GB2115537A - Shaft furnace for producing combustible gas from organic material - Google Patents

Abstract

Two separate supplies (13, 14) of air or other gasifying medium are provided for a shaft furnace used for the continuous production of combustible gas from organic material, especially organic waste. The material is fed in at the top of the furnace and descends under gravity as a bed of material (2). This bed is supported by a conical delivery device (3) which is cooled and rotatably mounted. The material is first dried, and then degassed with a primary supply (13) of gasifying medium. A secondary supply (14) is introduced 25 to 30 cm above a delivery gap (5) between the delivery device (3) and the furnace wall (6). A fire bed (16) forms above this delivery gap (5) and serves to crack the higher molecular weight components of the gas produced, which is drawn off through a duct (11). The two air supplies have independent regulators (15, 15a) and the secondary supply (14) is regulated in dependence upon the fire bed (16) temperature. <IMAGE>

Description

SPECIFICATION Shaft furnace for producing combustible gas from organic material The invention relates to a shaft furnace for producing combustible gas from organic material, especially but not necessarily from organic wastes. The invention is especially applicable to a furnace serving for continuous production of combustible gas. The furnace has a provision for charging the material, situated at the top of the shaft furnace and in use has a heap or bed of material in the shaft, in which bed the organic material descends under the action of gravity. The heap or bed of material is supported by a delivery device conically tapering towards the bed of material and centrally disposed in the shaft at the ash discharge region of the shaft furnace. This delivery device is cooled and rotatably mounted.

the shaft furnace comprises means for giving up heat for drying and subsequently degassing the material in an upper region of the said bed of material. It also has ducting for the supply of gasifying medium, which opens above a fire bed in a lower region of the heap or bed of material charged to the furnace. The throughflow of gasifying medium can be adjusted by a regulator controlled in dependence upon the temperature in the fire bed. There is provided at the lower edge of the delivery device a delivery gap, the width of which has to be determined in dependence upon the iumpiness developing in the material in the fire bed. Connected to the ash discharge region of the shaft furnace is a combustible-gas duct through which the combustible gas produced in the shaft furnace is discharged.

It is known to produce combustible gas from wood, coal or coke. The combustible gas is produced by incomplete combustion of the material in a shaft furnace into which air, oxygen and/or water vapour is introduced as gasifying medium. The combustible gas produced comprises as combustible gas components substantially carbon monoxide and hydrogen, as well as a small I proportion of methane. the gasproducing process is endothermic. It is desirable to carry out the formation of the producer gas at the highest possible temperatures in order to obtain high proportions of carbon monoxide and hydrogen.

With continuous delivery of material from the shaft furnace, it is known to introduce the gasifying medium into the shaft furnace from below, in order that the fuel gas formed may be drawn off from the top of the shaft furnace. With this procedure, the combustible gas contains considerable proportions of tar.

If the flow takes place in the reverse direction, i.e. if the gasifying medium flows through the bed of material in the direction in which the solid material travels and the combustible gas is withdrawn from the bottom of the shaft furnace, tarry and oily constituents formed in the gasification of the material are extensively cracked into volatile compounds as low temperature carbonisation gas passes through.

The breaking-up of high-molecular-weight compounds is temperature-dependent. The higher the temperature of the fire bed zones through which the gases formed are to pass, the higher is the calorific value of the combustible gases which can be extracted from the shaff furnace.

It has been established that the quality of combustible gas produced in a shaft furnace decreases in proportion to the magnitude of the temperature gradient within the bed of material, between the tempertaure of the material in the inner shaft region and the temperature of the material in the region close to the wall. Only a part of the gases withdrawn from the bottom have then flowed through the zones of highest temperature. The remainder of the gas is only incompletely cracked.

