GB2187397A - Combustion method - Google Patents

Abstract

In the fluidised bed combustion of a solid fuel, typically of high ash content or low volatile content, a catalyst is employed that promotes the oxidation of carbon monoxide to carbon dioxide. The catalyst may be a Group VIII metal, or Ni, Ti, Cr or V, or a compound thereof. The surface of a non-combustible particulate substance included in the bed may be provided with a coating of the catalyst. <IMAGE>

Description

SPECIFICATION Combustion method This invention relates to a method of combustion. In particular it relates to combustion of fuels in a fluidised bed.

The combustion offuels in a fluidised bed has been practised as follows: a bed of inert particulate or granular material such as sand, ash or limestone is established. The bed isfluidised by air. The temperature in the bed is raised to a chosen value by heating the bed. When the bed has attained the chosen temperature, the substance to be burned is introduced into the bed. The temperature is chosen that it is not so lowthata self-sustaining combustion reaction cannot take place orso high that ash produced in the bed as a resultof the combustion fuses. Typically, the fuel is fed continuously into the bed and there is continuous extraction of ash. If desired, heat may be extracted from the hot gases passing out of the bed orfrom the bed itself and may for example be used to raise steam.

It is known that some fuels are difficu It to burn in a fluidised bed. Into this category come fuels with relatively high contents of non-combustible solids (i.e. above 10% by weight), fuels of low calorificvalue and fuels including relatively low proportions of volatile combustibles (i.e. less than 15% by weight). It has been proposed to improve the combustion of such fuels in fluidised beds by enriching the fluidising air in oxygen.

This increases the partial pressure of oxygen in the bed and thereby accelerates oxidation of carbon to carbon dioxide. Experiments that we have had performed using such oxygen enrichment in the combustion of high ash Arigna coal having on a dry basis an approximate analysis of 52% by weight of ash, 15% byweight ofvoiatile combustible matter and 33% by weight of fixed carbon have shown that oxygen enrichment does little to facilitate combustion. In particular, it is found that the ash which is extracted from the flue gases has a relatively high carbon content. In addition, only a minimal increase in bed temperature is achieved by oxygen enrichment.Similar results are obtained when coke (having an approximate composition of 7% byweight of ash, 11% by weight of volatile matter and 82% by weight of fixed carbon) is burnt. On the other hand, when coal was the fuel, oxygen enrichment of the bed was found to give a relatively substantial increase in temperature and no problem so far as carry over of unburntcarbon in the ash was concerned.

Essentially, ignoring reactions involving hydrogen, when, after combustion of volatiles,the residual carbon content of the fuel is burnt the following chemical reactions take place: C+ 2 = CO2 ... 1 2C+O2 = 2CO ... 2 CO2+C = 2CO ... 3 2CO+O2= 2CO2 ... 4 We believe that difficulties in obtaining complete combustion in the bed are attributable to the presence in the bed of an undesirably large concentration of carbon monoxide as a result of reaction 2 taking place to an undesirable extent.

We further believe that fluidised bed combustion of fuels can be improved by including in the fluidised bed a promoter of the reaction between carbon monoxide and oxygen. The use of such a substance is particularly effective if oxygen or oxygen-enriched air is supplied to the bed in addition to thefluidising air. Particular benefit can be obtained in the examples of the combustion of a solid fuel having a low volatiles content (i.e.

less than 15% by weight of the fuel) and/ora high ash content (i.e. 10% or more by weight ofthefuel).

According to the present invention, there is provided a method of burning a combustible solid comprising the steps of establishing a bed of non-combustible particulate or non-combustible granular material,fluidising the bed with air, raising the bed to a combustion temperature, supplying said fuel to the bed separately from the said granular or particulate material, and extracting any ash from the bed, wherein in that the bed additionally includes a catalyst of the oxidation of carbon monoxide to carbon dioxide, said catalyst providing combustion promoting surfaces available to the gaseous phase in the bed.

Preferably, the said air is enriched in oxygen upstream of the bed or in the bed itself.

