GB2120118A - Fluidized bed gasification using bed material containing a calcium compound and silica - Google Patents

Abstract

In the gasification of a sulphur- containing fuel in a fluidized bed, bed material is used which contains a calcium compound and silica. The calcium compound reacts with the sulphur generated as a result of the gasification of fuel introduced to the bed (22), and the presence of silica permits the gasification process to be carried out at higher temperatures than would be possible using a calcium compound alone.

Description

SPECIFICATION Method of gasification utilizing a bed material containing calcium and silica This invention relates to the gasification of a sulphur containing fuel and, more particularly, to such a method in which the gasification takes place in a fluidized bed of particulate material.

The Environmentai Protection Agency of the United States and various other state agencies have established standards of performance that define maximum allowable sulphur dioxide emission levels for fossil fueled power stations. In response to these standards, a generation of stack gas clean up equipment has been designed to remove or scrub, sulphur dioxide from the steam generator flue gases prior to release into the atmosphere.

Since large volumes of gas with dilute sulphur dioxide concentrations are encountered at the steam generator exit, the stack gas clean up equipment becomes large and expensive.

Instead of controlling sulphur dioxide emissions by treating the stack gases it is advantageous to remove sulphur from the fuel prior to combustion in the steam generator, since at this stage the volume of gases requiring treatment is significantly reduced. To this end a gasification process has evolved that involves the partial combustion of fuel, such as heavy fuel oil or particulate coal, in a fluidized bed of lime particles. Desulphurization is accomplished through reaction with the lime particles and a combustible off-gas is produced that is used in external apparatus such as a steam generator where combustion is completed in commercially available gas burners.

In these systems the maximum sulphur removal efficiency is obtained if the fluidized bed temperature is controlled in the range of 81 5,C to 870"C (1 500 F to 1 600 F), since at temperatures in excess of 870"C (1 600 F), the sulphur retention ability of the lime falls off sharply. However, this relatively low temperature limits the efficiency of the gasification system process to the extent that it is difficult to operate at commercially practical rates.

It is an aim of the present invention to provide a gasification method in which a chemically active fluidized bed is provided for producing a product gas substantially free of sulphur and at good efficiency levels.

We have found that a gasification process of the above type may be carried out at higher temperatures, thereby increasing the gasification efficiency but without compromising the sulphur removal, if the bed material additionally includes silica. Accordingly, the present invention provides a method of gasifying a sulphur-containing fuel comprising establishing a bed of material containing calcium and silica, and introducing the sulphur-containing fuel thereto; passing air through the bed to fluidize same and promote the combustion of the fuel therein; regulating the introduction of fuel and passage of air so that the fuel gasifies to release a sulphur-containing gas; selecting the quantity of calcium in the bed so that it reacts with said sulphur to remove a substantial portion of sulphur from said gas; and selecting the quantity of silica in the bed to enable gasification to take place at higher temperatures without a reduction in sulphur removal than would be possible without such quantity of silica. The calcium reacts with the sulphur from the product gas, and the presence of silica in sufficient quantity enables the temperature in the bed to be maintained at temperatures in excess of 87"C (1600"F) without equivalent loss in gasification efficiency. A typical bed temperature in methods of the invention is around 98"C (1800"F).

The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a sectionai view of a gasifier utilized in the present invention; and Figure 2 is a cross-sectional view taken along the like 2-2 of Fig. 1.

In the drawings a gasifier is shown in general by the reference numeral 10. The gasifier 10 includes a perforated floor, or grate 12, a front wall 14, a rear wall 1 6 and two sidewalls 17, (Fig. 2) extending between the grate and a roof 18. A vertical partition 20 extends between the front wall 1 4 and the rear wall 1 6 to divide the interior of the gasifier into a gasifier section 22 and a regenerator section 24. The floor 12, the walls 14 and 16, the roof 1 8 and the partition 20 are all insulated in a conventional manner.

An air duct 26 extends below the floor 1 2 and has an inlet 28 for receiving air from an external source. A plurality of T-shaped air distributor pipe assemblies 30 extend through the perforations in the floor 1 2 and receive air from the duct 26 and introduce the air into the gasifying section 22 and into the regenerating section 24. As better shown in Fig. 2, each pipe assembly 30 includes a vertical pipe 30a which extends through an opening in the floor 1 2 and a horizontal pipe 30b connected in registry with the vertical pipe for receiving air from the vertical pipe. Although not shown in the drawings, the horizontal pipes 30b have a series of perforations formed in their upper surfaces for distributing the air into the sections 22 and 24.

A plurality of fuel distributor pipe assemblies 32 extend through other perforations in the floor 1 2 below the gasifying section 24 with each assembly including a horizontal pipe 32a extending below the floor 1 2 and a vertical pipe 32b extending through a perforation in the floor and connected in registry with the vertical pipe. An end portion of each horizontal pipe 32a extends through a respec tive sidewall 1 7 and is adapted to be connected to a source of fuel (not shown) which could be oil or particulate coal.

A feeder 34 extends through a sidewall 1 7 and is adapted to feed a particulate material containing calcium and silica into the gasifying section 22. As an example, the material can be in the form of portland cement having the following composition by weight: CaO 60-65% Awl203 6% Fe203 3% SiO2 21-26% Na2O 1% SO3 2% MgO 2% The presence of the silica in the above composition produces the unexpected result of permitting the gasification process to proceed at higher temperatures without a reduction in sulphur recovery than is possible using a composition that does not include silica.

It is understood that the above composition is not limited to the particular configuration set forth, but can vary as long as, for a given quantity of calcium needed to react with the sulphur to remove predetermined quantities thereof from the hydrogen sulphur gas, enough silica is present to enable the gasification process to proceed at an elevated temperature without any substantial reduction in sulphur recovery when compared to similar processes utilizing only lime or limestone. As an example of an alternative composition, the particulate material could consist of a mixture of calcium and silica in substantially the same proportions as set forth above.