The object of the invention is to provide a shaft furnace for producing combustible gas, from which a combustible gas of high quality can be extracted, even when the temperature differences which must be accepted over the cross-section fo the shaft furnace within the gasification zone after addition of the gasifying medium are such that a uniform treatment of all the gases formed in the bed of material becomes difficult Broadly, the invention provides primary and secondary supply paths for gasifying medium, leading to different places, and with the flow in one regulatable independently of the other.

According to this invention there is provided a shaft furnace for continuously producing combustible gas from organic material, with provision for material charging at the top of the shaft furnace and having a cooled, rotatably mounted delivery device which tapers conically upwards and is disposed centrally in the shaft at the ash discharge region of the shaft furnace so that in use a bed of material forms in the shaft, supported by the delivery device which tapers towards the bed, in which bed the organic material progresses downwardly under gravity, the furnace further having:: means for giving up heat to dry and subsequently degas the material in an upper region of the bed; a delivery gap at the lower edge of the delivery device; an outlet duct for the combustible gas produced, connected to the ash discharge region of the shaft furnace; a primary supply path for gasifying medium opening into the said bed of material, proximate the tip of the conical delivery device; and a secondary supply path for gasifying medium opening above, but in the region of the delivery gap, with flow through one of the supply paths being regulatable independently of flow through the other.

By division of the quantity of gasifying medium to be supplied into a primary gas stream introduced into the bed of a material at the cone head of the delivery device and a secondary gas stream which is introduced into the bed of material in the region of the delivery gap, the result is obtained that a fire bed at high temperature is formed immediately before the material to be degassed escapes through the delivery gap. The secondary gas stream is preferably introduced into the bed of material at a distance of 25 to 30 cm above the delivery gap.

The gases escaping through the delivery gap in the direction of transport of the material are forced to flow through the fire bed formed, so that a uniform treatment and substantial cracking of all high-molecular-weight substances in the combustible gas is achieved. Tarry and oily constituents are converted into volatile compounds. By producing the fire bed immediately before the point at which the material escapes into the ash space, an homogenisation of the waste substance leaving the shaft furnace is achieved. This facilitates the adjustment of the width of the delivery gap in dependence upon the lumpiness of the gasified material to be discharged from the fire bed.

The furnace may also have at least one regulator for controlling the through flow of gasifying medium through a said supply path, preferably the secondary supply path in dependence upon the temperature in the fire bed.

In order to achieve a uniform distribution of the secondary stream of gasifying medium in the fire bed, it is preferable that a secondary supply duct opens into a distributor channel which is situated at the base of the conical delivery device and which is formed with apertures distributed around the periphery of the base for the introduction of gasifying medium into the fire bed. Such apertures are preferably of slit form.

A variant of the construction of the distributor channel which facilitates the supply of the stream of gasifying medium consists in that the distributor channel is formed of a number of gas ducts extending radially below the cone of the delivery device. In order that the heat of the escaping combustible gas may be utilised for a preheating of the gasifying medium, wall regions of the gas ducts are constructed as heatconducting walls for heat exchange between gasifying medium and combustible gas flowing in the ash delivery region.

In the following, embodiments of the invention will be described by way of example with reference to the drawings, in which: Figure 1 diagrammatically illustrates a shaft furnace having a secondary supply of gasifying medium via the conical delivery device; and Figure 2 diagrammatically illustrates a shaft furnace having a secondary supply of gasifying medium in the region of the shaft wall.

Figures 1 and 2 each show shaft furnaces 1 to which organic substances to be gasified, more particularly organic wastes, can be supplied. The material which is charged to the furnace forms in the interior of each shaft furnace a bed of material 2. This bed is a randomly packed heap of the material in bulk, charged to fall on top of any material already present. The material moves downwards in the shaft furnaces under the action of gravity. It is warmed in the upper region (not shown in the drawings) of each shaft furnace, dried and heated to about 2000C.

