Method of optimizing combustion and the capture of pollutants during coal combustion in a cyclone combustor

• US 4,765,258 A
• Filed: 02/26/1986
• Issued: 08/23/1988
• Est. Priority Date: 05/21/1984
• Status: Expired due to Term

• Current Assignee: Coal Tech Incorporated
• Original Assignee: Coal Tech Incorporated
• Inventors: Zauderer, Bert
• Primary Examiner(s): Yuen, Henry C.
• Application Number:
  US06/834,143
• Time in Patent Office: 909 Days
• Field of Search: 110/245, 110/347, 110/260-265, 432/210
• US Class Current: 110/347
• CPC Class Codes: F23C 3/008 for pulverulent fuel
• CPC Class Codes: F23J 7/00 Arrangement of devices for supplying chemicals to fire

Abstract

Coal combustion and the capture of pollutants are optimized by a method which applies two mechanisms for sulphur capture, one in which pulverized coal particles suspended in the gas stream in the injection zone of the combustor are affected by reaction with a suspended sorbent, and another in which the particles are reentrained in the gas stream by a "sand storm" effect near the wall of the combustor. Use of the two mechanisms results, in commercial scale cyclone combustors, in 70 to 90% sulphur capture at economical Ca/S ratios. The method also minimizes emission of ash by removal from the pulverized coal fuel particles too small to be retained in the combustor and too large to be completely burned in the combustor, minimizes reevolution of sulphur compounds from slag by rapid and continuous removal of slag from the combustor, minimizes emission of NO_x pollutants by maintaining a favorable overall fuel-rich stoichiometry.

Claims

1. A method for the combustion of coal in a slagging cyclone combustor while minimizing the emission of ash particles and other pollutants, comprising the steps of:

pulverizing a quantity of coal;

removing from the pulverized coal particles too large to readily be burned in the combustor;

injecting the remaining pulverized coal particles into the combustor adjacent to a closed end wall of the combustor together with a stream of primary combustion air so as to form an air-fuel stream;

injecting into the combustor adjacent to the closed end wall a stream of secondary air in such a manner as to cause said air-fuel stream to flow helically within the combustor;

maintaining the velocity of said secondary air stream such that centrifugal force on said coal particles due to tangential gas velocity drives said particles toward the cylindrical wall of the combustor so that combustion of said coal particles is apportioned between and occurs both in the gas stream and on the walls of the combustor with most of said combustion occurring in the gas stream;

injecting into the combustor adjacent to the location at which the fuel-air stream is injected a pulverized sorbent capable of capturing sulphur compounds, whereby the sorbent being injected into a region within the combustor in which the local gas conditions are oxidizing and the temperature is lower than the average temperature of the combustor, the average stoichiometry of said combustor being reducing;

maintaining a liquid slag layer on the cylindrical wall of the combustor, the sulphur captured by the sorbent being removed from the cyclone as a result of the impingement and retention thereof on the slag layer, and the further steps of controlling by air cooling the temperature of the slag layer, and removing slag from the combustor in a time less than the time required for evolution of sulphur gas from the slag.

2. A method in accordance with claim 1, wherein said air-fuel stream is injected in an annular configuration, spaced from the central axis of the combustor, and the secondary air stream is injected between said air-fuel stream and the cylindrical wall of the combustor.

3. A method in accordance with claim 1, wherein said secondary air is regeneratively preheated by heat exchange from the combustor.

4. A method in accordance with claim 1, wherein the combustor is operated at overall fuel rich conditions of approximately 50 to 70 percent of stoichiometric conditions, so as to reduce NO_x emissions.

5. A method in accordance with claim 1, and the further step of injecting into the combustor coal particles smaller than about 10-20 microns, said air-fuel stream, said secondary air stream and said coal particles smaller than about 10-20 microns being injected into the combustor through a closed end wall thereof and near the cylindrical wall of the combustor.

6. A method in accordance with claim 1, and the further step of maintaining the tangential and axial velocities of the gases in the combustor such that unburned coal particles deposited on slag-covered walls of the combustor are reentrained, so that combustion of said particles continues.

7. A method in accordance with claim 1, wherein substantially all of said coal is burned in a region adjacent to the walls of the combustor.

8. A method in accordance with claim 1, wherein the temperature of the sorbent while the sorbent is in the sorbent-carrying air stream is less than 2000°

F., said sorbent being a compound of a group comprising limestone, dolomite and calcium hydroxide, sulphur capture proceeding in said region by the steps of calcination of the sorbent and combination of the calcined sorbent particles with compounds of sulphur.

9. A method in accordance with claim 8, wherein the sorbent particles are in suspension in said oxidizing region for less than about 0.1 seconds, sulphur capture taking place within said oxidizing region and said time.

10. A method in accordance with claim 1, and the further steps of maintaining the tangential and axial velocities of the gases in the combustor such that sorbent particles and unburned coal particles deposited on slag-covered walls of the combustor are reentrained so that combustion of coal particles continues, and sulphur capture also continues under oxidizing or reducing gas conditions until the temperatures of the sorbent particles exceeds about 2000°

F.

11. A method for the combustion of coal in a slagging cyclone combustor while minimizing the emission of ash particles and other pollutants, comprising the steps of:

pulverizing a quantity of coal;

removing from the pulverized coal particles too fine to be retained in the combustor and too large to readily be burned in the combustor;

injecting the remaining pulverized coal into the combustor adjacent to a closed end wall of the combustor together with a stream of primary combustion air to form an air-fuel steam;

injecting into the combustor adjacent to the closed end wall a stream of secondary air in such a manner as to cause said air fuel stream to flow helically within the combustor with said secondary air stream surrounding said air-fuel stream so that centrifugal force on said coal particles drives said particles toward the cylindrical wall of the combustor;

maintaining the velocity of said air-fuel stream such that gasification of the coal particles is apportioned between and occurs both in the gas stream and on the walls of the combustion chamber;

gasifying most of the injected coal while the coal is suspended in the gas stream, said step of gasifying in the gas stream being performed under fuel rich conditions of about 50 to 70% stoichiometric conditions; and

gasifying substantially all of the remainder of the coal on the wall of the combustor so that the ash is retained in slag on the wall of the combustor; and

the further step of removing the slag from the combustor, said step of removing being performed in a time less than the time required for the revolution of sulphur gas from the slag.

12. A method in accordance with claim 11, and the further steps of maintaining the tangential and axial velocities of the gases in the combustor such that unburned particles deposited on slag-covered walls of the combustor are reentrained in the gases, so that combustion of said particles continues.

13. A method in accordance with claim 11, wherein a slag layer is maintained on the cylindrical wall of the combustor, and the temperature of the slag layer is controlled by air cooling said cylindrical wall.

14. A method in accordance with claim 11, and the further steps of injecting into the combustor adjacent to the air-fuel stream a pulverized sorbent, the temperature of the sorbent while the sorbent is in the sorbent-carrying air stream being less than 2000°

F., said sorbent being a compound of a group comprising limestone, dolomite and calcium hydroxide, sulphur capture proceeding in said region by the steps of calcination of the sorbent and combination of the calcined sorbent particles with compounds of sulphur, and a slag layer being maintained on the walls of the combustor, the sulphur gas captured by the calcined sorbent being removed from the cyclone as a result of the impingement and retention thereof on the liquid slag layer.