PROCESS FOR THE UPGRADING OF LOW-GRADE SOLID FUEL

Abstract: A process for the upgrading of low-grade solid fuel by heating at a temperature above 300°C in the presence of water, in which before or during the heating above 300°C acid is added to the fuel.

Description: PROCESS FOR THE UPGRADING OF LOW-GRADE SOLID FUEL The invention relates to a process for the upgrading of low-grade solid fuel by heating at a temperature above 3000C in the presence of water. In this specification the term lowgrade solid fuel is meant to denote carbon-containing material of which the carbon originates from photo-synthesis and which can be available in various degrees of coalification (such as biomass, vegetable material, refuse, manure, peat and brown coal); the term low-grade solid fuel is meant to denote also a material, mentioned above, which has already undergone a pretreatment. For the sake of brevity, such materials will in this specification be denoted by the term "fuel". As a rule, this fuel contains much water. The water is partly physically absorbed, partly bound in gel structures and partly chemically bound. The fuel also contains many oxygen-containing groups. The calorific value of the fuel can be considerably increased by removing as much water as possible, by means of a dewatering process, and oxygencontaining groups, by means of a decarboxylation process. This removal can very conveniently be carried out by heating above 3000C in the presence of water. A considerable amount of the water present in the fuel is thus removed and a high degree of decarboxylation is effected. The result is a fuel with a greatly increased value, with a low water content and a high calorific value. The heating may take place in the presence of liquid and/or vaporous water, but the presence of water is of importance for the decarboxylation. In the untreated state fuel may already contain tar, which can be separated from it by extraction; heating the fuel above 3000C may in certain cases increase the amount of tar separated from the upgraded solid fuel. It has now been found that the amount of tar formed in the above heating can be considerably increased by reducing the pH of the water present in the fuel. According to the invention, in the process for the upgrading of low-grade solid fuel by heating to a temperature 0 above 300 C in the presence of water, acid is added to the fuel before or during heating. In addition to a high-grade fuel a considerable amount of tar can be obtained in this way. Thus, more than 10%w of the low-grade fuel can be converted into tar. At least part of the tar formed is suitably separated from the upgraded fuel and/or is advantageously used for pelletizing or briquetting the upgraded solid fuel. The tar obtained typically has a highly aliphatic character and a low content of polyaromatic constituents. The nature of the acid added is not very important. It may be an inorganic acid such as hydrochloric acid or sulphuric acid. Organic acids such as lignosulphonic acid may also be used; very suitable are formic acid and acetic acid. Aromatic alcohols, for instance phenol, may also be used. The desired effect of the heating is achieved at a temper 0 ature above 300 C. Depending on the fuel to be treated, it may be useful to choose a temperature well above 3000C. The heating may be carried out at a pressure lower than 85 atm. The water which is liberated from the fuel will then evaporate. When the heating is carried out at a temperature below the critical temperature of water (3740C), evaporation of the water can be prevented by heating at a pressure which is higher than the water vapour pressure at the temperature chosen. The water liberated from the fuel will then remain in the liquid state and can be separated as such from the upgraded fuel. During the heating part of the tar formed can be entrained by steam or liquid water and be recovered from it. The tar present in the upgraded fuel after heating may be separated from it by various known methods, for instance by extraction with solvents such as toluene, by supercritical extraction, or by azeotropic distillation with steam. It is preferred to add so much acid to the fuel that the pH of the water present in it becomes 6 or lower. Further reduction of the pH to values varying from 3.5 to 5 may in many cases lead to an additional rise in tar yield. It may be advantageous to impregnate the fuel with the acid before heating. This enables the acid to reduce the pH of the water present in tb fuel. The reactions leading to a higher tar yield can thus start earlier. By subjecting the fuel to the heat treatment without an excess of acid the acid consumption can be considerably reduced and the risk of corrosion of the equipment used will be smaller. After the addition of the acid it may be useful not to start the heating until after some time, so that the diffusion of the acid in the fuel, especially when the latter consists of fairly large lumps, becomes more complete, whilst reactions between the fuel and the acid are already under way. This leads to a further decrease of the amount of acid required. In case the heating at a temperature above 3000C is carried out at a pressure below 85 atm. (which means that no liquid water is present during the heating), the acid should be added before the heating. It is in this case preferred to subject the fuel to a pretreatment aiming at the removal of the greater part of the water before the heating above 3000C; the acid is then added before or during the pretreatment of the fuel at P temperature between 150 and 3000C and a pressure which is higher than the water vapour pressure at the temperature used, and the pretreated fuel is separated from expelled water before being heated to above 300 C. Such a process has the great advantage that prior to the heating above 3000C a considerable part of the water present in the fuel is removed at a relatively low pressure without evaporating the water, whilst during the heating above 300 C a very high degree of decarboxylation, up upgrading and tar formation takes place at a much higher temperature without the necessity of increasing the pressure. The evaporation of the small amount of water still present when heating above 0 300 C is no real drawback. The invention will now be explained with reference to four Examples. EXAMPLE I An Australian brown coal with a water content of 60.o%w and an ash content of 1.0%w was suspended in water (6 parts of water to 10 parts of brown coal) to which technically pure glacial acetic acid had been added until a p11 of 3.5 was obtained (14 parts of glacial acetic acid to 1000 parts of brown coal), and subsequently heated in an autoclave to 3400C (heating rate 80C/min.). The autoclave was then opened and water (pX 4.0) and coal were separated by means of a sieve; a hard black coal with a water content of 15%w was obtained. This hard black coal was then extracted with toluene, whereupon an amount of tar went into solution corresponding to 6.8%w of the original brown coal. For comparison, the original brown coal was extracted with toluene, which yielded an amount of tar equal to 1.2%w of the brown coal. For further comparison, the above upgrading of the brown coal was repeated with omission of the glacial acetic acid; the ultimate tar yield was 2.4%w of the original brown coal and the pH of the water after completion of the upgrading was 7.5. The above example, together with the comparative tests, shows that there are circumstances under which acidification 0 of a brown coal - before subjecting it to heating above 300 C - has an effect on the tar yield. EXAMPLE II Cow manure was suspended in water to which so much glacial acetic acid had been added that the pH was 4.5, and the suspension 0 was heated in an autoclave to 325 C, after which a coal was sieved off. In this upgrading process per 100 pbw ash-free and water-free material present in the cow manure, 40 pbw ash-free and water-free coal were obtained from which 25 pbw tar could be recovered by extraction. When this test was repeated with omission of the glacial acetic acid (the pH of the cow-manure suspension in water was in this case 7.5), 34 pbw ash-free and water-free coal were obtained per 100 pbw ash-free and water-free material present in the cow manure, from which 11 pbw tar could be recovered by extraction. By extraction of the cow manure itself only 4 pbw tar were obtained per 100 pbw ash-free and water-free material present in the cow manure. This example shows that the process according to the invention increases the tar yield in the upgrading of cow manure. EXAMPLE III 2500 g of the Australian brown coal used in Example I, containing 1500 g water, 975 g organic material and 25 g ash, were subjected, in an aqueous suspension after acidification to a p11 of 3.5, to a pretreatment at 2400C. This pretreatment was carried out in an autoclave at a pressure higher than the water-vapour pressure at 2400C. Thus, after separation of the liquid water, 1300 g partly coalified product was obtained, containing 300 g water, 975 g organic material and 25 g ash. This product was subjected at atmospheric pressure to a further treatment at 3400C, in which heating was effected directly by superheated steam. Thus, 645 g water-free coal were obtained, containing 25 g ash and 140 g tar. The tar had been entrained by the steam and was recovered from it by condensation. So, in this case the tar yield was 5.6%w of the original brown coal. EXAMPLE IV 1000 g of the Australian brown coal used in Example I, containing 600 g water, 390 g organic material and 10 g ash, were subjected, in an aqueous suspension after acidification to a pH of 3.0, to a pretreatment by heating it to 2500C (heating rate 100C/minute). This pretreatment was carried out in an autoclave at a pressure higher than the water 0 -vapour pressure at 250 C and was continued during 30 minutes. Thus, after separation of the liquid water, 520 g partly coalified product uere obtained, containing 120 g water, 390 g organic material and 10 g ash. This product was subjected at a pressure of 50 bar to a further heating to 3400C, in which heating was effected directly by superheated steam (heating rate 80C/minute, followed by cooling immediately after the temperature of 0 340 C had been reached). Thus, 258 g water-free coal were obtained, containing 10 g ash and 56 g tar. Without separation of the tar the product was pressed to briquettes with a diameter of 11.5 mm. The crushing strength of the briquettes formed was 29.9 Newton. For comparison reasons the two-stage process was repeated using identical conditions with the exception of the pH which was kept 8 in this case. The crushing strength of the briquettes formed was now only 6.1 Newton. By briquetting of the Australian brown coal itself (without applying a heat treatment at all) briquettes were obtained with a crushing strength of only 4.0 Newton. By briquetting of the first-stage product of the two-stage heat treatment the crushing strength of the briquettes formed was 6.0 Newton and 7.5 Newton respectively when a pIf = 8 and a pre = 3 was applied.

