Patent Application
20120192773
The invention provides for the use of a burner for carbon dioxide-oxygen-coal combustion processes. More particularly, the invention provides for a lance in connection with a burner to more effectively combust a solid fuel, such as coal, with an oxidant mixture comprising predominantly of carbon dioxide and oxygen, such as may be formed by the addition of substantially pure oxygen with flue gas recirculated from said combustion process.
Air-coal burners and steam boilers for electrical power generation have been developed over many years through a process of trial and error. Typically, the motive air that is used to pulverize the coal is also used as the primary air in the air-coal burners. Primary air and pulverized coal typically flow through an annular conduit that is mesial to coaxial annular conduit for the secondary air. A helical secondary oxidant flow patter is often used to increase the mixing rate with the primary air-coal stream with the secondary air stream.
In order to more cost effectively recover carbon dioxide from coal fired power stations, electrical power utilities are evaluating substituting a predominantly carbon dioxide-oxygen oxidant for the traditional air oxidant. The carbon dioxide-oxygen oxidant is formed by mixing a CO2 rich flue gas from the combustion process with industrial oxygen, and as such may also contain water vapor, nitrogen, and minor constituents arising from the combustion process. However, substitution of the nitrogen diluent in the air oxidant with carbon dioxide decreases the coal combustion flame velocity more rapidly than the coal flammability limit and flame temperature. If one tries to increase the carbon dioxide-oxygen-coal flame velocity to acceptable values by increasing the primary oxidant oxygen in carbon dioxide concentration, then the fire hazard in the coal grinding mill also increases. If one increases the flame velocity to acceptable values by increasing the oxygen in carbon dioxide concentration in the secondary oxidant stream, then excessive heat fluxes are observed in the near burner region.
An additional trend is in the utilization of increasing quantities of biomass mixed with coal, or even 100% biomass firing. The biomass feed may be any organic material, including agricultural crops and agricultural wastes and residues, wood and wood wastes and residues, animal wastes, municipal wastes, algae and aquatic plants. The biomass feed may be dried, chopped, ground, or pelletized. Typical biomass feed examples include wood chips, wood charcoal, pelletized grass, and similar materials Biomass, particularly wet biomasses can further lead to reductions in flame speed and more unstable combustion conditions. As such the addition of biomass can lead combustion related problems that may not be overcome by increasing the oxygen content of the secondary air.
The invention seeks to limit these concerns by adding an oxidant lance to a conventional air-coal burner assembly.
The invention provides for the use of a predominantly carbon dioxide-oxygen oxidant in place of air oxidant in a solid fuel burner. The solid fuel herein after referred to as coal may be any solid hydrocarbon fuel, such as various grades of coal, peat, coke, or biomass The invention further provides for an improved coal burner assembly which comprises an air-coal burner equipped with an oxidant lance, thereby providing a better combustion with an oxygen-carbon dioxide-coal mixture than an air-coal mixture.
In an embodiment of the invention, there is disclosed a burner for combusting fuels such as coal comprising a lance having one or more nozzle feeds and one or more nozzle outlets concentrically surrounded by a primary oxidant passage which is concentrically surrounded by a secondary oxidant passage wherein said primary and secondary oxidant passages communicate at their proximal ends with a gas supply, said lance having a distal and proximal end and said one or more nozzle feeds is in communication with a gas supply.
In another embodiment of the invention there is disclosed a method for oxidizing coal comprising feeding a mixture of oxygen, carbon dioxide and coal to a burner comprising a lance having one or more nozzle feeds and one or more nozzle outlets concentrically surrounded by a primary oxidant passage which is concentrically surrounded by a secondary oxidant passage wherein said primary and secondary oxidant passages communicate at their proximal ends with a gas supply, said lance having a distal and proximal end and said one or more nozzle feeds is in communication with a gas supply.
The solid fuel that can be used are selected from the group consisting of coal, coke, peat, biomass and mixtures thereof. The solid fuel is typically transported via a primary oxidant stream through the primary oxidant passages. The primary oxidant stream will have a molar oxygen concentration less than 32%, preferably less than 25% and more preferably less than 21%.