There is introduced into the shaft furnaces 1 from below a rotatably mounted delivery device 3 which supports the bed of material 2 charged to the furnace. The delivery device 3 is disposed centrally in the shaft and tapers conically towards the bed of material 2 oppositely to the direction of travel of this material. Situated at the base 4 of the conical delivery device is a delivery gap 5 which is an annular gap between the edge of the base 4 of the device 3, and the shaft furnace wall 6. The degassed material falls through the delivery gap 5 into an ash discharge space 7 and on to an ash bed 8. During rotation of the delivery device 3, and ash is positively transported from the bed 8 to an ash chute 10, by means of blades 9 which are secured to the underside of the delivery device 3 and extend to the bottom of the ash bed 8.The ash chute opens into an ash box (not shown in the drawings) suitable for the intermediate, i.e. temporary storage of the ash.

Also connected to the ash discharge space 7 is a suction duct 11 for the combustible gas formed in the shaft furnace. the gases formed in the bed of material 2 flow downwards in the shaft furnace in the direction of travel of the material, and after conversion by reaction in the fire bed, are passed as combustible gases through the suction duct 11, to a combustion chamber, optionally by way of an intermediate store. The intermediate store and the combustion chamber are not illustrated in the drawings.

A primary supply duct 12 for a primary gas stream of the gasifying medium extends centrally through the delivery device 3. The primary gas stream is indicated by chain-lined flow arrows in the drawings. The primary supply duct 12 leads into the shaft furnace above a gasification zone 13 formed in tyd bed of material 2. In the gasification zone, part of the low-temperature carbonisation gas formed in the preheating zone in the shaft furnace is burned with a supply of gasifying medium, and the temperature in the bed of material is increased. The organic waste material is gasified at a temperature in the range between 4000 and 10000C.

In the embodiment of Figure 1, a secondary supply duct 14 carries a secondary gas stream of the gasifying medium to the base 4 of the conical delivery device 3, below the gasification zone 13.

This is indicated by solid-lined flow arrows. The secondary gas stream is introduced into the bed of material 2 at a distance of 25 to 30 cm above the delivery gap 5. The quantity of gasifying medium flowing in the secondary gas duct 14 can be adjusted by means of a flow regulator 15, independently of the primary gas stream of the gasifying medium. The latter stream is regulated by the flow regulator 1 5a. The quantity of secondary gasifying medium to be supplied is controlled in response to a temperature which is set up in the fire bed 16 above the delivery gap 5.

The quantity of gasifying medium is so adjusted that the temperature of the fire bed is sufficient for the cracking of the tarry and oily constituents of the gas formed in the bed of material 2.

In the embodiment of Figure 2, a secondary supply duct 1 7 formed with apertures 17' and extending to the wall 6 of the shaft furnace opens into the bed of material in the shaft furnace, at a point above the fire bed 16. This secondary supply duct is also connected to a flow regulator 1 8 by means of which it is possible to control the quantity of secondary gasifying medium separately from the primary gas flow. Again control is in dependence upon the temperature in the fire bed 1 6. The apertures 17' also open into the shaft furnace at a distance of 25 to 30 cm above the delivery gap 5. It has been found that this distance between the supply of secondary gasifying medium and the delivery gap ensures a formation of the fire bed in accordance with the requirements and ensures readily managable regulation of the fire bed temperature.

Furthermore it does so independently of the other dimensions of the shaft furnace, more particularly independently of the diameter of the shaft furnace and the opening angle of the conical delivery device 3.

In the production of combustible gas, the gases formed in the bed of material 2 in the shaft furnace during the drying and degassing of the material initially flow through the gasification zone 1 3. This zone, the extent of which is diagrammatically indicated in the drawings, exhibits a temperature gradient from the inside towards the outside. At the point of entry of the primary gas stream of the gasifying medium at the cone head 1 9 of the delivery device 3, the material is heated to about 1 0000C, while in the wall region of the shaft furnace temperatures in the range between 6000 and 7000 are reached, depending upon the cross-sectional area of the shaft. Only part of the gas formed in the bed of material 2 is extensively cracked.This is the part which flows through that part of the gasification zone 13 which is a fire zone in the 900-1 0000C temperature range. The other part of the gases initially remains incompletely decomposed. Only in the fire bed 16, through which all of the gas is passed before it leaves by way of the delivery gap 5, are the high-molecular-weight constituents of this part of the gas also cracked. The introduction of the secondary gas stream into the bed of material and the resultant development of the fire bed 16, the extent of which is diagrammatically illustrated in the drawings in the same way as the extent of the gasification zone 13, thus results in a high quality of the combustible gas withdrawn from the shaft furnace.

