New Light on Carbon-monoxide Production
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Strong Evidence is Put Forward to Disprove the Twozone Theory of Gas-producer Reactions. Reactivity of Fuel is a Function of its Micro-porosity. Arguments are Based on Phenomenon of the Specific Heat of Wetting
By J. Hoy Robertson,
Ph.D., A.I.C.
GAS producers installed on road vehicles, because,of their small size and fluctuating loads have to meet problems not normally encountered by industrial units Thus the depth of fuel bed is limited the fuel must be such that the volatiles are reduced to a minimum, and it is not possible to take advantage of the sensible heat of the gases evolved. These factors impose limitations both on the fuel to be used and on the design of the producer itself.
Whilst p rac ti c al experience, naturally, must play an important part in the improvement in design of roadtransport producers, it is, nevertheless. equally true that a precise knowledge of the reactions taking place in the fuel is of immense importance. Unless accurate information about these reactions is available, there will be no satisfactory basis for further development.
Basis of Two-zone Theory Questioned Until recently the so-called " twozone" thkry of gasification has been generally accepted, but this theory is based on the assumption that the fuel behaves as a monolithic structure,' whereas investigations which have been undertaken by the British Coal Utilization Research Association, at its experimental station, indicate that this is by no means an accurate picture. The fuel is, indeed, of a highly porous nature.
Before the influence of the porosity of the fuel can be appreciated fully, it is desirable to consider briefly the reactions which can take place when air and steam are passed through the fuel bed and how these reactions are influenced by temperature If air alone is first considered, it will be found that there are five possible reactions. The carbon can be oxidized either to carbon-monoxide .(CO) or carbon-dioxide (C05); carbon-monoxide can be further oxidized to carbondioxide; carbon-dioxide coming into contact with carbon can be reduced to
carbon-monoxide; and the last-named reaction can be reversed. All these reactions, with the exception of the ,carbon-dioxide reduction, are exothermic, that is, heat is given out, and the relative probability of their occurrence is influenced to a large extent by the temperature and character of the bed.
The position is complicated further by the admission of steam, which leads to the formation of both carbon oxides and of hydrogen. On the balance, the steam reactions are endothermic, that is, heat is absorbed, and it is thus possible to enrich the gas mixture at the expense of the sensible heat imparted to the gas stream by direct oxidation reactions.
The influence of temperature on the gasification reactions has been stressed, but this is again affected by the time factor. It is known, for example, that, given sufficient time for a state of equilibrium to be reached, a mixture of carbon oxides in contact with carbon at a temperature of 800 degrees C. will contain 87 per cent. of' carbon-monoxide.
State of Equilibrium Not Achieved This, however, assumes that there is sufficient time for equilibrium conditions to be reached, a state of affairs not encountered in a producer when gases and steam are in rapid movement through the fuel bed. In these circumstances the gas stream may differ in temperature from that of the fuel bed, and the structure of the fuel—which, as will be seen later, approaches molecular dimensions—may prevent direct contact between the gases and the fuel from taking place.
Therefore, a study of the equilibrium conditions at various temperatures does not reveal an accurate picture of the reactions occurring in the fuel bed. Further experimental evidence is essential, but this can be obtained only after overcoming technical difficul-ties, because secondary reactions in the sampling apparatus may disguise those at the fuel surface. Efforts have been. made to eliminate secondary reactions by employing low pressures and low fuel-bed temperatures and, from the results so obtained, several conclusions have been drawn:
Of these, the more important are that high' temperatures favour the formation of carbon-monoxide, that a complex is formed on the surface ofthe fuel which is capable of dissociation into either of the carbon oxides, and that high temperatures also favour the formation of carbon-monoxide from this complex. _Furthermore, it appears that the reduction of carbon-dioxide proceeds with extreme slowness below 1,200 degrees C. and that this oxide May not be a primary product of the steam-carbon reaction.
The conventional two-zone theory of gasification was advanced to account for the results of sampling the gases, formed at various points in the fuel bed, because these analyses indicated that carbon-dioxide was formed almost quantitatively in the first few inches, and that, only after all the oxygen was exhausted, was the monoxide present in any large amount. In consequence, it was assumed that gasiecation took place in two stages, the initial reaction producing carbondioxide, which was subsequently reduced to the monoxide.
Misleading Effect of Unsound Technique Such an explanation has the merit of simplicity, based as it is on the observations of a number of workers, but it is open to criticism if the sampling technique be not above suspicion.
Whilst the two-zone theory found general acceptance, it Was noted that conditions in the first zone bad a profound influence on the final gas composition, a fact somewhat contrary to what might have been expected. Furthermore, fuel-bed temperatures were lower than would correspond with the exclusive formation of carbondioxide. Doubt was thus cast on the applicability of the two-zone theory and this doubt was further strengthened by several observations made during the course of research on small producers at the B.C.U.R.A. experimental station.
