WO2016179823A1

WO2016179823A1 - System and method for combustion of pulverized solid fuels in small and medium scale boilers

Info

Publication number: WO2016179823A1
Application number: PCT/CN2015/078945
Authority: WO
Inventors: Zheng Shi
Original assignee: Zheng Shi
Priority date: 2015-05-14
Filing date: 2015-05-14
Publication date: 2016-11-17

Abstract

A system and method for combustion of pulverized coal in small and medium boilers. The system includes a combustion chamber (101) comprising an internal chamber and an external chamber separated by an inner wall (103) and connected at the apex of the combustion chamber, one or more nozzles (105), a primary air pumping mechanism, and a secondary air pumping mechanism for supplying additional combustion air into the combustion chamber. During the combustion of the pulverized solid fuel, the primary and secondary air pumping mechanisms creat a reflux of fuel-air flow within the combustion chamber, improving the overall efficiency of combustion of the fuel.

Description

SYSTEM AND METHOD FOR COMBUSTION OF PULVERIZED SOLID FUELS IN SMALL AND MEDIUM SCALE BOILERS

TECHNICAL FIELD

The present invention relates to combustion of powdered solid fuels such as pulverized coal within small and medium scale boilers. More particularly, the invention is directed to a system and method that improves the overall efficiency of combustion of powered solid fuels.

BACKGROUND

Combustion chambers, e.g. industrial boilers, 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 the combustion chamber are well known. In this type of systems, the pulverized fuel and air are blown into the combustion chamber as an air-fuel-flame mixture. The fuel-air-flame mixture subsequently travels through the chamber as the fuel is further burned to near completion. The walls of the combustion chamber are often lined with a water tubing system that absorbs the heat produced for productive purpose and keeps the walls of the combustion chamber relatively cool throughout the combustion 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 combustion chamber at an appropriate rate and direction, and keeping the fuel in the combustion chamber for a sufficient time and optimal spatial distribution 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 and air 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 powdered fuel spends a protracted amount of time in the combustion chamber and thus assures adequate combustion of the powdered fuel. For example, elaborate systems such as tangentially-fired boilers are designed in such a way that four nozzles are arranged at four corners of the cross section of the combustion chamber and the powdered fuel is projected into a vortex flow which creates a stable flame and protracts the time the fuel spends within the chamber.

Unfortunately, the above is not applicable to small and medium scale (e.g. boiler capacity less than 30MW) combustion of solid fuel whereby a small installation is utilized and arrangements for fuel preheating are limited. Consequently, in China, most small and medium scale industrial boilers are based on the grated chain technology that was invented a couple of hundreds of years ago, operating at low thermal efficiency.

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

SUMMARY OF INVENTION

The present invention discloses a system and method for combustion of solid fuels, such as pulverized coal in small and medium boilers.

In accordance with one embodiment of the present invention, the system of described in the present invention includes a combustion chamber aligned vertically to the ground, divided into an internal chamber and an external chamber by the inner walls, one or more nozzles that is affixed at the bottom of the combustion chamber and made to face substantially upwards in the direction of the apex of the combustion chamber, a primary air pumping mechanism for supplying a stream mixture of pulverized solid fuel and combustion air into the combustion chamber, and a secondary air pumping mechanism for supplying additional combustion air into the combustion chamber.

In accordance with the aforementioned embodiment of the present invention, the method of the present invention involves injecting both the stream mixture of pulverized solid fuel and combustion air and additional combustion air into an internal chamber of a combustor vertically mounted, through one or more nozzles that are affixed at the bottom of the combustion chamber and are made to face substantially upwards in the internal chamber. Since both the primary stream mixture and the secondary air are blown upwards against the natural falling downwards of the fuel particles due to the force of gravity, a reflux flow is created within the internal combustion chamber, which prevents heavier particles from falling to the bottom and allows lighter particles to exit the internal chamber at the apex into the external chamber.

In accordance with the aforementioned embodiment of the present invention, the system further includes one or more sets of adjustable swirl vanes, affixed at the bottom of the combustion chamber, through which the secondary air is blown upwards into the combustion chamber.

In accordance with the aforementioned embodiment of the present invention, the directions of both primary and secondary air are at an angle ranging from 100 to 80 degrees relative to the ground surface.

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.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be better understood and its 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, a combustion chamber 101 is positioned vertically to the ground (depicted in FIG 1. as G) . The combustion chamber 101 further includes an internal chamber and an external chamber separated by an inner wall 103 and connected at the apex of the combustion chamber. It should be noted that although the embodiment illustrated in FIG. 1 shows a combustion chamber of rectangular shape, the present invention also embodies other potential designs such as cylindrical shaped combustion chambers. Furthermore, the relative sizes and shapes of each of the present invention’s components depicted in FIG. 1 are for illustrative purposes only and in no way should be seen as a restriction of the invention in itself.

In the embodiment depicted in FIG. 1, a combustion air nozzle 102 that projects a secondary air within the confines of the inner wall 103 of the internal combustion chamber is arranged at the bottom of the chamber 101. The apex, also known as the outlet, of the combustion chamber is open so as to allow air streams and solid fuel ash to pass through after adequate combustion is achieved. A fuel nozzle 105 (or a plurality of fuel nozzles) is arranged within the combustion chamber 101, the outlet of which is arranged so as to face the apex of the combustion chamber. An igniter 104 is also arranged within the combustion chamber to start the flame. An outer combustion wall 108 surrounds the combustion chamber on its sides as well as on top, leaving a gap between the inner and outer walls so as to accommodate the unrestricted flow of air, flue gas and solid fuel ash. The top of the outer combustion wall’s main purpose is to serve as a deflector 109 for the flow of solid fuel ash particles leaving out of the apex of the combustion chamber. The present embodiment also provides inner water shells 111 and outer water shells 112 that line up along the sides of the internal combustion chamber and the external combustion chamber, respectively. The main purpose of the outer water shell 112 and inner water shell 111 is for cooling of chamber walls as well as heat collection.

