GB2193200A - A kiln and a method of operating a kiln - Google Patents

Abstract

The invention concerns a kiln for producing charcoal, activating charcoal produced therein or turning charcoal or other material into a gas which may be burnt or otherwise usefully consumed. A charcoal producing kiln comprises a carbonisation chamber (1) having means for introducing woody material, organic matter and/or wastes and a gas inlet (1D) and a gas outlet (1B), a gas circulation system (6) connecting the gas outlet to the gas inlet and including a condenser (2), means (3) for enforcing gas circulation through the circulation system, outlet means (12) for removing condensed products from the condenser and a controlled inlet (4) to the gas circulation system/chamber for introducing a controlled quantity of oxygen. <IMAGE>

Description

SPECIFICATION A kiln and a method of operating a kiln Conventional kilns for making charcoal from wood cuttings, saw dust, organic matter or other waste are usually large installations capable of producing hundreds of tons of charcoal per year or small portable devices suited to temporary location in forest clearings. In both cases, the processing times required for charcoal making run into whole days or even weeks. Charcoal production is slow because the pyrolysis of wood is maintained at a relatively low temperature (400 to 500 ) and in the presence of very limited quantities of oxygen which are supplied without much movement of gases through the mass of woody or other material in the kiln.

An object of the present invention is to provide a kiln in which the movement of gases is accelerated by forced re-circulation so helping to remove the products of combustion from their points of origin as soon as they are generated. By this means the pyrolytic reactions are accelerated and charcoal production occurs more rapidly. Furthermore, re-circulation of gases permits many of the products of pyrolysis (pyroligeneous acid) to be carried into a condenser attached to the kiln where these products may be collected.In one aspect the invention provides a charcoal producing kiln comprising a carbonisation chamber having means for introducing woody material, organic matter and/or wastes and a gas inlet and a gas outlet, a gas circulation system connecting the gas outlet to the gas inlet and including a condenser, means for enforcing gas circulation through the circulation system, outlet means for removing condensed products from the condenser and a controlled inlet to the gas circulation system/chamber for introducing a controlled quantity of oxygen.

Heat from the condensation process can be transferred to a fluid circulating in a separate heat exchanging coil so providing a heat source for various purposes. Forced circulation of gases within the kiln and the condenser also enables the gas stream to be monitored for oxygen content and temperature. These measurements are useful in controlling the access of air (oxygen) to the kiln, upon which the rate of pyrolysis depends, as does the temperature level.

Acceleration of characoal production should make batch processing of wood and other products more profitable, even in kilns of modest size. Thus the proposed kiln could be a size easily installed,on farms, where perhaps heat from the condenser would be useful for domestic purposes such as warming animal sheds, grain drying and pre-drying of timber before conversion to charcoal.

In another aspect the invention provides a method of making charcoal which comprises pyrolysis of woody materials, organic matter and/or wastes in a kiln in the presence of a limited and controlled supply of oxygen which includes enforcing circulation of gases through the kiln and preferably passing the gases leaving the kiln through a condenser.

In a further aspect the invention provides a method of using a kiln for gasification which includes enforced circulation of flue gases through the charcoal and other materials therein, at high temperatures (at least 900" at the base of the kiln), and burning a proportion of the flue gases.

One embodiment of kiln in accordance with the invention will now be described with reference to the accompanying diagrammatic sectional drawing of a kiln, given by way of example only.

The kiln has a cylindrical carbonisation chamber 1 which will contain packed wood, organic matter or other wastes sitting on a grate 1A. The woody matter may be introduced into the chamber by means of a hinged porthole not shown. Charcoal 13 collects beneath the grate and can be removed. The chamber has an outlet 1B connected to a central flue pipe 6 leading away to a condensing chamber 2. At the upper end of the chamber is a central baffle plate 1C. The chamber has a gas inlet 1D and gases from the kiln and condenser are pumped by a pump 3 from the outlet through the condenser and back to the inlet. The flue gases on their way back to the base inlet pick up some air through a controlled inlet valve 4 if that valve is open.On re-entry of the flue gases beneath the grate a cyclone effect is generated by virtue of a tangential arrangement of the inlet relative to the cylindrical carbonisation chamber. This cyclone effect forces dust from the wood and charcoal into the centre of the chamber and along its vertical axis. The baffle-plate 1C prevents dust escaping into the flue pipe while permitting gases to pass around its margin and into the flue stream. A coil of tubing 8 in the condenser having an inlet 9 and an outlet 10 carries a fluid which absorbs heat from the stream of condensing substances in the condensing chamber 2 while both kiln chamber and condenser are surrounded by insulation 11 to prevent excessive heat loss to the immediate surroundings. The condensed liquids are removed at outlet 12.