In addition to high quality of the combustible gas, the introduction of the secondary gas stream and the formation of the fire bed 16 bring about at the same time an homogenisation of the processed material. This facilitates the adjustment of the width of the delivery gap 5, which is determined in dependence upon the lumpiness of the material which develops in the fire bed 16, in the lower region of the bed 2, and upon the necessary throughput of material. For the gasification of fine material, for example of rice husks, a gap width between the edge of the base 4 and the wall 6 of the shaft furnace of about 5 to 20 mm is appropriate. The bed material forms a structure having a sieve-like action in the fire bed 1 6 so that only sufficiently small particles of material which have been substantially degassed are delivered through the delivery gap 5.

The maximum temperature set up in the fire bed 1 6 is preferably in the range between 9000 and 1 0000C. A temperature sensor 20 introduced into the bed of material 2 measures a reference temperature in the shaft furnace above the fire bed 1 6 which varies analogously to the reaction temperature in the fire bed. In the embodiment illustrated in Figure 1, the temperature sensor 20 is operatively connected to the flow regulator 1 5 of the secondary supply duct 14 for the gasifying medium.The throughflow of the gasifying medium in the secondary supply duct 14 may alternatively be regulated in another manner, for example by varying the suction pressure in the suction duct 11 or in combination with the flow regulator 1 5 and adjustment of the negative pressure in the ash discharge space 7. When the desired temperature in the fire bed 1 6 is exceeded, the supply of gasifying medium is cut down, and when the temperature falls the flux of air or oxygen is increased. the temperature in the fire bed 1 6 may alternatively be regulated by the composition of the gasifying agent.With a metered supply of air, oxygen, water vapour or carbon dioxide, a reliable and-owing to the direct supply of the gasifying medium into the bed of material 2 above the fire bed 1 6-also rapidly reacting control of the gasifying process can be achieved.

In the embodiment illustrated in Figure 2, a temperature sensor 21 is operatively connected to the flow regulator 1 8 for the secondary supply duct 1 7. The variants indicated in the preceding paragraph for the regulation of the temperature of the fire bed, i.e. variation of the suction pressure of metering of the components of the gasifying agent, apply in like manner to the embodiment illustrated in Figure 2.

As shown in sectional view at the left hand side of Figure 1, the secondary supply duct 14 of the shaft furnace opens into a distributor channel 23 which is disposed at the base 4 of the conical delivery device and which is formed with slit-like apertures 24 distributed around the periphery of the base for the introduction of gasifying medium.

An alternative to this construction is diagrammatically shown in section at the right hand side of Figure 1. For the introduction of the secondary gas stream into the fire bed 16, a number of gas ducts 25 are provided, extending radially in the lower part of the conical delivery device 3, instead of the distributor channel 23 shown in the left-hand sectional view of the shaft furnace. The gas ducts 25 desirably consist of channels of rectangular form having identical cross-sections, which extend from the orifice of the secondary supply duct 14 to the edge of the base 4 of the delivery device 3. In the gas ducts 25, the secondary gas stream of the gasifying medium is guided along the bottom 26 of the delivery device 3.The bottom 26 forms a wall region of the gas ducts 25 which serves as a heatconducting wall for heat exchange between the hot combustible gas flowing in the ash discharge space 7 and the secondary gasifying medium. The gasifying medium is preheated in the gas ducts 25 in counter-current to the combustible gas before entering the bed of material 2.