The fuel and oxidant mixture comprises oxygen and carbon dioxide and is typically formed by mixing flue gases from a combustion process with industrially pure oxygen.
A secondary oxidant stream is fed through the secondary oxidant passage and the secondary oxidant stream has a molar oxygen concentration less than 40%, preferably less than 30% and more preferably less than 28.
The nozzles at the distal end of the lance are angled in a manner whereby the discharge streams from the lance do not intercept an outer wall of the primary oxidant passage. Further, these nozzles are positioned to direct oxygen into a flow path of the primary oxidant stream containing solid fuel.
The burner further comprises a quad and the nozzles at the distal end of the lance are arranged complementary to the primary and secondary oxidant streams being directed through the quarl. In some instances, the distal end of the lance is angled so that the projected axis intercepts an outer edge of the burner quarl.
The burner can be adapted for external oxidant staging.
The oxidant stream that issues through the distal end of the lance is at a higher velocity than the primary oxidant. Typically, this velocity is less than four times the velocity of the primary oxidant stream.
The coal channel 38 and primary oxidant passage concentrically surround the closed end 35 of the air-coal burner. As seen in
In Table 1 below, the differences between an air-coal burner using air versus an air-coal burner using a mixture of carbon dioxide and oxygen are shown.
The overall oxygen in carbon dioxide concentration was set at about 28 molar percent to yield roughly equivalent adiabatic flame temperatures for the air and carbon dioxide-oxygen oxidants. The burner outside diameter was held constant so that the carbon dioxide-oxygen burner can use the air burner mounting system. The overall oxidant flow rate was held constant which would result in an approximate 30% increase in thermal output. Similar techniques could be used for the less demanding constant thermal output basis. The coal transport gas oxygen in carbon dioxide content was set at 21 molar percent which would decrease the coal grinding mill fire hazard concern at higher oxygen concentrations.
The addition of oxidant lance oxidant increased the primary oxidant oxygen in carbon dioxide content to 31 molar percent in order to roughly match the air-coal flame velocity. Sixteen oxidant lance nozzles as noted in
In the above embodiment of the invention a primary oxidant was thus provided by the enrichment of the coal transport gas with a lanced oxidant to a higher level of oxygen within the discharge end of the burner and, as such, delivering a ready mixed oxygen concentration suitable for a stable combustion into the burner quarl.
In a further embodiment of this invention the lance nozzles 75 are located so as to inject an oxidant directly into the burner combustion space 93 within the burner quarl 60, as shown in
The flow patterns of the expanding oxidant streams (75A, 80 and 85) and the region for stabilization of the flame root 91 may be determined by CFD modeling or by visual observation. A further method for determining the angle of the lance oxidant nozzles 75 is for them to be angled in a divergent manner towards the outer lip or edge 65 of the burner quarl 60. The nozzles 75 may further be angled to induce a swirling motion complimentary to the swirling motion in combustion space 93 of any of the transport and secondary streams 80 and 85. In order to direct the lance oxidant jets 75A into and mix with the fuel laden transport oxidant stream it is advantageous to operate the lance oxidant nozzles 75 at a greater velocity than the transport oxidant stream 80. By orienting the lance oxidant nozzles 75 in such a divergent manner and by operation at velocities greater than that of the transport oxidant stream the fluid recirculation patterns 100 important for a stable combustion process in such a burner are reinforced.
As exemplified in Tables 2a and 2b a central lance delivering relatively modest quantities of pure oxygen can deliver significant increases in the oxygen concentration when mixed with the transport oxidant. In these cases the secondary oxidant oxygen content has been maintained at a constant level of 26 mol. % which has resulted in a variation in the overall oxygen content, however the secondary oxidant oxygen content can be raised or lowered slightly to maintain an overall or global desired oxygen concentration. In these cases the lance oxidant nozzles are operated at a velocity of 45 m/s or approximately 3.5 times the velocity of the transport oxidant
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.
This application claims priority from U.S. provisional patent application Ser. No. 61/369,894 filed on Aug. 2, 2010.
| Number | Date | Country | |
|---|---|---|---|
| 61369894 | Aug 2010 | US |
