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Title:
SYSTEM AND METHOD FOR SMALL-SCALE COMBUSTION OF PULVERIZED SOLID FUELS
Document Type and Number:
WIPO Patent Application WO/2014/179956
Kind Code:
A1
Abstract:
Provided are a system and a method for small-scale combustion of pulverized solid fuels. A fuel stream is injected into the flow field of the secondary air in a direction that is substantially opposite to the direction of the secondary air. The reflux of the fuel stream creates a zone of stability within the combustion chamber, which prolongs the amount of time the fuel stream spends within the combustion chamber. The flame is formed substantially near and along the central axis of the flow field of the secondary air, or near the center of the combustion chamber and away from the walls of the combustion chamber, which improves the temperature profile of the combustion chamber.

Inventors:
SHI, Zheng (Suite 1208, West Tower Twin Towers, B12 Jianguomenwai Avenue, Beijing 2, 100022, CN)
Application Number:
CN2013/075367
Publication Date:
November 13, 2014
Filing Date:
May 09, 2013
Export Citation:
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Assignee:
SHI, Zheng (Suite 1208, West Tower Twin Towers, B12 Jianguomenwai Avenue, Beijing 2, 100022, CN)
International Classes:
F23D14/64; F23D14/32; F23D17/00
Domestic Patent References:
WO2008133695A1
Foreign References:
EP0874194A2
EP2199674A1
US20090123882A1
CN1880848A
Attorney, Agent or Firm:
SHANGHAI SAVVY INTELLECTUAL PROPERTY AGENCY (Room 341, Building 1789 West Tianshan Road, Changning District, Shanghai 5, 200335, CN)
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Claims:
We Claim:

1. A method of burning pulverized solid fuel, comprising the step of generating a flow field of a secondary air substantially in a first direction; fluidizing the pulverized solid fuel with a primary air to generate a stream of pulverized solid fuel; injecting the stream of pulverized solid fuel into the flow field of the secondary air in a second direction that is substantially opposite to the first direction, wherein a reflux stream of pulverized solid fuel substantially in the first direction is formed; and igniting the stream of pulverized solid fuel to form a flame substantially near and along the central axis of the flow field of the secondary air.

2. The method of claim 1, wherein the first direction and the second direction are at an angle ranging from 120 to 180 degrees.

3. The method of claim 1, wherein a volume of the secondary air is greater than a volume of the primary air.

4. The method of claim 1, wherein the flame comprises a reflux zone that enables flame stability.

5. The method of claim 1, wherein the flame has a peak temperature substantially near and along the central axis of the flow field of the secondary air.

6. The method of claim 1, wherein the flow field of the secondary air is restricted in a combustion chamber.

7. The method of claim 1, wherein the pulverized solid fuel is pulverized coal.

8. The method of claim 1, wherein the pulverized solid fuel is pulverized biomass.

9. The method of claim 1, wherein the primary air or the secondary air is oxygen-enriched.

10. A burner for burning pulverized solid fuel, comprising: a combustion chamber; a secondary air pumping mechanism for generating in the combustion chamber a flow field of a secondary air substantially in a first direction; a blow pipe for supplying a stream of pulverized solid fuel; a nozzle for injecting the stream of pulverized solid fuel in the flow field of the secondary air in a second direction that is substantially opposite to the first direction, wherein a reflux stream of pulverized solid fuel substantially in the first direction is formed substantially near and along the central axis of the flow field of the secondary air; an auxiliary nozzle for igniting the stream of pulverized solid fuel.

11. The burner of claim 10, further comprising a feed pipe for fluidizing the pulverized solid fuel with a primary air to generate the stream of pulverized solid fuel.

12. The burner of claim 10, wherein the first direction and the second direction are at an angle ranging from 120 to 180 degrees.

13. The burner of claim 10, wherein, for a nameplate power capacity of 5MW, the nozzle has an inner diameter of 8cm, the combustion chamber has a length of 1 meter and an inner diameter of 0.5 meters, the primary air has a speed ranging from 15 to 20 m/s, and the secondary air has a speed ranging from 1 to 10 m/s.

14: The burner of claim 10, wherein the pulverized solid fuel is pulverized coal.

15. The burner of claim 10, wherein the pulverized solid fuel is biomass.

16. The burner of claim 10, further comprising an ignition device for triggering the combustion of the pulverized solid fuel.

Description:
System and Method for Small-scale Combustion of Pulverized Solid

Fuels

Technical Field

The present invention relates to small-scale combustion of powdered solid fuels such as pulverized coal. More particularly, the invention is directed to a system and method that improves ignition and flame stability in the combustion of powered solid fuels.

Background of the Invention

Combustion chambers wherein a single or a plurality of fuel nozzles are arranged to project a mixture of air and solid fuel, such as powdered coal, into a furnace combustion chamber are well known. In this type of systems, the powdered coal and pressurized air are blown into a combustion chamber as an air-fuel mixture and ignited. The coal-air flame subsequently extends into the chamber. The walls of the furnace chamber are often lined with a water tubing system that keeps the chamber relatively cool through a heat transfer process.