Any conventional catalystforthe oxidation of carbon monoxideto carbon dioxide may be employed. Such catalysts are conventionally used as promoters on fluidised catalytic cracking catalysts. The catalyst for use in the invention may, for example, be platinum, palladium, rhodium, iridium, osmium or rhenium, typically, but not necessarily in the form ofthe metal itself or a coordination compound of such metal. As an alternative to such Group 8 elements, other catalysts that can be used in this invention include nickel, titanium, chromium orvanadium as a metal or combined with other element or elements in a compound (for example an oxide or a salt). Unlike catalytic cracking catalysts there is no need to avoid substances such as vanadium which have a strong dehydrogenation effect.Preferably the catalyst is provided in the bed by coating the said granular or particulate substance with such promoter prior to introducing said granular or particulate substance into the vessel in which the fluidised bed is established. If the catalyst is a Group 8 metal, the said granular or particulate substance may be provided at its surface with less than 50 parts per million byweight of the catalyst. It may also be possible to choose as the granularor particulate substance one that naturally contains such a catalyst in sufficient quantities to have a beneficial effect in operation of the fluidised bed.

Alternatively, granules of a promoter such as, say, nickel oxide orchromium oxide may be prepared as the bed material. Assuming a uniform distribution of the catalyst in such a substance, it will be appreciated that considerably more than 50 parts permillion by weight ofthe promoterwill be required in such an example.

Where the catalyst is not naturally contained in said substance, conventional methods can be employed to incorporate the catalyst into or onto the said particulate or granulate substance. Examples of suitablecatalysts and impregnating methods are described in US patent 4072600.

Preferably, commercially pure oxygen is used as the source of the oxygen forthe said enrichment. It is also preferred that the amount of such oxygen that is added is sufficient to increase the concentration of oxygen in the fluidising airto a level in the range 22 to 35% by volume, most preferably in the range 25 to 30% by volume.

It is believed that oxygen enrichment of the fluidising air and provision of the catalyst in the bed both have the effect of increasing the amount of carbon monoxide converted to carbon dioxide in the bed, thereby increasing the amount of heat released per unit quantity of fuel bed into the bed. This makes it possibleto achieve adequate self-sustaining combustion of fuels such as coke which have low proportions of combustible volatiles, and fuels such as Arigna coal which have high proportions of ash.

The fluidised bed is preferably operated at the highest possible temperature consistent with avoiding fusion ofthe said non-combustible granular or particulate substance or fusion of the ash. In this and other details (apartfrom the provision of the promoter in the use of oxygen to enrich the fluidising air) the oper ation of the bed may be conducted in a conventional manner. However, it will generally be possible to operatethe bed with relatively low volumes of excess air, for example 2 to 10% of excess air. This facilitates the recovery of heat from the flue gases.

The method according to the present invention will now be described by way of example with referenceto the accompanying drawing which is a schematic diagram of an experimental fluidised bed combustion apparatus.

Referring to the drawing, combustion vessel 2 generally square cross-section has towards its base a hori zontally disposed grate 4through which in operation of the apparatus air is provided. Fluidising air isprovided from blower 6 to a plenum chamber8 bounded by the grate 4 and the walls of the vessels 2, there being a pipeline 10 placing the blower 6 in communication with the chamber 8. A pipe 12 terminates in the pipeline 10 and communicates with a source ofcommercially pure oxygen.

In order to establish the fluidised bed in the vessel 2, a screw feeder 14 is employed to feed into the vessel 2 sufficient number of particles or granules of non-combustible materials such as silica to enabie a bed of chosen depth to be established. The silica particles typically have an average size greaterthan 6 mm and are provided at their surfaces with a coating of a promoterofthe oxidation of carbon monoxide to carbon dioxide. The screw feeder 14 may also be employed during operation of the apparatus shown in the drawing to feed solid fule to be burnt into the bed. If fuel particles to be burnt have a size less than 6 mm then a pneumatic feeding device shown generally by reference numberal 16may be employed forthis purpose.

Generally, we prefer not to burn particles of less than 6 mm in size. The use of oxygen enrichment helpsto meet thins objective even with fuels that are difficult to burn. In orderto extract heat from thefluidised bed during operation ofthe apparatus shown in the drawing, a steam raising coil 18 is located in that region ofthe vessel 2 where the bed is established. The coil 18 has an inletforwater and an outlet for steam. The vessel 2 is also provided with a start-up burner 20. Air and gas may be provided to this burner so as to raisethetem- perature ofthe bed to a desired operating temperature. Once this has been achieved, operation of the burner 20 may be discontinued.

The vessel near its top is provided with an outlet 22 of the flue gas. The outlet 22 communicates with the cyclone 24 which is adapted to extractfrom the fuel gas ash formed by combustion of a solid fuel in the fluidised bed. The ash is collected in a bin 26 while the thus cleansed flue gases are exhausted through a stack 28.

