced through nozzles in the rear wall of the furnace, so placed that the air forms a curtain which protects the rear wall. The fuel-lean initial stage of combustion inhibits formation of nitrogen oxides.

Description

METHOD OF NITROGEN OXIDES ( NO > AND FIRE-SIDE CORROSION REDUCTION USING AN AIR CURTAIN.

This-invention relates to an improved method of combustion on front wall firing fosel fuel boilers.

1 INTRODUCTION Nitrogen oxides ( NOx ) are produced as a by-product of the boiler combustion process by the chemical combination of oxygen and nitrogen. The term NOX Refers to all the oxides of nitrogen, from NO to N20, N02 and N204. Nitrogen is present in both the atmosphere (thermal NO-) and in the pulverised fuel (fuel NOx) whilst oxygen is present in the primary and secondary air streams, and in the fuel. The nitrogen oxides produced in the combustion process are transmitted to atmosphere as part of the flue gases emitted by the station.When the nitrogen. oxides combine with atmospheric water vapour, the result, is rain polluted with NOx, forming one of the types of 'acid rain'. The low NOX firing system, by inhibiting the production of nitrogen oxides, aims to reduce the levels of 'acid rain' which may be attributable to the station.

The walls of the furnace sections of the boiler are formed with the evaporator tubes, and the cooling effect of the steam/water mixture limits the temperature at the fire-side metal surface to about 450or. The outer surfaces of scale and ash deposits on these tubes can exceed 1200or as they are subjected to radiant heating from the flame as well as to the hot gases. In some circumstances the flame may impinge directly upon the surface of these tubes. The corrosive action of fire-side deposits on boiler tubes cause external thinning of the tube walls. The consequence of the conjoint corrosion and internal pressure from the steam filled tubes may rupture well before their design lifetime is reached. With the prospect of higher chlorine coals the level of fire-side corrosion will become more significant.

1. 1 Combustion Cycle ( fig. 1 ) The combustion process uses pulverised fuel (PF) which mixes with hot primary air, before being admitted into the boiler furnace via the PF burners. A further supply of air, termed secondary air, which is necessary to advance the combustion process, is admitted into the furnace via the windbox. The coal combustion in the furnace takes place in three stages: coal nitrogen devolatisation ( nitrogen driven off from parent fuel ), combustion of volatiles ( ignition of fuel ) and char combustion ( ignition of residual material left by the removal of the coal volatiles ). It is during the latter two stages of combustion that NOx formation takes place, with the distribution of nitrogen between volatiles and char a major factor of NOX formation.The amount of oxygen and nitrogen within the fuel is another deciding factor which influences the level of NOX formation.

2 METHOD OF NO REDUCTION ( fig. 2 ) Research has shown that to reduce the levels of NOM formation, the combustion process must be carried out in two stages : 'fuel rich' operation and a 'fuel lean' operation. The 'fuel rich' stage is achieved by reducing the amount of oxygen available during the early part of the combustion cycle and introducing the balance of oxygen as tertiary air.

The 'fuel lean' stage is achieved by diverting part of the secondary air flow to the otherside of the furnace away from the fuel rich centre. As NOX formation increases with combustion temperature, the low NOX firing system allows combustion to generally take place at a lower temperature than a conventional system.

2. 1 Reduction of Oxygen Level To reduce the level of oxygen available during the eariy part of combustion, it is necessary to decrease the amount of secondary air which flows through the fuel burners. Reducing the air flow increases the fuel to air ratio causing a decrease in the amount of fuel nitrogen which is converted to nitric oxide. Operating the first stage at a high temperature devolatises the coal nitrogen in the fuel rich environment, as any nitrogen retained in the char may be converted to NOX during the latter stages of combust ion.

Typically the air register, through which the burner passes, is cylindrical in shape and there are eight swivel type doors equally spaced around its circumference. The doors all moving together, regulate the air from the windbox to the burner and movement is obtained from a single shaft via cranks and links. Air control doors are set manually and each group of three burners is supplied from an individual windbox. Each windbox has an automatically controlled damper.

