EP0026011B1

EP0026011B1 - Process for the upgrading of low-grade solid fuel

Info

Publication number: EP0026011B1
Application number: EP80200805A
Authority: EP
Inventors: Leopold Van Raam; Herman Petrus Ruyter
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1979-09-25
Filing date: 1980-08-27
Publication date: 1983-08-17
Also published as: ATE4462T1; DK400980A; GR70066B; SU1056908A3; FI802993A; RO78331A; PL226878A1; YU241880A; ES495250A0; IE801976L; AR223729A1; CS222691B2; TR21076A; HU186751B; PL125118B1; YU40885B; IE50128B1; NZ195028A; AU6262480A; DE3064580D1

Description

The invention relates to a process for the upgrading of low-grade solid fuel with simultaneous formation of tar, by heating at a temperature above 300°C in the presence of water.
In this specification the term low-grade 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 oxygen-containing groups, by means of a decarboxylation process.
This removal can very conveniently be carried out by heating above 300°C in the presence of water. Such a process is described in US-A-3,660,054, according to which the heating is carried out at a temperature of from about 315°C to 375°C. A considerable amount of the water present in the fuel is thus decreased and a high degree of decarboxylation is effected. Furthermore, the sulphur and/or ash content of the fuel are (is) decreased. 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 300°C may in certain cases increase the amount of tar separated from the upgraded solid fuel. The solid fuel, after heating in the presence of water, may be pressed to briquettes, the tar still being present in the solid fuel. However, the briquettes thus obtained have a low crushing strength, only little tar being present.
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.
The invention provides a process for the upgrading of low-grade solid fuel with simultaneous formation of tar, by heating at a temperature above 300°C in the presence of water, characterized in that before or during the heating above 300°C acid is added to the fuel.
It was already known to heat peat to a temperature between 100°C and 180°C in the presence of water (See DE-C-899,493). This known method of upgrading is carried out to make the peat more suitable for pressing to shaped articles, heating at a temperature below 180°C resulting in removal of colloidally bound water and a chemical change in the structure of the fibres. Tar is not formed at temperatures below 180°C. It was known from this specification that carrying out this treatment in the presence of an acidic solution considerably reduces the time required for this upgrading.
In addition to a high-grade fuel it has surprisingly been found that a considerable amount of tar can be obtained by means of the process according to the invention. 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. In this manner, briquettes having a high crushing strength are obtained. Moreover, the calorific value of the former upgraded fuel is still much higher than that of the starting low-grade 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 temperature above 300°C. Depending on the fuel to be treated, it may be useful to choose a temperature well above 300°C.
The heating may be carried out at a temperature lower than 8.6 MPa. 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 (374°C), 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 the 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 300°C is carried out at a pressure below 8.6 MPa (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 300°C; the acid is then added before or during the pretreatment of the fuel at a temperature between 150 and 300°C 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 300°C 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, 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 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 60.0% by weight and an ash content of 1.0% by weight 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 pH of 3.5 was obtained (14 parts by weight of glacial acetic acid to 1000 parts by weight of brown coal), and subsequently heated in an autoclave to 340°C (heating rate 8°C/min.).
The autoclave was then opened and water (pH 4.0) and coal were separated by means of a sieve; a hard black coal with a water content of 15% by weight was obtained.
This hard black coal was then extracted with toluene, whereupon an amount of tar went into solution corresponding to 6.8% by weight 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% by weight 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% by weight 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 of a brown coal - before subjecting it to heating above 300°C - increases the tar yield and the calorific value.

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 -was heated in an autoclave to 325°C, after which a coal was sieved off. In this upgrading process per 100 parts by weight ash-free and water-free material present in the cow manure, 40 parts by weight ash-free and water-free coal were obtained from which 25 parts by weight 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 parts by weight ash-free and water-free coal were obtained per 100 parts by weight ash-free and water-free material present in the cow manure, from which 11 parts by weight tar could be recovered by extraction.
By extraction of the cow manure itself only 4 parts by weight tar were obtained per 100 parts by weight 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 pH of 3.5, to a pretreatment at 240°C. This pretreatment was carried out in an autoclave at a pressure higher than the water-vapour pressure at 240°C. 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 340°C, 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% by weight 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 250°C (heating rate 10°C/min.). This pretreatment was carried out in an autoclave at a pressure higher than the water-vapour pressure at 250°C, and was continued during 30 min. Thus, after separation of the liquid water, 520 g partly coalified product were obtained, containing 120 g water, 390 g organic material and 10 g ash.
This product was subjected at a pressure of 5 MPa to a further heating to 340°C, in which heating was effected directly by superheated steam (heating rate 8°C/min, followed by cooling immediately after the temperature of 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 pH = 8 and a pH = 3 was applied.

Claims

1. A process for the upgrading of low-grade solid fuel with simultaneous formation of tar, by heating at a temperature above 300°C in the presence of water, characterized in that before or during the heating above 300°C acid is added to the fuel.

2. A process as claimed in 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 as claimed in claim 1 or 2, characterized in that the heating is carried out at a pressure below 8.6 MPa and in that the acid is added before the heating.

4. A process as claimed in claim 3, characterized in that the acid is added before or during a pretreatment of the fuel at 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 300°C.

5. A process as claimed in any one of the preceding claims, characterized in that at least part of the tar formed is separated from the upgraded fuel.

6. A process as claimed in any one of the preceding claims, characterized in that at least part of the tar formed is used for pelletizing or briquetting of the upgraded solid fuel.