Gasification of fuel in a slagging gasifier

Abstract

A process for gasification of carbon-containing fuel is taught. The process includes heating carbon-containing fuel in a container, introducing air into the container in controlled quantifies to maintain substoichiometric conditions; removing syngas from the container and, removing slag from the container.

Description

FIELD OF THE INVENTION

The present invention relates to a gasifiers, including single-stage, entrained flow, slagging type coal gasifiers and a method for operation thereof.

BACKGROUND OF THE INVENTION

Generally speaking, gasification is a process that converts carbonaceous materials, such as coal, petroleum, petroleum coke or biomass, into fuel gas, or synfuel, which is predominantly carbon monoxide and hydrogen. The synfuel is suitable for firing steam generators and similar devices or gas turbines. In a gasifier a variety of complex chemical reactions occur, but essentially, the carbonaceous material undergoes three processes, pyrolysis, combustion and gasification.

The pyrolysis (or devolatilization) process occurs as the carbonaceous particle heats up. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.

The combustion process occurs as the volatile products and some of the char reacts with oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. Pyrolysis and combustion are very rapid processes.

The gasification process occurs as the char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen. The resulting gas is called fuel gas or syngas (or wood gas when fueled by wood) and may be more efficiently converted to energy such as electricity than would be possible by direct combustion of the fuel, as the fuel is first combusted in a gas turbine and the heat is used to produce steam to drive a steam turbine. Also, corrosive ash elements such as chloride and potassium may be refined out by the gasification process, allowing high temperature combustion of the gas from otherwise problematic fuels.

Four types of gasifiers are currently available for commercial use: counter-current fixed bed, co-current fixed bed, fluid bed and entrained flow.

The counter-current fixed bed (“up draft”) gasifier consists of a fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the “gasification agent” (steam, oxygen and/or air) flows in counter-current configuration. The ash is either removed dry or as a slag. The co-current fixed bed (“down draft”) gasifier is similar to the counter-current type, but the gasification agent gas flows in co-current configuration with the fuel (downwards, hence the name “down draft gasifier”). Heat needs to be added to the upper part of the bed, either by combusting small amounts of the fuel or from external heat sources.

In the fluid bed gasifier, the fuel is fluidized in oxygen (or air) and steam. The ash is removed dry or as heavy agglomerates that defluidise. The temperatures are relatively low in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly suitable. In the entrained flow gasifier a dry pulverized solid, an atomized liquid fuel or a fuel slurry is gasified with oxygen or air in co-current flow. The gasification reactions take place in a dense cloud of very fine particles. Most coals are suitable for this type of gasifier because of the high operating temperatures and because the coal particles are well separated from one another.

There are a number of examples patents concerning the gasification process. One example of a fixed bed gasifier is the subject matter of U.S. Pat. No. 3,920,417 entitled “Method of Gasifying Carbonaceous Material”, which issued on Nov. 18, 1975. Another design of fixed bed gasifiers forms the subject matter of U.S. Pat. No. 4,069,024 entitled “Two-Stage Gasification System”, which issued on Jan. 17, 1978.

Turning to entrainment-type gasifiers, one example of an entrainment gasification process is that which forms the subject matter of U.S. Pat. No. 4,158,552 entitled “Entrained Flow Coal Gasifier”, which issued on Jun. 19, 1979. Another example of an entrainment gasification process is that which forms the subject matter of U.S. Pat. No. 4,343,627 entitled “Method of Operating a Two-Stage Coal Gasifier”, which issued on Aug. 10, 1982. Yet another example of an entrained gasification process is that which forms the subject matter of U.S. Pat. No. 4,610,697 entitled “Coal Gasification System with Product Gas Recycle to Pressure Containment Chamber”, which issued on Sep. 9, 1986. Still another example of an entrained gasification process is that which forms the subject matter of U.S. Pat. No. 4,680,035 entitled “Two Stage Slagging Gasifier”, which issued on Jul. 14, 1987.

Although there are various types of gasifiers currently on the market, there still is a need to improve the gasification process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of the present invention.

DETAILED DESCRIPTION

An embodiment of the present invention is described herein. Those of ordinary skill in the art will realize that the following detailed description of the present embodiment is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to an implementation of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts.

In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.

In the embodiment shown in the FIG. 1 schematic, a combustor 10 is shown. An opening 16 into the combustor 10 receives pulverized coal. A pulverized coal bin 18 supplies coal via a conveyor belt 20 to a pulverizer 22. Air from a transport air blower 24 is mixed with the coal in a venturi eductor 23 and pneumatically conveyed to a solids mixer/splitter 30. Alkali, such as hydrated lime [Ca(OH)₂] is supplied from bin 26 via conveyor 28 to a transport line 29 where air from the transport air blower 24 is mixed with the hydrated lime in a venturi eductor 31. Other alkali that can be employed are lime, limestone, dolomite, nacholite, potassium carbonate and trona. The pulverized coal and hydrated lime are thoroughly mixed in the mixer/splitter 30 and then are delivered to opening 16 in the combustor 10.

Controlled gasification of the coal takes place in the combustor 10 by regulation of air flow 40 from air blower 42. Air is introduced to maintain a substoichiometric ratio and particularly at a stoichiometric ratio of air to coal of about 0.55 to 0.80, and most preferably at about 0.60.

Optionally the injection of steam 52 into the combustor 10 is used to enhance sulfur capture. The steam can be added at a 0.1 to 0.3 steam to fuel weight ratio. The syngas leaves the combustor 10 via conduit 54 at a temperature of about 2000 to about 3000 degrees Fahrenheit. The syngas can then be transferred to a boiler for further combustion.

It should be understood that substantial quantities of nitrogen, sulfur, mercury and other components of the coal are captured in molten slag so that they do not contaminate the syngas. Fuel-bound nitrogen which is not captured in the slag forms nitrogen dioxide, NO₂, but not other nitrogen compounds known as NO_X. Sulfur in the coal is captured by the hydrated lime so that it does not form sulfur dioxide, SO₂, in the syngas. Molten slag from the ash portion of the coal plus the inorganic alkali compounds are separated in a cyclone chamber 60, and a molten slag eutectic 34 containing calcium sulfide and other alkali sulfides from reaction with alkali compounds in the coal ash are collected at the bottom opening 36 of the combustor 10. The molten slag is quenched and the ash is sluiced to a settling pond.

Claims

1. A process for gasification of carbon-containing fuel comprising:

heating carbon-containing fuel in a container;

introducing air into the container in controlled quantifies to maintain substoichiometric conditions;

removing syngas from the container; and,

removing slag from the container.

2. The process of claim 1 wherein the carbon-containing fuel is coal.

3. The process of claim 2 wherein air is introduced to maintain a stoichiometric ratio of air to coal of about 0.55 to 0.80 percent.

4. A system for gasification of carbon-containing fuel comprising:

a container;

an introduction system for introducing air into said container in controlled quantities to maintain substoichiometric conditions;

a gas removal system for removing syngas from said container; and,

a slag removal system for removing slag from said container.