Before introduction into the gasifier section 22, the material containing the calcium and silica is preferably formed into hardened spheres or pellets, such as by introducing water to the material and utilizing a rotating disc, a screen, or the like. This increases the handleability of the material and thus adds to the efficiency of the process.

A divider wall 36 is disposed in the gasifying section 22 to divide the latter section into chambers 22a and 22b (Fig. 2). The divider wall 36 extends from the partition 20 (Fig. 1) to an area spaced from the rear wall 1 6 to define a passage 22c communicating with the chambers 22a and 22b.

An inlet slot 38 and an outlet slot 40 are formed in the partition 20 with the former communicating the chamber 22a with the regenerating section 24, and the latter communicating the chamber 22b with the regenerating section.

As a result of this arrangement, the particulate material introduced into the gasifier section 22 via the feeder 34 and a fuel, such as oil, introduced into the gasifier section via the pipe assemblies 32, mix and continually flow from the chamber 22b, around the passage 22c through the chamber 22a and the slot 38 and into the regenerating section 24; and, from the latter section, through the slot 40 and into the chamber 22b for recirculation.

A pair of outlets 42 and 44 are formed in the roof 1 8 in communication with the upper portions of the gasifier section 22 and the regenerating section 24, respectively, to dis- charge the product gas and the sulphur gas produced in the respective sections.

In operation, a quantity of the particulate material is placed in the gasifier section 22, and the temperature in the bed is maintained at approximately 980"C (1800"F) by control of the fuel entering the beds from the fuel distributor pipe assemblies 32. Air from the duct 26 is admitted into the gasifier section 22 through the air distributor pipe assemblies 30 in substoichiometric proportions to fluidize the bed material and to limit the amount of combustion and heat release; while flue gas is used as an inert, heat absorbing medium to control the overall process temperature.

The fuel and air inputs are regulated so that partial combustion of the fuel in the gasifying section 22 with approximately 25 to 30% stoichiometric air occurs which furnishes sufficient heat to partially combust the fuel, and, when applicable, to vaporize and crack the remaining oil. This partial combustion results in the formation of hydrogen sulfide which reacts with the calcium in the bed material to form calcium sulfide. The gaseous product of this process is an essentially sulphur free and vanadium free fuel gas possessing a heating value of approximately 1 78 IT keal/ms (200 BTU/ft3). This gas rises in the gasifying section 22 by natural convection and exits from the gasifier through the outlet 42 for passage to external equipment such as burners for a steam generator, or the like.

Air from the duct 26 is also admitted into the regenerating section 24 through the pipe assemblies 30 in quantities to produce an oxygen-rich environment and the calcium su Bfide formed in the gasifying section 22 is circulated through the regenerating section 24, as discussed above. The regenerating section is maintained at a predetermined elevated temperature, such as 1 040 C (1 900'F), to convert the calcium sulphide to calcium oxide while producing an off-gas with a high sulphur dioxide concentration, in accordance with the following reaction: CaSO4 + CaS + 022CaO + 2SO2 The sulphur dioxide formed by the above reaction discharges from the regenerating section 24 through the outlet 44, and can be recovered by external equipment from the gas stream in the form of elemental sulphur, while the calcium oxide is recirculated back to the gasifying section 22 for reuse as a sulphur absorbent.

The capacity for sulphur retention in the gasifying section 22 is maintained by the continuous removal of the spent bed material through a drain pipe, or the like, (not shown) and the replenishing of the bed material through the feeder 34.

The amount of particulate material needed will vary depending on the quantities of sulphur that are to be removed from the gas.

This can be controlled by varying the input of the particulate material to the bed and the output of spent material from the bed.

As a result of the foregoing, the calcium present in the bed material enables a substantially sulphur-free product gas to be produced in an efficient manner while the silica enables the gasifying process to be carried out at a relatively high temperature compatible with increased gasifier efficiency, without any substantial reduction in sulphur recovery.

It is understood that several variations can be made in the foregoing without departing from the scope of the invention. For example, the fuel is not limited to oil, but can be in the form of other fuels such as particulate coal which would be introduced to the gasifier in a stream of air.

Claims (8)

1. A method of gasifying a sulphur-containing fuel comprising establishing a bed of material containing calcium and silica, and introducing the sulphur-containing fuel thereto; passing air through the bed to fluidize same and promote the combustion of the fuel therein; regulating the introduction of fuel and passage of air so that the fuel gasifies to release a sulphur-containing gas; selecting the quantity of calcium in the bed so that it reacts with said sulphur to remove a substantial portion of sulphur from said gas; and selecting the quantity of silica in the bed to enable gasification to take place at higher temperatures without a reduction in sulphur removal than would be possible without such quantity of silica.

2. A method according to Claim 1 wherein the percentage of weight of calcium in the bed material is in the range 60% to 65%.

3. A method according to Claim 1 or Claim 2 wherein the percentage of weight of silica in the bed material is in the range of 21% to 26%.

4. A method according to any preceding Claim including the step of forming the bed material into hardened spheres or pellets before establishing the bed therewith.

5. A method according to any preceding Claim wherein the temperature of the bed is maintained in excess of 870"C (1600to).

6. A method according to Claim 5 wherein the temperatures of the bed is maintained at approximately 980"C (1800 F).

7. A method according to any preceding Claim wherein the quantity of air passed to the bed is in substoichiometric proportions to limit the combustion and heat release therein

8. A method of gasifying a sulphur containing fuel substantially as herein described with reference to the accompanying drawing.