WO1981002581A1

WO1981002581A1 - Procedure for pulsed gasification

Info

Publication number: WO1981002581A1
Application number: PCT/SE1981/000070
Authority: WO
Inventors: O Lindstroem
Original assignee: Lindstroem Ab Olle; O Lindstroem
Priority date: 1980-03-07
Filing date: 1981-03-06
Publication date: 1981-09-17
Also published as: SE8001804L

Abstract

Solid fuels are gasified to a fuel gas for subsequent efficient combustion causing no harm to the environment by supply of a reaction gas-like oxygen or preheated air, eventually with an addition of recirculated fuel gas and/or water vapor. The fuel is thereby disposed in a uniform way in a limited zone of gasification (8). Gasification is carried out in a pulsed manner by pulsations in the supply of reaction gas at a comparatively high frequency, said pulsation being produced by means of pulsing means (13) or by cyclic variations in the composition of the reaction gas.

Description

Procedure for pulsed gasification

The present invention is concerned with a procedure for gasification of solid fuels. The purpose with the in¬ vention is to generate a fuel gas in an useful manner. The fuel gas may then be combusted efficiently in a flame space. The procedure is particularly suited for small combustion plants to the effect^'that emissions of unburnt material in the flue gas may also be eliminated in the case of small scale combustion processes.

Combustion of wood e.g. in a wood stove may at first sight seem to go on continuously and at a constant rate of burning. If one follows the combustion process some¬ what closer one will however observe pulsations or oscillations in the process of combustion, partly between different parts of the hearth, partly in one and the same element' of the fire.. These pulsations give life to the fire.

Combustion of solid fuel like wood is taking place in several stages. Frequently one differentiates between (a) the drying stage, (b) the pyrolysis stage, (c) the gas combustion stage and (d) the char combustion stage. One stage follows the other but these partial processes are also to some extent going on simultaneously depend¬ ing on the rate of combustion and other conditions.

The partial processes are also to some extent separated physically. Drying is taking place deeper in the fuel whereas char combustion is taking place near the sur¬ face of the fuel. Char combustion may of course take place only after the fuel has become pyrolysed to a pyrolys residue mainly consisting of carbon.

If one could follow the process of combustion in a very small element of the fuel one might find that these pulsations are taking place around a kind of equilibrium state. A momentary increase in the rate of char com¬ bustion gives heat for an increased production of pyro- lysis gas which in its turn again reduces the rate of char combustion depending on, among other things, the temperature reduction during the pyrolytic process. Combustion of pyrolysis gas in the gas phase near the fuel causes the temperature in the char layer to rise again. At the same time more air is drawn to the ele¬ ment in question which again accelerates the char com¬ bustion. - This hypothesis relating to pulsations in the process of combustion, which has not been verified experimentally or theoretically, has led to the.present scope of invention.

Conceiving the overall process in the whole fire one may assume that the hypothetical pulsations in the dif¬ ferent parts of the fire occur with different frequen¬ cies and so-to-say out of step. If it would be possible to control the combustion process in such a way that the pulsations were taking place in phase and with the same frequency would it then not be possible to utilize this feature in some way for a more efficient gasification process? The invention constitutes an an- swer to this question. The invention is concerned with a procedure for pulsed gasification for generation of a fuel gas for furnaces and combustion engines.

The new procedure is governed by a number of cooperating measures. The process of combustion is to begin with separated in two steps, one step of gasification for generation of a fuel gas, and one step with combustion of the fuel gas e.g. in a furnace or a combustion engine. The second measure is to generate the fuel gas under geometricaaly uniform conditions ^*in the whole

OM zone of gasification. This necessitates a design of the furnace or the gasgenerator which limits and de¬ marcates the size of the zone of gasification. A third and necessary measure is to generate the fuel gas in the zone of gasification in a pulsed way by having the fuel to change between primarily char com¬ bustion and pyrolysis/gasification. This kind of pulsed gasification for generation of fuel gas to the flame space or the engine cylinder is brought about by cyclic variations in the supply of the reaction gas, which may be oxygen or preheated air eventually with an addition of recirculated flue gas (exhaust gas) and/or water vapor to the zone of gasification.

The invention is thus characterized in that the fuel is brought to be gasified in a pulsed manner by pulsations in the supply of reaction gas at a comparatively high frequency - or corresponding cyclic variations of the composition of the reaction gas - whereby the pulsed gasification is carried out at a frequency in the range, 0,1 - 100 Hertz in a uniform manner in a limited zone of gasification. The new procedure may be used with many different kind of fuels and in many types of fur¬ naces and gas generators. The invention is particularly well suited for solid fuels which give off large quan- titles of volatile matter during heating. The procedure may, however, also be used for .combustion of degassed fuels like coke whereby the fuel gas which is generated in the gasification zone consists of water gas or pro¬ ducer gas.