The primary gasifying medium flowing through the delivery device 3 acts as a coolant and prevents overheating of the cone of the delivery device. At the same time, the primary gasifying medium is warmed up before entering the bed of material 2. A covering 27 on the cone head 19 prevents the penetration of material from the bed 2 into the primary supply duct. The delivery device 3, the primary supply duct 12 and the secondary supply duct 14 in the embodiment illustrated in Figure 1, and the delivery device 3 with the primary supply duct 12 in the embodiment illustrated in Figure 2, form in common, in each case, a unit rotatable about an axis 28. The motor and the gearing for the drive of this unit are not shown in the drawings. In the illustrated embodiment, the rotation of the unit takes place stepwise.

In a shaft furnace of the kind diagrammatically illustrated in Figure 1, having a diameter of 260 mm and a shaft height of 1 m, chopped straw was gasified. The straw fragments had on average a length of about 10 cm. Up to 10% by weight of wood pieces were added to the straw, as is preferred. In the shaft furnace, the introduced material was heated to about 2000C in heat exchange with combustible gas produced. Air was introduced into the bed of material 2 as primary and secondary streams of gasifying medium. The reaction zone between the cone head 1 9 and the fire bed 16 was about 350 mm long. The highest temperature in the fire bed 1 6 was 10000C. The delivery device was rotated stepwise at intervals of about 2 minutes, about 2 to 10 revolutions being completed in 1 hour.With a throughput of 10 kg per hour of material to be gasified, it was possible to produce from 30 to 40 cubic metres of combustible gas per hour.

A further shaft furnace of the type illustrated in Figure 1, having a diameter of 400 mm and a height of 1.5 m served for the gasification of sawdust, which was also mixed with pieces of wood before being introduced into the shaft furnace. The pieces of wood had various sizes up to 1 50 mm long blocks or logs. The reaction zone between the cone head 19 and the fire bed 1 6 in the case of this shaft furnace was 450 mm long and the maximum temperature in the fire bed 1 6 was 10000 C. The delivery device performed one complete revolution, also in steps, 5 times per hour. The interval time between steps was about 4 minutes. With a throughput of 30 kg per hour of material to be gasified, about 100 cubic metres of combustible gas were produced per hour.

On average, the combustible gas produced in the aforesaid shaft furnaces had the following composition: 2% by volume of CH4,15% by volume of H2,25% by volume of CO, 5% by volume of CO2,53% by volume of N2. The calorific value of the combustible gas was in the range of 5000 to 6000 kilo-joules per cubic metre.

The gasifying process can be optimised by controlling the supply of primary and secondary gasifying medium and regulating the discharge of material. The gases set up in the bed of material 2 react with the gasifying medium and, on introduction of the secondary gas stream, form directly above the delivery gap a fire bed through which the low-temperature carbonisation gases and the remaining gas components are forced to pass. The high-molecular-weight components of the gas are thereby cracked. By division of the supply of gasifying medium into a primary gas stream and a secondary gas stream, it is possible to control the extent of the reaction volume in the bed of material 2 and hence the quantity of combustible gas to be produced. The maximum quantity of primary gasifying medium to be supplied is determined by the desired capacity of the shaft furnace.The quantity of the secondary gas stream is controlled in dependence upon the temperature which is necessary in the fire bed 1 6 for splitting up the gas components. When the amount of combustible gas required is low, the supply of secondary gasifying medium is sufficient. The reaction volume in the shaft furnace is then confined to the fire bed immediately above the delivery gap. The range of regulation for the quantity of combustible gas to be produced is about 5:1 with constant combustible gas quality.

For different materials, the cone form and the gap width of the delivery device, as well as the reaction zone are adapted. It is possible in one and the same shaft furnace to feed in materials of the most varied structure and differing gasification behaviourforthe production of combustible gas. Adaptation to different materials is preferably accomplished simply by exchanging the delivery device.