Claims: CLAIMS 1. A process for the upgrading of low-grade solid fuel by heating at a temperature above 3000C in the presence of water, characterized in that before or during the heating above 3000C acid is added to the fuel. 2. A process according to claim 1, characterized in that so much acid is added that the pH of the water present in the fuel becomes 6 or lower. 3. A process according to claim 1 or 2, characterized in that the heating is carried out at a pressure below 85 atm. and in that the acid is added before the heating. 4. A process according to claim 3, characterized in that the acid is added before or during a pretreatment of the fuel at 0 a temperature between 150 and 300 C and a pressure which is higher than the water-vapour pressure at the temperature used and in that the pretreated fuel is separated from expelled water before being heated above 3000C. 5. A process according to any one or more of claims 1-4, characterized in that at least part of the tar formed is separated from the upgraded fuel. 6. A process as claimed in any one or more of claims 1-5, characterized in that at least part of the tar formed is used for pelletizing or briquetting of the upgraded solid fuel. 7. A process as claimed in claim 1, substantially as described hereinbefore with special reference to the Examples. 8. Upgraded solid fuel whenever prepared with the aid of the process as claimed in any one or more of claims 1-7.

Inventors: RUYTER, HERMAN PETRUS, VAN RAAM, LEOPOLD
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1980
Main classification: C10L9
IPC codes: C10L9, C10F5