To restrict the secondary air flow through the fuel burners the swivel type doors should be adjusted. It is anticipated that a reduction in secondary air flow of approximately 25% to 35% would be required.

Diversion of the secondary air causes a reduction in the formation of NOX by introducing a 'fuel lean' stage into the combustion process. The 'fuel lean' stage is preferred at a low temperature with the amount of excess air kept to a minimum thus inhibiting the formation of NOx 2.3 Tertiary Air Tertiary air is admitted into the furnace by rows of nozzles installed in the rear wall. The nozzles shall supply the balance of secondary air not admitted by the PF burners. It is injected up the rear wall of the furnace and partially into the flame in order to create a degree of turbulence. The injection of tertiary air completes the combustion process and reduces the flame temperature in the main combustion area, thus limiting the production of NOx.

This proces will introduce an air curtain up the rear wall acting as a protective barrier preventing flame impingement upon the rear wall tubes and restricting ash build up on them.

2.4 Benefits of Rear Wall Curtain Low NOx System The low NOx system has the following beneficial effects.

(1) An estimated reduction in NOx levels of approximately 25% (2) A lowering of the sensible heat losses in the flue gases ( caused by lowering the amount of excess air ) (3) No adverse effects upon unit performance (4) No increase in the emission levels of other pollutants such as carbon or hydrocarbons (5) A reduction in fireside corrosion of the water-walls which is caused by the removal of the protective 'oxide skins' from the boiler tubes (6) Flame impingement directly upon the surface of the tubes will be eliminated.

(7) Reduces the requirement of rear wall blowers.

(8) Reduces maintenance costs.

Claims

A CATM -

I Nitrogen oxides and fire-side corrosion reduction using an air curtain comprising of the installation air nozzles in the rear wall.

2 Nitrogen oxides and fire-side corrosion reduction as claimed in claim 1 wherein the nozzles provide a means of directing the air.

3 Nitrogen oxides and fire-side corrosion reduction using an air curtain as claimed in claim 1 or claim 2 provides a means of preventing flame impingement directly upon the rear wall tubes.

4 Nitrogen oxides and fire-side corrosion reduction using an air curtain as claimed in claim 1 or claim 2 will reduce NOx levels 5 Nitrogen oxides and fire-side corrosion reduction using an air curtain as claimed in claim 1 or claim 2 will produce a reduction in fireside corrosion on the furnace rear wall.

Amendments to the claims have been filed as follows 1. A method of burning fuel in a furnace, the method comprising introducing the fuel and secondary combustion air through one side of the furnace and introducing tertiary combustion air through an opposite side of the furnace.
2. A method according to Claim 1, in which the tertiary air is introduced so as to form an air curtain against said opposite side.
3. A method according to Claim 1 or 2, in which the fuel is pulverised prior to introduction into the furnace.
4. A method according to any of Claims 1 to 3, in which primary combustion air is introduced into the furnace in direct contact with the fuel.
5. A method according to Claim 4, in which the primary air is heated prior to introduction into the furnace.
6. A method according to any of the preceding Claims, in which the secondary and tertiary air is heated prior to introduction into the furnace.
7. A method according to any of the previous Claims, in which the ratio of secondary to tertiary air is between 3:1 and 13:7.
8. A method according to any of the preceding Claims, in which the tertiary air is partially injected into the flame in the furnace.
9. A furnace comprising means in a front wall for introducing fuel and secondary combustion air, and means in a rear wall for introducing tertiary combustion air.
10. A furnace according to Claim 9, in which the means for introducing tertiary air comprises a plurality of nozzles aligned to inject the tertiary air across the rear wall of the furnace and partially into the flame.
11. A furnace according to Claim 9 or 10, in which means are provided for introducing primary air into the furnace in direct contact with the fuel.
12. A method of burning fuel in a furnace substantially as hereinbefore described with reference to the accompanying drawing.
13. A furnace substantially as hereinbefore described with reference to the accompanying drawing.