One of the important tasks of the abovementioned systems is to provide the proper conditions for the combustion of the fuel. This is mainly achieved by projecting the fuel supply fed into the furnace at an appropriate rate, and keeping the fuel in the furnace for a sufficient time for it to combust.

In large scale industrial coal-fired facilities, complete or near complete combustion of the pulverized solid fuel is almost assured as these installations have large combustion chambers and sophisticated fuel preheating systems. Typically, in these large installations, preheating arrangements and the design of the combustion chamber itself is made in such a way that the fuel dust spends a protracted amount of time in the combustion chamber and thus assures adequate combustion of the fuel dust. For example, elaborate systems such as tangentially-fired furnaces are designed in such a way that the fuel dust is projected into a vortex flow which protracts the time the fuel spends within the chamber.

Unfortunately, the above is not applicable to small-scale (<30MW) combustion of solid fuel whereby a small installation is utilized and arrangements for fuel preheating are limited.

For example, consider a small cylindrical shaped combustion chamber of around a meter in length and having an inner diameter of around 0.5 meters wherein the fuel/air nozzle is arranged at one end of the chamber. Assuming that the fuel/air mix is projected into the furnace chamber at a speed of around 15 m/s, the fuel dust will pass through the furnace chamber in less than 10 milliseconds, which is insufficient for the fuel to combust completely.

Accordingly, there is a need to improve ignition and flame stabilization in small scale combustion of pulverized solid fuels, while simultaneously providing a system that is simple to construct and inexpensive to produce.

Summary of the Invention

The present invention provides a system and method for combustion of solid fuels, such as pulverized coal. In accordance with an embodiment of the present invention, the method comprises the step of generating a flow field of a secondary air substantially in a first direction; fluidizing the pulverized solid fuel with a primary air to generate a stream of pulverized solid fuel; injecting the stream of pulverized solid fuel into the flow field of the secondary air in a second direction that is substantially opposite to the first direction, wherein a reflux stream of pulverized solid fuel substantially in the first direction is formed; and igniting the stream of pulverized solid fuel to form a flame substantially near and along the central axis of the flow field of the secondary air.

In a preferred embodiment, the first direction and the second direction are at an angle ranging from 120 to 180 degrees. In another preferred embodiment, the volume of the secondary air is greater than the volume of the primary air. In yet another preferred embodiment, the flame comprises a reflux zone that enables flame stability. In yet another preferred embodiment, the flame has a peak temperature substantially near and along the central axis of the flow field of the secondary air.

In accordance with another embodiment of the present invention, a burner comprise a combustion chamber; a secondary air pumping mechanism for generating in the combustion chamber a flow field of a secondary air substantially in a first direction; a blow pipe for supplying a stream of pulverized solid fuel; a nozzle for injecting the stream of pulverized solid fuel in the flow field of the secondary air in a second direction that is substantially opposite to the first direction, wherein a reflux stream of pulverized solid fuel substantially in the first direction is formed

substantially near and along the central axis of the flow field of the secondary air; an auxiliary nozzle for igniting the stream of pulverized solid fuel.

In accordance with one embodiment of the present invention, a fuel stream is injected into the flow field of the secondary air in a direction that is substantially opposite to the direction of the secondary air. The reflux of the fuel stream creates a zone of stability within the combustion chamber, which effectively prolongs the amount of time the fuel stream spends within the combustion chamber, and improves the combustion of the fuel.

In accordance with another embodiment of the present invention, the flame is formed substantially near and along the central axis of the flow field of the secondary air, or near the center of the combustion chamber and away from the walls of the combustion chamber, which improves the temperature profile of the combustion chamber.

Brief Description of the Drawings

FIG. 1 is an exemplary cross-sectional view of a combustion chamber in accordance with one embodiment of the invention. FIG. 2 is another exemplary cross-sectional view of a combustion chamber in accordance with one embodiment of the invention.

FIG. 3 depicts an exemplary temperature distribution within a combustion chamber in accordance with one embodiment of the invention.

Detailed Description of the Invention

The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings.

FIG. 1 is an exemplary cross-sectional view of a combustion chamber in accordance with one embodiment of the invention. As shown in FIG 1, the combustion chamber 101 is of a cylindrical shape. A combustion air nozzle 102 that projects a secondary air into the combustion chamber 101 is arranged at one end of the chamber 101. A plurality of fuel nozzles 103 are arranged along the walls of the chamber 101, and the outlets of the fuel nozzles 103 substantially face the outlet of the combustion air nozzle 102. The arrows numbered 111, 112 and 113 graphically depict the flow of air formed in the combustion chamber. An ignition device 105 (e.g., natural gas torch, plasma torch, fossil fuel pilot torch, or arc igniter) is arranged near fuel nozzles 103 for triggering the combustion process.

The combustion process within the combustion chamber 101 can be described as the following. A fuel/air stream is blown through pipes leading up to the fuel nozzles 103 and projected into the combustion chamber 101. The general direction of the flow is depicted as arrows 111. The flow of fuel/air 111 runs substantially in the opposite direction of the flow of secondary air 112 (i.e., combustion air) at an angle that can range anywhere from 120 to 180 degrees.