Secondary air can also be pumped into the internal chamber through one or more sets of adjustable swirl vanes affixed at the bottom of the combustion chamber 101(not shown in FIG. 1) .

The arrows numbered 113, 114, 115, 116, 117 and 118 graphically depict the flow of air within the combustion chamber.

The combustion process within the combustion chamber 101 is described as the following. A fuel/air stream mixture is blown through pipes leading up to the (or a plurality of) fuel nozzle 105 and projected into internal combustion chamber. The general direction of the flow is depicted as the arrows labeled 113. The secondary air is also blown upwards into the combustion chamber through the combustion air nozzle 102. The flow of the fuel/air mixture 113 is made to move approximately in the same direction as the flow of the secondary air 114 (i.e., combustion air) . The directions of both primary and secondary air can be at an angle ranging from 100 to 80 degrees relative to the ground surface.

As the fuel particles are combusted, they lose mass and weight, and become easier to be carried by the airflow upwards. Once the fuel particles are burned to near completion, they exit the internal combustion chamber at the outlet of the internal chamber that is located at the apex of the combustion chamber, before eventually leaving the combustion chamber.

As seen in FIG. 1, the flow of both the primary air/fuel mixture 113 and the secondary air 114 subsequently exits the apex of the combustion chamber and enters the external combustion chamber. At this point the combined flow of the primary air/fuel mixture 113 and the secondary air 114 is deflected by the deflector 109 towards the sides of the outer combustion walls 108. This redirected flow is depicted as the arrows labeled 116. The flow of the primary air/fuel and the secondary air subsequently moves down along the lining of the combustion chamber walls in the space in-between the external and internal walls (depicted as the arrows 115) and eventually exits the combustion chamber where the arrows labeled 117 are depicted.

Since the flows of the primary air/fuel 113 and the secondary air 114 within the internal combustion chamber are facing upwards relative to the ground, a reflux flow is created due to the natural force of gravity counteracting against these flows. The reflux flow is depicted as the arrows 118. According to the dynamics of the reflux flow 118 occurring within the internal combustion chamber, the relatively lighter solid fuel ash particles are carried by the flow and exit the internal combustion chamber (depicted by the arrows 116) whereas the heavier solid fuel particles, which are not adequately combusted yet, stay within the reflux flow 118 located within the internal combustion chamber and continue to combust until they are light enough to leave the internal combustion chamber.

The reflux of the fuel-air mixture improves the flame stability and the completion of the fuel combustion in the following manner. Within the reflux, completely burned fuel particles are the lightest and are carried by the flow of the primary and secondary air to exit the internal combustion chamber at the apex of the combustion chamber, and then move down along the lining of the combustion chamber walls in the space in-between the external and internal walls and eventually exit the combustion chamber. Partially burned fuel particles are heavier than completely burned fuel particles and consequently they fall down or suspend against the flow of the primary and secondary air, such falling down or suspension effectively elongated the time that such partially burned fuel particles spend inside the combustion chamber. As the partially burned fuel particles become more thoroughly burned, they lose more mass and become lighter, and they become easier to be carried upwards by the flow of the primary and secondary air, until they become completely burned and exit the internal combustion chamber at the apex.

Those already skilled with the art will recognize that through the flow dynamic effect described above, relatively heavier solid fuel particles which are partially burned spend a protracted amount of time within the confines of the internal combustion chamber until they are burned up to be light enough to leave the combustion chamber. This process effectively allows the combustion process to increase in overall efficiency. Furthermore, given the simplicity of the system as well as the effective use of a natural occurring force (i.e., gravity) , this increase in efficiency is very cost effective.

Claims

A system for burning pulverized solid fuel, comprising:

-a combustion chamber vertically mounted, and comprising an internal chamber and an external chamber separated by an inner wall and connected at the apex of the combustion chamber；

-one or more nozzles affixed at the bottom of the combustion chamber and made to face substantially upwards in the internal chamber；

-a primary air pumping mechanism for supplying a stream mixture of pulverized solid fuel and combustion air into the internal chamber through the one or more nozzles； and

-a secondary air pumping mechanism for supplying additional combustion air into the internal chamber；

wherein, during the combustion of the pulverized solid fuel, the primary and second air pumping mechanisms create a reflux of fuel-air flow in the internal chamber, preventing heavier particles from falling to the bottom of the combustion chamber, and allowing lighter particles to exit the internal chamber at the apex into the external chamber.
The system of claim 1, further comprising, one or more sets of adjustable swirl vanes affixed at the bottom of the combustion chamber, wherein the secondary air is pumped into the internal chamber through these vanes.
A method for burning pulverized solid fuel, comprising:

-pumping, by a primary air pumping mechanism, a stream mixture of pulverized solid fuel and combustion air into an internal chamber of a combustion chamber vertically mounted, through one or more nozzles that are affixed at the bottom of the combustion chamber and are made to face substantially upwards in the internal chamber, during the combustion of the pulverized solid fuel；

-pumping, by a secondary air pumping mechanism, additional combustion air into the internal chamber, during the combustion of the pulverized solid fuel；

-creating a reflux of fuel-air flow in the internal chamber, preventing heavier particles from falling to the bottom of the combustion chamber, and allowing lighter particles to exit the internal chamber at the apex of the combustion chamber into the external chamber of the combustion chamber.
The method of claim 3, further comprising, pumping the secondary air into the internal chamber through one or more sets of adjustable swirl vanes affixed at the bottom of the combustion chamber.