In operation of the apparatus shown in the drawing, a fluidised bed 30 of silica coated or otherwise provided with a catalyst in accordance with the invention is established in the vessel 2 by passing fluidised air6 into the vessel at a chosen rate and feeding a chosen quantity of the silica into the vessel 2 from the screw feeder 14. Once the bed is established, the burner 20 is operated to raisethetemperature to a chosen value, say, in the range 800 to 85Q"C. Water is passed through the steam raising coil 18 and steam is extracted therefrom. When the bed has reached a temperature of 800"C, particulate fuel is fed from the screw feeder 14 into the bed. At the same time, the air in the pipelines 10 is enriched with oxygen by a chosen amount.A self-sustaining combustion reaction takes place in the bed, being in part catalysed by the catalyst provided on the surface of the particles of non-combustible material. The flue gases and unreacted part ofthefluidising air (mainly nitrogen) pass upwardly through the vessel 2 and leave itvia the outer 22. These gaseswill include entrained therein ash formed as a result of the combustion. They will also contain a small amountof othersolidsthat elutriatefrom the bed. Preferably, thefluidising velocity is selected to as to keep such elutriation to a minimum. The ash and other solids are collected in the bin 26 and may be analysed fortheir carbon content. It is an advantage ofthe method according to the present inventionthat by employing a promoter ofthe combustion of carbon monoxide such ash will contain a lower proportion of residual carbon than ifthe bed is operated conventionally without the use of such promotor and without the use of oxygen enrichmentofthefluidising air.

One possible consequence of the operation of the bed with oxygen-enriched fluidising air is that the propensity of oxides of sulphur and nitrogen to be formed in the bed is increased. Such propensity can be counteracted by injecting into the bed a gaseous reducing agent such as methane or ammonia that will reactwith the oxides of nitrogen or oxides of sulphur.

If desired, the fluidised bed combustion method according to the invention may be employed in the gasification of solid fuels.

The invention is further illustrated bythefollowing examples.

Example 1 A bed of 250g of hot material (e.g. silica) was fluidised by air and heated to a temperature of 7500C. 5g of foundry coke (fixed carbon 81.9% by weight; volatiles 11.3% by weight; and ash 6.8% by weight) were mixed into the bed. It was found that ittook29 minutes two burn outthecoke, during which period a maximum temperature of 785"C and a maximum carbon dioxide concentration of 5.55% in the gas mixture leaving the bed were obtained.

Example2 The procedure of Example 1 was repeated save that in addition to mixing 59 of coke with the bed, 1 5g of fluidised catalytic cracking (FCC) catalyst, comprising a platinum-impregnated zeolite available under designation Crosfield CCZ-1 1 from Joseph Crosfield and Sons was also mixed into the bed. It was found that it took22 minutes to burn outthecoke, during which period a maximum temperature of 802"C and a maximum carbon dioxide concentration of 6.75% in the gas mixture leaving the bed were obtained.

Claims (10)

1. A method of burning a combustible solid comprising the steps of establishing a bed of noncombustible particulate or non-combustible granular material, fluidising the bed with air, raising the bed to a combustion temperature, supplying said fuel to the bed separately from the said granular or particulate material, the extracting ash from the bed, wherein the bed additionally includes a catalyst of the oxidation of carbon monoxide to carbon dioxide, said catalyst providing combustion promoting surfaces available to the gaseous phase in the bed.

2. A method as claimed in claim 1, in which the air is enriched in oxygen.

3. A method as claimed in claim 1 or claim 2 in which the catalyst comprises a Group VIII metal (or compound thereof).

4. A method as claimed in claim 1 or claim 2, in which the catalyst comprises nickel, chromium orvanadium (or compound thereof).

5. A method as claimed in any one of the preceding claims in which the catalyst is provided at the surface of said non-combustible substance.

6. A method as claimed in claim 1,2, or 4 in which the said non-combustible substance consists of a substancethat is said catalyst.

7. A method as claimed in any one of the preceding claims, in which the combustible solid has an ash content of at least 10% by weight of the fuel.

8. A method as claimed in any one ofthe preceding claims, in which the combustible solid has a content of volatile matter of less than 15% by weight of the fuel.

9. A method of burning a combustible substance substantially as herein described with reference to the accompanying drawing.

10. A method of burning a combustible substance substantially as herein described in Example 2.