A thermocouple or thermal sensor 7 senses the temperature in the carbonisation chamber 1 and when this temperature exceeds an operating temperature (400 to 500"c), an electrical signal is sent to the integrator and control unit 5 which controls the air inlet valve 4.

Since further heat accumulation and temperature rise caused by combustion or pyrolysis, would cause excessive combustion and gas production, the control unit now closes down the air inlet valve in order to cut off or reduce air and thus oxygen supply to the kiln base.

Though the gases already present continue to re-circulate, oxygen depletion reduces combustion and so returns the kiln temperature to the desired operating level. About 25% of the energy content of woody matter is required for maintaining the reactions of pyrolysis, which are endothermic when considered as an overall group. This 25% of energy is released by oxidation, and an oxygen supply is therefore required. However, little or no oxygen should remain in the mixture of gases by the time it has passed through the woody material. An oxygen sensor 6A in the central flue pipe 6 measures the quantity of oxygen in the flue gases leaving the kiln.These gases should be more or less free of oxygen and if a certain level of oxygen is exceeded, indicating that combustion in the kiln has not consumed it all, then the sensor sends an electrical message to the control unit and the air inlet valve is opened in order to admit more oxygen to the kiln base. Increased oxygen supply consequently restores the combustion process to its proper level and reduces the amount of oxygen in the outlet flue pipe.

Although ordinary characoal, in powdered or lump form, is a useful product, so called "activated" charcoal is more valuable. Activation is produced by increasing the porosity and absorbtive properties of charcoal. One method of achieving this is to expose ordinary charcoal, at a temperature of 800 to 1000 C to steam carbon dioxide, or flue gas for a short time. Clearly the proposed kiln would be suitable for this activation process (by flue gas) if the temperature is allowed to rise to 800 to 1000"C for the required period of time. Thermocouple 7 could be readily switched out of operation and another with a higher setting brought into action for this purpose.Of the several types of oxygen sensor currently available, a ceramic device which becomes conductive for oxygen atoms at about 300"C would probably be suitable for the kiln (e.g.

Lambder model 2LS, made by Lucas, normally monitors unburnt oxygen in petrol engine exhaust gases).

For initial heating to the charcoal making temperature, or heating up to activation temperature, if the air control valve (or another valve situated between the condenser and the circulating pump) were opened to the atmosphere, then free introduction of oxygen into the gas stream would facilitate rapid combustion in the kiln chamber. Once the required temperature had been reached, the valve could be closed and put on automatic control.

Instead of supplying oxygen in the form of air oxygen could be supplied by other means.

For example, a heated vaporiser admitting alcohol (e;g. methanol or ethanol) in the gas phase, at inlet valve 4 could replace the air supply and then atmospheric nitrogen would be excluded and oxygen would be admitted only in chemical combination as alcohol. Reactions between alcohol and hot carbon (charcoal) would be endothermic, as in the "water gas" reaction, but the exclusion of nitrogen would enhance the calorific value of the resulting gas mixture.

The kiln could also be used as a gasifier.

Normally, the kiln would be used at an operating temperature of 400 to 500"C to make ordinary lump or powdered charcoal, or at 800 to 1000 C to convert the charcoal into its activated form. However, should the need for gas exceed that for charcoal into its activated form. However, should the need for gas exceed that for charcoal then the re-circulation process may be continued at a high temperature, (preferably at least 900"C at the base of the kiln and with advantage in the range 900 to 1000"C) until all the charcoal in the kiln had been converted into gases. Recycling increases the proportion of methane in the gases and also degrades tars and higher molecular weight compounds through a "cracking effect".The gas mixture produced in this way has a high calorific value despite admixture with some atmospheric nitrogen, and may be piped off for burning elsewhere or supplying fuel gas for engines. External combustion engines (e.g. Sterling engines) would be particularly suited to running on gas from the kiln in view of the fact that gas cleaning and cooling equipment is not essential to such engines, and this equipment is not included in the kiln.