Cyclic processes are known since long time in the fuel chemistry for production of producer gas etc. These known processes are discontinuous, which is the case e.g. in the state of art water gas generator where the temperature in the bed is stepped up by combustion where- after water vapor is added for generation of the water- gas

A decisive difference between the invention and this known technique is that changes in the reaction environ- 5 ment with the procedure according to the invention is taking place at a comparatively high frequency compared to the state of art processes. The procedure according to the invention therefore gives a process which is continuous from a practical combustion point of view.

0 It is also known that advantages can be obtained by so-called pulsed combustion e.g. of fuel oil. This has, however, nothing to do with the procedure according to the invention which may be described as a pulsed gasification of a solid fuel.

5 . Against this background it is necessary to describe the the object of the invention in a general way. The artisan may then use the procedure in practice for all different applications which are possible with simple modification of existing plants etc.

0 The invention will now be described further by means of the drawing, Figure 1, which shows the principle of the procedure according to the invention quite schematically,

Figure 1 shows completely schematically a furnace which _ has been modified for pulsed gasification according to 5 the procedure of the invention. The fuel container (1) is ending -at the bottom with an inclined grate (2) . The fuel charge (3) , in this case boiler wood, is resting towards the grate (2) . The grate space (4) is con¬ nected to the flame space (5) by means of a slot or 0 constriction (6) . Formed combustion gas is leaving via the chimney (7) after its utilization with heat ex-

^Q?-!PI changing means which are not shown. Fuel gas is gene¬ rated in the gasification zone (8) near the grate (2) . The formed fuel gas (9) is combusted in the flame zone (10) in the flame space (5) by supply of com- bustion air (11) .

The pulsed gasification in the gasification zone (8) is brought about by pulsed supply of reaction gas (12) . This reaction gas consists frequently of air or oxygen, possibly with an additive of recirculated hot combus- tion gas and/or water vapor. (The same effect is ob¬ tained by pulsations in the content of the reaction gas of reactive components like oxygen, water vapor etc. Such pulsations can be produced in a similar way.) The pulsations are obtained by means of the pulsing means (13) . These pulsing means or pulsator can comprise many different means e.g. a piston pump for air or si¬ milar means, a valve or a damper which is governed by mechanical pulsing means, etc. The supply of combustion air (11) to the flame space may be governed in a related way by the pulsing means (14) in such a way that the supply to the flame zone (10) increases when the supply of reaction gas to the gasification zone (8) is re¬ duced and vice versa. Such control of the supply of combustion air is however not necessary to achieve the technical effect of the invention. If a high stack gas temperature is desired it is however frequently useful to introduce such control of the combustion air parti- cularly for larger plants. _

The supply of reaction gas e.g. preheated air is thus taking place in a pulsed way. Pulsations may be sinus¬ oidal but may also have a different character e.g. tri¬ angular or rectangular wave form. The control of the supply of the reaction gas causes pulsations in tempera- tur, char combustion rate and pyrolysis rate

O PI W WIIPPOO with a certain phase difference with respect to the variations in the supply of reaction gas. When the gas supply is increased the temperature is also in¬ creased because of increased combustion which leads to an increased production of pyrolysis gas.

The supply of reaction gas, e.g. air with an additive of recirculated combustion gas (exhaust gas) is charac¬ terized by its average value, frequency and amplitude for the pulsations. Here only general rules can be given for these values since the figures depend very much on the kind of fuel and the reaction environment in general.

The average value should be slightly below the corre¬ sponding value for constant supply of primary air in. the case of so-called two stage combustion or combus¬ tion with a pregasifier. The increased methane content of the fuel gas obtained with gasification of wood and peat according to the invention has the effect that the need of combustion energy for the gasification process can be reduced compared to non-pulsed supply of primary air. Wood- and peat fuel produce quite large quantities of methane during the fast pulsed pyrolysis/gasification according to the invention.

The average value for the supply of reaction gas is in practice determined so that the desired average tempera¬ ture in the gasification zone is obtained, said tempe¬ rature being determined by the desired production of fuel gas for the flame zone. The average value may be as low as around 300-400°C with wood fuels and up to about 800-1000°C with coke and other different fuels.

Suitable average values for biofuels like wood and peat and lignite is in the range 600-800°C which gives a rapid and well controlled gasification of subsequent combustion.

The supply of combustion air to the flame space can be constant and follow the variations in the supply of reaction gas to the gasification zone. The supply is primarily determined by the fuel gas supply to the flame space and can be controlled in known way by means of sensors for the stack gas composition etc.

