The world produces enormous amounts of organic wastes, both agricultural and synthetic and they provide a major problem of disposal. Disposal is met in many different ways, ranging from crude dumping procedures to sophisticated incineration processes.
In general, these methods represent the local cheapest solution rather than an attempt to utilise any potential energy which may exist in these wastes. Pyrolysis, or the destructive distillation in which organic wastes are split into solid, liquid and gaseous by-products, offers the opportunity of recovering some of this energy.
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The destructive distillation of wood and coal has been practised for centuries and a great deal of information is available, but outside this area the information is much more meagre. The pyrolysis of several cellulostic waste materials, e.g., sawdust, peanut shell and rice husks, is considered and also the pyrolysis of rubber as an example of a polymeric or synthetic type waste.
The work has been carried out under conditions which simulate fluidised bed conditions since it seems probable that this type of process is more likely to have industrial significance, rather than static bed pyrolysis. Under fluidised bed conditions, the products of decomposition are rapidly removed from the hot zone and the product distribution is different from that obtained in a static system?
Organic wastes vary widely in type, but in considering their pyrolysis it is important to see if common ground can be found to make processing simpler. The first broad division considered has been into cellulostic and polymer type wastes, since it seems possible that the cellulostic materials will show strong similarities during pyrolysis and similarly for polymer materials. This investigation has gone some of the way to examine the validity of such a division.
Pyrolysis of Cellulostic Materials:
Wood, peanut shells and rice husks are all cellulostic materials. Wood has been considered to consist of 3 components, cellulose (50%) hemicellulose (25%) and lignin (25%). Other cellulostic materials are comprised basically of these components but in different proportions.
It has been found that the yield of products when wood is completely pyrolysed is about that obtained by pyrolysing the proportional amounts of the major wood constituents. The hemicellulose is said to decompose first, mainly between 200 and 260°C, followed by cellulose at 240-300°C and finally by lignin at 280-500°C.
This belief in the additive nature of the products is not supported by some who considers that more interaction takes place between the products of pyrolysis from the wood components. It seems probable that the conditions of pyrolysis such as surrounding atmosphere, residence time of products, etc., may be expected to affect the interaction between components. No decisive answer is available at present, and no systematic work on this problem has been done on other cellulostic materials.
Pyrolysis of Polymers:
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Considerable investigation on the pyrolysis of polymers has been undertaken extending back for over a century, work being reported on rubber fairly early. It seems likely that in the majority of cases depolymerisation is a primary reaction and that the size of the fragments obtained is related to the residence time in the reaction zone. In many cases therefore it could be expected that a high yield of monomer and dimer would be produced and only small amounts of char and gas.
Pyrolysis of Organic Mixtures in Practice:
Practical pyrolysis of synthetic waste materials and urban refuse has received attention only in the last few years. The investigations have been carried out usually in batches in non-agitated retorts. The work has mostly been non-selective with respect to composition but has examined the effect of pyrolysis temperature.
Union Carbide has been experimenting with a process of compressing, melting and pumping waste plastic into a heated pyrolysis tube. The pyrolysis yielded hard and soft waxes and greases as well as a carbonaceous residue and gases.
On pilot plant scale, destructive distillation of tyres has been shown to be a feasible method of obtaining valuable products from natural and synthetic rubber sources. The product distribution is shown to be strongly temperature dependent.
Experimental Work:
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The apparatus used in our work is a batch-type reactor stirred by a paddle stirrer; the material to be pyrolysed is introduced from a hopper, after the reactor has reached operating temperature. The recovery train is quite simple, a cylindrical knock-out pot which catches the bulk tar, liquor and any fines carryover, followed by a tar mist trap and a gas collection system. The details are supplied in Fig. 5.31.
Experimental runs on cellulostic wastes were carried out at constant temperature (600°C). The runs on rubber considered the effect of varying the temperature. The results are examined separately for the cellulostic and polymeric materials even though the products formed have some points of resemblance.
Results on Cellulostic Materials:
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The analyses of the waste materials are shown in Table 5.35. Also included is an approximation of the proportions of cellulose, hemicellulose and lignin. In Table 5.36 are shown the yields of pyrolysis products.
In yield of products, sawdust and peanut shells seem similar when compared on a moisture and ash-free basis; rice husks appear quite different. This is illustrated more clearly by comparing the distribution of the elements in the products, shown in Table 5.37.
It seems probable that the process of decomposition of the rice husks is quite different; however, the sample of rice husks had a high ash content due to the inclusion of fine soil. Reported typical values for the ash content of rice husks range from 19% to 24%. The results obtained here may therefore be unusual. In any event more information is required on other cellulostic materials to see if some useful generalisation may be made.
Results of Rubber Material:
The ultimate analysis of the rubber material is shown in Table 5.38; an assessment is also made of the proportions of natural rubber material and other substances such as carbon black, sulphur and inorganic fillers. This assessment permits some insight into the decomposition which takes places during pyrolysis.
The overall material balances for the pyrolysis runs are given in Table 5.39 and the results suggest that the yield of char declines as the temperature increases; the converse occurs with the gas. It seems likely, although more data is required, that the tar yield goes through a maximum. This is not unexpected since the pyrolitic reactions may be divided into primary and secondary stages.
At relatively low temperatures only primary cracking occurs and at 400°C even this may be incomplete whereas at 500°C primary cracking should be rapid and comprehensive and produce a lot of material in the liquid range. At higher temperatures, secondary reactions take place and convert much of the liquid material into products of lower molecular weight.
The formation of char as a result of secondary cracking is probably not significant under the operating conditions. The residual solid material may be expected to comprise essentially all the inorganic filler plus the carbon black, about 35% and the weight % of char plus losses for the runs at 500°C and 650°C are 35.3 and 33.4 respectively. Within experimental tolerances it seems probable therefore that the natural rubber substance is converted to liquid and gaseous products only, at these temperatures.
The structure of the rubber material was completely changed during pyrolysis and the residue was a very fine lightweight black char. Much of this char was of dust size and carryover was a significant problem and would presumably be so in a commercial fluidised bed operation. In static bed operation the residue could be anticipated to be a flaky bulk but in fluidised bed operation the size of the residue would probably be independent of the feed size.
To be able to predict the extent of the overall reaction of pyrolysis on an engineering scale is a formidable task. For polymer-type materials, similar to rubber, it seems possible that a consideration of primary and secondary pyrolysis reactions related to such factors as temperature, residence time, particle size and atmosphere could prove adequate.
For the cellulosic materials the picture is less clear and no discernible pattern has emerged. The fluidised bed type reactor, with its high heat transfer rates and short residence times for the volatile products, offers a simpler system for study of the pyrolysis process than is possible by static bed experiments.