Perhaps the most striking evidence against the assumption—essential to the two-zone theory—that carbondioxide is the primary-combustion product rests on the observed temperatures of the fuel bed. • Here, the B C.0 R.A. results record temperatures between 25 and 30 per cent. below those which would be expected from direct combustion to the dioxide; such temperatures can be explained only il a considerable quantity of the primary product is the monoxide. This fact, in itself, Would be strongly adverse to the two-zone theory, but it is suppotted by other no less interesting observations. ' It has been found that fuel consumption is appreciable only in the first inch or two of the fuel bed and that the thickness cal the bed can be drastically reduced—from 12 ins. to 6 ins.—without any appreciable effect
on gasification. Also, the temperature gradient is less steep than would be expected if the endothermic reduction reaction—carbon-dioxide to carbon-monoxide—were confined to a region in which the only source of heat was the sensible heat of the •gases.
Although it is probable that the gases, during their passage through the fuel bed, are enriched by the formation of carbon-monoxide, the evidence cited above is in conflict with the assumptions of the simple two-zone theory and suggests that the reactions taking place at the point of impact of the blast in reality determine the gas composition. That, this should be so is, perhaps. not surprising when -the true physical structure of . coal, es elucidated at the B.C.U.R.A. station, is considered.
Coal Wrongly Regarded as Solid Far from being a " solid " material, it has been shown that a lump of coal is an aggregate of an enormous number of minute particles of a size so small that up to the present it has not been possible to observe them on microscopes of even the highest power and in cOnsequence, gasification reactions take place over an area vastly greater than the superficial area of the fuel. How this internal area is measured will be explained shortly; for the moment we are interested only in the influence of this discovery on the theory. of gasification.
It will be seen that ..air passing through the combustion area can take two courses. Either it will come in contact with only the external surface, or it may penetrate the minute interstices of the fuel and there react with carbon to form the dioxide. In the latter event, conditions favourable for the reduction to the monoxide are present as the gas, of necessity, must be in the closest contact with the internal carbon surface which, in turn, is maintained at a high temperature by both radiation and convection. It ie thus unlikely that carbon-dioxide, as such, will escape without being reduced.
Carbon-monoxide so formed will then meet again the main air stream where it will be re-oxidized so long as there is any oxygen left, Only when all the oxygen has been utilized will the formation of the rtionoxid,es be permanent'. Therefore the main gasification reactions will be confined to a
• single, zone where the production of carbon-monoxide is the predominating reaction:
The revised theory of gasification, which, as has been shown, accords with observations of the behaviour of fuel beds, is based' on the asgumption that the fuel is of a highly porous nature and is composed of an aggregate of particles of ‘a. minute site,,exposing a total area far in excess of the superficial area. This view of the structure of carbonaceous fuels is founded on the phenomenon of specific heat of wetting, namely, the heat evolved when a material is immersed in a liquid.
By measuring the heat of wetting, it is possible to calculate the area of a substance, as the heat is proportional to the surface. With the aid of a suitable calorimeter, the heat of wetting of coal immersed in an appropriate liquid —in this instance methyl alcohol—can be measured and calculations made of the corresponding surface..
• Effective Area a Million Times Greater
It is from experiments of this nature at the B.C.U.R.A.station that estimates of the surface, external and internal, of fuels have been made and these have revealed that the total area is of the order of 104 sq. ft. per lb. or about one million times the superficial area. The whole aspect of the behaviour of fire during gasification is changed when the scale of the areas exposed in even very limited sections of the fuel bed is realized, and it is readily understandable that conditions in the initial stages will have a profound influence on the behaviour of the entire producer.
Gasification in small producers-proceeds satisfactorily only with what is known as a highly "reactive " fuel. although the precise meaning of reactivity is somewhat obscure. Factors which must be considered are the type of apparatus used for testing, and whether the reaction is towards oxygen, carbon-dioxide or steam. Whilst previously it was thought that reactivity was connected with the giosser voids in the fuel,. there now seems to be good reason to believe that. on the contrary, the micro-structure is the controlling factor.
Reactivity is, accordingly, an inherent property of the coal and not a function of the size of the lumps. Fuel with a coarse micro-structure will tend to be unreactive and special treatment will be necessary, if such a fuel is to be used satisfactorily. One such means is to operate at a high temperature in a localized combustion zone, when the difference between the behaviour of reactive and unreactive coals is reduced to a minimum, and another is to activate the fuel by treatment with an alkali carbonate.
Design of Single-zone Producer Different The single-zone theory of gasification has important repercussions on the design of small producers, as certain conditions, applicable only to the two. zone theory, will not have to be taken into consider,ation. With the almination of the reducing vine, the need for a continuous feed to this section of the producer—to maintainthe fuel in a suitable condition for the riducing reaction—no longer exists. On the other hand, conditions in the combustion zone are critical and the assumption, madein the two-zone theory, that an unreactive fuel will behave satisfactorily in this zone does' not hold good.
Reactions taking' place during the gasification of a fuel in a producer are complex and even yet not fully understood. The two-zone theory of gasification is open to a number of serious objectiont which render it unacceptable as an, accurate pictufe of events.' These objections, coupled with the work on the micro-structure of coal carried out by the B.C.U.R.A., have led to the development of the single-zone theory which has an important bearing on the design of the small producers suitable for...road transport vehicles. '