The frequency for the pulsations in the supply of reac¬ tion gas have to be in the range from about 0,1 Hertz up to 100 Hertz preferably around 1-10 Hertz. The le¬ vel 50 Hertz gives a possibility to control the supply of air by means of simple pulsators powered by alterna¬ ting current. This means that the time period for the fluctuations is in the range from one hundreds.of a second to approximately 10 seconds with a preferred value around 1 second corresponding to frequency on the level of 1 Hertz.

The amplitude in the supply depends on the frequency. Rapid pulsations in the range up to 100 Hertz reduces the amplitude whereas the opposite is the case for slower pulsations down to below 1 Hertz. A frequently useful value for the amplitude in the range around 1 Hertz is 5-30% of the average value. A particularly useful value in this case is 10-20% of the average value. With higher frequencies the amplitude is reduced about pro¬ portionally with the inverted frequency.

The associated changes of temperature in the active layer of the fuel which control the delivery of pyrolysis gas varies and they may amount from a few C up to 100 C or more depending on frequency and amplitude and other con¬ ditions.

OMPI The conditions during the stage of .pyrolysis will then to some extent correspond to the conditions during very fast pyrolysis so-called flash pyrolysis which gives a fuel gas with high methane content.

I have tried to explain in the description above how the spirit of invention has emerged from the hypothesis about oscillations in a naturally burning fire and on the basis of the special conditions which characterize so-called flash pyrolysis. If these attempts to expla- nations and interpretations are correct or not has how¬ ever no relevance for the object of the invention. The surprising technical effect of the procedure with pulsed gasification of solid fuels may have other causes. ' It is thus possible that the combustion chemical conditions are influenced by the pulsations from one conditi-on to another and that reactivity of the solid fuel is in¬ creased during the controlled changes from char combus¬ tion to pyrolysis.

The technical effect of the invention is evident from the following example. Wood, small sticks of willow, so-called energy forest was gasified in a gasification zone of a furnace which was in principal designed accord¬ ing to Figure 1. (Such a furnace is described in more detail in the Swedish patent application 8001801-3 "Furnace" which was filed simultaneously with the pre¬ sent application.) The reaction gas was recirculated combustion gas with a temperature of 800 C and an oxygen content of 3%. The conditions during non- pulsed supply of reaction gas then correspond to the conditions with a procedure for combustion which is de¬ scribed in the Swedish patent application 8001803-9 "Procedure for two stage combustion of solid fuels".which was filed simultaneously with the present application.

O .. WI The supply pipe for recirculated combustion gas was furnished with pulsing means comprising a door which was brought to oscillate around an averaqe positio in a mechanical way by a camshaft driven by a motor. The composition of the fuel gas was analysed in the conventional way by a gas chromatograph with a hot wire sensor e.g. Shimadzu GC-3BT-^'.

The content of methane of the fuel gas is one important process parameter. When the door was non-oscillating

3 the methane content was 0,02 -Nm per kilogram fuel. When the door was brought to oscillate one could observe an increase in the methane content already at 0,1 Hertz.

3 The methane content reaches a maximum around 0,04 Nm at around 1 Hertz and is then again reduced with increased frequency. Around 100 Hertz the methane conten,t.is still higher than with a non-oscillating door.

I have tried to describe in the above description the principle nature of the object of the invention and I have used one example of an-embodiment. The invention may,however,as has been pointed out,be used with many different solid fuels by modification of the existing combustion plants or by new design in order to maintain the conditions for the new procedure in a better way. It should be no difficulty for the artisan to design, manufacture and use furnaces for the new procedure with the knowledge of the object of this invention accord¬ ing to the description above- and using state of art combustion technology so as to realize the big advan¬ tages with the invention,above all the possibility to more complete and more environmentally acceptable com¬ bustion of solid fuels. It should not be any difficulty for the artisan to design, manufacture and use gas generators for the new procedure for stationary as well as fractionary applications.

Claims

Patent..Claims

Claim 1

Procedure for gasificationof solid fuels to a fuel gas for subsequent combustion whereby the fuel is brought to be gasified in a pulsed way by pulsations in the supply of reaction gas at a comparatively high frequency or corresponding cyclic variations of the composition of the reaction gas c h a r a c t e r i z e d in that the pulsed gasification is taking place at a frequency in the range 0_#1 - 100 Hertz in a uniform way in a limited zone of gasification.

Claim 2

Procedure according to claim 1 c h a r a c t e r i z e d in that the frequency is in the range 1-10 Hertz.

Claim 3

Procedure according to claim 1 or 2 c..h a r a .c t e r- i z e d in that the reaction gas consists of preheated air.

Claim 4

Procedure according to claim l or 2 c h a r a c t e r - i z e d in that the reaction gas consists of recirculated combustion gas.

Claim 5

Procedure according to claim l or 2 c h a r a c t e r- i z e d in that the solid fuel consists of wood . Claim 6

Procedure according to claim l or 2 c h a r a c t e r¬ i z e d in that the solid fuel consists of peat_/