For drying the material introduced into the shaft furnace through material charging means (not illustrated in the drawings) at least a part of the hot combustible gas flowing away through the suction duct 11 is used as heating medium in the preheating zone. For this purpose the upper region of the shaft furnace is preferably made in double-walled form. The combustible gas is introduced into the space between the two walls and exchanges its entrained heat in countercurrent to the waste material descending in the bed of material 2 in the shaft furnace. With this arrangement for drying and degassing, a recovery of heat takes place, which is especially significant when the combustible gas is to be intermediately stored before it is used as a heating medium.

Claims (9)

Claims

1. A shaft furnace for continuously producing combustible gas for organic material, with provision for material charging at the top of the shaft furnace and having a cooled, rotatably mounted delivery device which tapers conically upwards and is disposed centrally in the shaft at the ash discharge region of the shaft furnace so that in use a bed of material forms in the shaft, supported by the delivery device which tapers towards the bed, in which bed the organic material progresses downwardly under gravity, the furnace further having:: means for giving up heat to dry and subsequently degas the material in an upper region of the bed; a delivery gap at the lower edge of the delivery device; an outlet duct for the combustible gas produced, connected to the ash discharge region of the shaft furnace; a primary supply path for gasifying medium opening into the said bed of material, proximate the tip of the conical delivery device; and a secondary supply path for gasifying medium opening above, but in the region of the delivery gap, with flow through one of the supply paths being regulatable independently of flow through the other.

2. A shaft furnace according to claim 1 also having a regulator for controlling the flow of gasifying medium through at least one of the said supply paths, in dependence upon the temperature in a fire bed which forms in use in a lower region of the said bed of material, above the delivery gap and below the opening of at least the primary supply duct.

3. A shaft furnace according to claim 1 also having a regulator for controlling the flow of gasifying medium through the said secondary supply duct, independently of flow through the primary supply duct and in dependence upon the temperature in a fire bed which forms in use in a lower region of the said bed of material, above the delivery gap and below the opening of at least the primary supply duct.

4. Shaft furnace according to Claim 1 Claim 2 or Claim 3 wherein the secondary supply path comprises a duct and a distributor channel into which the duct opens, which is situated at the base of the conical delivery device and is formed with apertures distributed around the periphery of the base for the introduction of gasifying medium into a fire bed.

5. Shaft furnace according to Claim 4, wherein the distributor channel has slit-form apertures.

6. Shaft furnace according to Claim 4 or Claim 5 wherein the distributor channel is formed of a number of gas ducts extending radially on the underside of the delivery device.

7. Shaft furnace according to Claim 6, wherein wall regions of the gas ducts on the underside of the delivery device are constructed as heatconducting walls for heat exchange between gasifying medium and combustible gas flowing in the ash delivery region.

8. Shaft furnace substantially as herein described with reference to either one of the accompanying drawings.

9. Shaft furnace for continuously producing combustible gas from organic material, especially from organic wastes, with provision for material charging situated at the top of the shaft furnace and a bed of material in bulk in the shaft, through which bed the organic material travels under the action of gravity, the said bed being supported by a delivery device conically tapering towards the bed of material in bulk and disposed centrally in the shaft at the ash discharge region of the shaft furnace, the said delivery device being cooled and rotatably mounted, the furnace further comprising means for-giving up heat for the drying and subsequent degassing of the material in the upper region of the bed of material in bulk as well as a supply duct for gasifying medium which opens above a fire bed in the lower region of the bed of material in bulk, the throughflow of the said gasifying medium being adjustable by a regulator controlled in dependence upon the temperature in the fire bed, while there is provided at the lower edge of the delivery device a delivery gap, the width of which is determined in dependence upon the lumpiness developing in the material in the fire bed, and there is connected to the ash discharge region a duct which discharges combustible gas produced, characterised in that, for the gasifying agent, there are provided primary supply opening into the bed of loose material at the cone head of the delivery device, and secondary supply regulatable independently thereof, the said secondary supply opening in the region of the delivery gap.