For such a use it may be particularly useful to supply the oxygen in the form of alchohol. A binary fuel system, derived from charcoal in the kiln/gasifier and alcohol in a separate tank, would yield a higher power output from that obtainable from ordinary producer gas.

Even when the kiln is not used as a gasifier but for making charcoal, during re-circulation of gases within the kiln the concentration of methane in these gases will increase. This methane may be used to supply energy by burning it, for example it could be used to additionally heat the fluid circulating in the heat exchanging unit of the condenser. The kiln may have a safety valve 14 which measures the methane concentration and when that concentration reaches a value corresponding to 5 to 15% methane in air the valve could be opened and the methane could be ignited. Movement of the valve mechanism, in the opening process could be used to operate a sparking device for gas ignition at the valve outlet.

Claims (23)

1. A charcoal producing kiln comprising a carbonisation chamber having means for introducing woody material, organic matter and/or wastes and a gas inlet and a gas outlet, a gas circulation system connecting the gas outlet to the gas inlet and including a condenser, means for enforcing gas circulation through the circulation system, outlet means for re moving condensed products from the condenser and a controlled inlet to the gas circulation system/chamber for introducing a controlled quantity of oxygen.

2. A kiln according to Claim 1 including means for measuring the oxygen content of gases leaving the carbonisation chamber and using this measure to control the oxygen intake.

3. A kiln according to Claim 1 or Claim 2 including temperature control means for measuring the temperature in the carbonisation chamber and using this measure to control the oxygen intake.

4. A kiln according to Claim 3 in which the temperature control means is arranged to maintain the temperature in the range 400-500 degrees C.

5. A kiln according to any of Claims 1 to 4 in which the controlled inlet is arranged to intake air for the oxgyen supply.

6. A kiln according to any of Claims 1 to 4 in which the controlled inlet is arranged to take in alcohol in the vapour phase for the oxygen supply.

7. A kiln according to any of Claims 1 to 6 including means for limiting the entry of dust into the gas circulation system.

8. A kiln according to Claim 7 including means for producing a cyclonic circulation of gases within the carbonisation chamber and baffle means for limiting the exit of dust through the gas outlet.

9. A kiln according to Claim 3 or Claim 4 or any claim dependent thereon including means for overriding the temperature control means and operating the kiln at a higher temperature (preferably in the range 800 to 1000 degrees C) for activating charcoal therein.

10. A kiln according to Claims 1 to 9 including a fluid circulating system for cooling the condenser and transferring heat to an external system.

11. A kiln according to any of Claims 1 to 10 including a gas safety valve having means for detecting methane in the flue gases and means for supplying the methane to a burner when this exceeds a predetermined amount.

12. A kiln according to Claim 11 together with Claim 10 in which the methane burner is arranged to heat the fluid in the fluid circulating system.

13. A kiln substantially as described herein with reference to or as illustrated in the accompanying drawing.

14. A method of making charcoal which comprises pyrolysis of woody materials, organic matter and/or wastes in a kiln in the presence of a limited and controlled supply of oxygen which includes enforcing circulation of gases through the kiln and preferably passing the gases leaving the kiln through a condenser.

15. A method according to Claim 14 which includes automatically maintaining the temperature in the carbonisation chamber of the kiln in the range 400 to 500 degrees centrigrade by controlled intake of oxygen.

16. A kiln according to Claim 14 or Claim 15 which includes meausuring the quantity of oxygen in the gases leaving the carbonisation chamber and automatically controlling the intake of oxygen according to this measure.

17. A method according to any of Claims 14 to 16 which includes activating charcoal already formed in the kiln by circulating flue or other gases through the kiln at a temperature in the range 800 to 1000 degrees centrigrade.

18. A method according to any of Claims 14 to 16 in which oxygen is introduced in the form of an alcohol vapour.

19. A method of using a kiln according to any of Claims 1 to 3 for gasification which includes enforced circulation of flue gases through the charcoal and other materials therein, at high temperatures (at least 900 at the base of the kiln), and burning a proportion of the flue gases.

20. A method according to any of Claims 13 to 19 in which, while raising the temperature in the kiln to a required level, material therein is burnt with a greater oxygen supply then the controlled supply and when the temperature has reached the required level the oxygen intake is controlled as required for charcoal-making, activation of charcoal or gasification.

21. A method of making charcoal substantially as described herein with reference to the accompanying drawings.

22. A method of activating charcoal substantially as described herein.

23. A method of using a kiln as a gasifier substantially as- described herein.