METHOD OF OPERATING A SYSTEM FOR BURNING PULVERISED FUEL

Abstract: Enhanced combustion of pulverised fuel in an oxy-fuel burner (10) is achieved by replacing the conventional pulverised fuel feed with a dense phase material flow transport system. This enables materials which would hitherto have been discarded as waste to be used as a viable energy source. In addition to greatly enhancing combustion the swirlers conventionally associated with the oxy-fuel burner (10) may be omitted.

Description: This invention relates to a method of operating a system for burning pulverised fuel and, more particularly but not exclusively, is concerned with a method of operating a system for burning pulverised coal. It has been proposed to burn pulverised fuel in an oxy-fuel burner. However, even with the use of pure oxygen it is difficult to obtain satisfactory combustion with certain fuels. In such cases the flame tends to be a relatively low temperature luminant flame and the ash may contain an appreciable quantity of uncombusted carbon. GB-A-2146758 discloses a method of operating a system for burning pulverised fuel, which system comprises an oxy-fuel burner, means for delivering oxygen to said oxy-fuel burner, and a dense phase material flow transport system for delivering pulverised material to said oxy-fuel burner. In this method the ratio (by weight) of pulverised material to propellant is between 0.5: 1 to 10: 1. The use of supplementary fuel gas and oxygen or oxygen enriched air is advocated at higher ratios within this range. This system has the same disadvantages as those described above. The present invention, at least in its preferred embodiments, aims to provide more efficient combustion. The present invention is characterized in that said dense phase material flow transport system is delivering between 20 and 30 kg of pulverised material to the oxy-fuel burner per 1 kg of propellant whereby substantially complete combustion takes place without the necessity of adding fuel gas to the burner. The propellant, for example air or nitrogen, will typically be at 2-2.5 bar A. Advantageously, the pulverised fuel will be between 100 and 200 mesh (150-75 µm). Preferably, 60% to 80%, advantageously 70 to 75%, and more advantageously 75% by volume of said pulverised material will pass through a 75 µm filter. Heretofore, it has been essential for oxy-fuel burners for burning pulverised fuel to incorporate swirlers either for the pulverised fuel or the oxygen. When using a dense phase material flow transport system this may be unnecessary. Furthermore, it has been found possible to obtain satisfactory combustion of pulverised materials hitherto regarded as having too low a carbon content for economic combustion. This is a most significant advantage since it enables materials which would hitherto have been discarded as waste to be used as a viable energy source. Furthermore, disposal of the fly ash produced by such materials will normally be far less expensive than disposal of the unburnt material. If desired, passageways may be provided in the oxy-fuel burner to convey natural gas to the outlet of the burner during start-up of a furnace. For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a simplified flow sheet of a system for burning pulverised fuel in accordance with the invention; Figures 2A and 2B together show a plan view, partly in cross-section, of an experimental oxygen/natural gas/pulverised fuel burner used in the system; and Figure 3 is an end view looking in the direction of line III-III in Figure 2. Referring to Figure 1 of the drawings 30t of pulverised coal (70% by volume of which will pass though a 75 µm filter) is stored in a fully enclosed silo 1. The pulverised coal is introduced into a coal injector 2 via an upper dome valve 3 and a lower dome valve 4. A motor 5 drives a mechanism which introduces the pulverised coal into a feed pipe 6 at a rate of 1000 kg/h. The pulverised coal is conveyed along the feed pipe 6 by air which is introduced through pipe 7 at a rate of 35 kg/h and at a pressure of about 2.3 bar A. The equipment thus far described is known as a "dense phase material flow transport system" and can be obtained from Simon-Macawber Limited of England. The pulverised coal is introduced into the central tube of an oxy-fuel burner 10. Oxygen is supplied to the oxy-fuel burner 10 through pipe 11 and mixes with the pulverised coal at the outlet 12 of the oxy-fuel burner 10. In tests the pulverised fuel and oxygen burnt with an intense high temperature flame. This was quite remarkable bearing in mind that the same pulverised coal had burnt with a low temperature luminant flare in a conventional oxy-fuel burner system. The ash had the appearance of silica sand and negligible residual carbon was found. Referring now to Figures 2 and 3, the oxy-fuel burner 10 comprises a central tube 13 which is made of stainless steel and extends from the inlet 14 to the outlet 15 of the oxy-fuel burner 10. The central tube 13 extends through a tube 16 which itself extends through a tube 17. The tube 17 defines the radial inner wall of an annular oxygen supply passageway 18 which extends from a radial wall 19 to the outlet 15 of the oxy-fuel burner 10. Twelve natural gas pipes 20 are disposed in the oxygen-supply passageway 18 and extend from a header 21 to the outlet 15 as shown. In use, pulverised coal from feed pipe 6 passes through the central tube 13 to the outlet 15 of the oxy-fuel burner 10. Oxygen from pipe 11 enters the burner 10 through oxygen inlet 22 and passes through the oxygen supply passageway 18 to the outlet 15 where substantially complete combustion takes place. The tip of the burner 10 is cooled by water which enters the burner 10 through water inlet 23, flows through passageways 24 and 25 and leaves through water outlet 26. If desired, natural gas can be admitted to the burner 10 via pipe 27, and natural gas inlet 28. The natural gas passes into the header 21 and thence passes to the outlet 15 via the natural gas pipes 20. The natural gas is used during start-up.

Claims: 1. A method of operating a system for burning pulverised fuel, which system comprises an oxy-fuel burner (10), means (11) for delivering oxygen to said oxy-fuel burner (10), and a dense phase material flow transport system (2) for delivering pulverised material to said oxy-fuel burner (10), characterized in that said dense phase material flow transport system (2) is delivering between 20 and 30 kg of pulverised material to said oxy-fuel burner (10) per 1 kg of propellant whereby substantially complete combustion takes place without the necessity of adding fuel gas to the burner. 2. A method as claimed in Claim 1, characterized in that said propellant is air or nitrogen. 3. A method as claimed in Claim 1 or 2, characterized in that said propellant is operated at 2-2.5 bar A. 4. A method as claimed in any preceding Claim, characterized in that said pulverised fuel is between 100 and 200 mesh (150-75 µm). 5. A method as claimed in any preceding Claim, characterized in that 75% by volume of said pulverised material will pass through a 75 µm filter.

Inventors: YOUNG, PHILIP JOHN
Applicants: AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1988
Main classification: F23K3
IPC codes: F23K3, F23D1, F23L7