In accordance to one embodiment of the invention, the volume of the secondary air 112 is larger than the volume of the fuel/air stream 111, effectively causing a reflux flow within the combustion chamber 101 whereby the stream 111 reverses direction towards the outlet of the combustion chamber. In FIG. 1, the direction of the reflux flow is show as arrows 113. A zone of stability is created from this reflux at point 104 where, once ignited, the flame stabilizes. This zone of stability effectively prolongs the amount of time the fuel stream (i.e. pulverized coal particles) spends within the combustion chamber. As a result, the combustion of the solid fuel is enhanced, as the solid fuel spends a prolonged time within the combustion chamber.

FIG. 2 is another exemplary cross-sectional view of a combustion chamber in accordance with one embodiment of the invention. The reference numeral 201 denotes the chamber's main body. In this embodiment, the furnace is of a conically widening shape (thereafter referred as muffle burner). The combustion air nozzle 202 that projects the secondary air is arranged at the smaller end of the chamber and the secondary air flows from the smaller end to the larger end of the chamber. A plurality of fuel nozzles 203 are arranged along the walls of the chamber, and the outlets of the fuel nozzles 203 substantially face the outlet of the combustion air nozzle 202.

In this embodiment, the volume of the secondary air 212 is larger than the volume of the fuel/air stream 211, which causes a reflux flow within the combustion chamber 201 whereby the stream 211 reverses direction towards the outlet of the combustion chamber. In FIG. 2, the direction of the reflux flow is shown as arrows 213. A zone of stability is created from this reflux at point 204 where, once ignited, the flame stabilizes. The creation of this zone of stability effectively prolongs the amount of time the fuel stream (i.e. pulverized coal particles) spends within the combustion chamber. As a result, the combustion of the solid fuel is enhanced, as the solid fuel spends a prolonged time within the combustion chamber.

FIG. 3 depicts an exemplary temperature distribution within a combustion chamber in accordance with one embodiment of the invention. In this embodiment, the combustion chamber is construed in accordance with FIG. 1, and has a given nameplate power of 5MW. The furnace or combustion chamber 101 has a cylindrical shape and is 1 meter in length and 0.5 meters in inner diameter. Each of the plurality of fuel nozzles 103 has an outlet inner diameter of 8cm.

The volume of air projected out of the fuel/air nozzles preferably comprises around 10% of the total volume required for the total combustion of the pulverized fuel; the remaining air, which comprises up to 90% of total volume required for the combustion of the pulverized fuel, is combustion air and is projected from the combustion air nozzle 102. The speed at which the air/fuel mix exits the fuel nozzles 103 is preferably between 15 to 20 m/s. The speed at which the combustion air exits the combustion air nozzle 102 is preferably between 1 to 10 m/s.

Based on experimentations, it was found that the combustion chamber described above achieved excellent fuel dust combustion results. The combustion chamber attains rapid ignition and stable flame, with combustion temperature ranged from 500°C to 1300°C based on user preferences and fuel ignitability, which in turn is mainly a function of fuel particle size, fuel stream velocity and air/fuel ratio, and the properties of the fuel in question (e.g., moisture, volatiles and ash content, etc.).

FIG. 3 shows an exemplary temperature distribution data for a device described in FIG. 1 wherein the secondary air speed is 4m/s. The data in FIG. 3 is provided for the purpose of illustrating the advantages of embodiments of the present invention, and should not be construed in any circumstances to limit the scope of the present invention. In FIG. 3, the points 0% to 100% are temperature measurement points relative to the distance from the axis of the cylindrically shaped combustion chamber at the outlet thereof, with 0% being the axis in the center, and 100% being the exterior wall of the combustion chamber.

The following data was obtained from one experiment: Measuring Point Temp °C

10% 926

28% 1025

48% 905

80% 620

94% 640

As shown in FIG. 3, the areas with the highest temperature are relatively close to the axis of the combustion furnace, i.e., in this case ranging from 926 to 1025 ° C at the points 10% and 28% respectively. In areas further away from the axis and closer to the wall of the combustion chamber, the temperature then decreases. The lowest temperature values are recorded close to the wall of the combustion burner, i.e., 640 ° C at 94% and 620 ° C at 80%.

Once ignited, the flame occurs primarily along and around the axis of the combustion burner and away from the walls of the combustion chamber. This temperature profile is different from what is known in the prior art, and has some distinctive advantages. Since the flame primarily burns around the axis of the combustion chamber and away from the walls, it reduces the accumulated damages typically associated with having high temperature along the walls of the chamber, and costly wall-cooling apparatus may be avoided.

While the invention has been described in connection with various embodiments, the invention is not limited to these specific embodiments. People skilled in the art will recognize that the system and method of the present invention may be used in many other applications, including but not limited to cement kilns and steam generators. In particular, while specific embodiments of the present invention is directed to small scale combustion of powered coals, the present invention can also be adapted to the combustion of other solid fuels, such as biomass. Furthermore, the present invention may also be adapted to media or large scale combustion of powered coal.