Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PROGESS FOR THE UPGRADI~G OF LOW-GRADE
SOLID FUEL
The invention relates to a process for the upgrading of
low-grade solid f`uel 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, ref~se, manure, peat and brown
coal)j 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 "~uel".
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 wa-ter 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. A considerable
amount of the water present in the fuel is thus removed and
a high degree of` decarbox~ylation 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 liauid and/or vaporous water, but
the presence o~ water is of importance for the decarbox~lation.
In the untreated state ~uel ma~ already contain tar,
which can be separated from it by extraction; heating the
fuel above 300C may in certain cases increase the amount
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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 up-
grading of low-grade solid fuel by heating to a temperature
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 separatea from the upgraded
fuel and/or is advantageousl~ 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 ver~ 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. Aroma-tic
alcohols, for instance phenol, may also be used.
The desired effect of the heating is achieved at a temper-
ature above 300C. Depending on the fuel to be treated, it may
be useful to choose a temperature well above 300 C.
The heating ma~ 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 belo~
the critical temperature of water (374C)~ 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 li~uid state and can be separated as such fro~ the
upgraded fuel.
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During the heating part of` the tar fo~ed can be entrained
by steam or liquid water and be reco~ered from it. The tar
present in the upgraded fuel after heating may be separated
fro~ it by various known methods, f~r 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
p~ 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 th~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
consu~ption can be considerably reduced and the risk of cor-
rosion of the equipment used will be smaller.
After the addition of the acid it may be use~ul 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 wa~. This leads
to a further decrease of the amount of acid re~uired.
In case the heating at a temperature above 300C is carried
out at a pressure below 85 atm. (Nhich 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 o~ the greater
part of the water before the heating above 300C; the acid is
then added before or during the pretreatment of the fuel at a
temperature between 150 and 300C 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 300C.
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Such a process has the great advantage that prior to the
heating above 30Q 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 abo~e 300C
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
o~ the small amount o~ water still present when heating above
300 C is no real drawback.
The invention will no~ be explained with reference to
~our 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 pH of 3.5 was ob-
tained (14 parts of gl&cial acetic acid to 1000 parts of brown
coal), and subsequently heated in an autoclave to 340C (heat-
ing rate 8C/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%w was obtained.
This hard black coal was then extracted with toluene,
whereupon an amount o~ 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 amou~t 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~ow of the original brown coal
and the pH o~ the water after completion of the upgrading
was 7.5.
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The above example, together with the comparative tests,
shows that there are circumstances under which acidification ;
of a brown coal - before subjectlng it to heating above 300C -
has an effect on the tar ~ield.
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 autocl&ve to 325 C, a~ter which a coal was
sieved off~ In this upgrading process per 100 pbw ash-free and
water-~ree 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 ma-terial present
in the COW manure, from which 11 pbw tar could be recovered by
extraction.
By extraction of the cow manure itself o~ly 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 in-
vention 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, 9~5 g organic material and 25 g ash,
were subjected, in an aqueous suspension after acidification
to a pH o~ 3.5, to a pretre~tment a-t 240C. ~his pretreatment
was carried o~t 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 340C, in which hea-ting was effected
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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 reco~ered 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 250C
(heating rate 10C/minute). This pretreatment was carried
out in an autocla~e at a pressure higher than the water-
vapour pressure at 250C and was continued during 30 minutes.
Thus, after separation of the liquid water, 520 g partly
15 coalified product ~re 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
fu~ther heating to 340C, in which heating was effected
directly by superheated steam (heating rate ôC/minute,
followed by cooling im~ediately after the temperature of
340C 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 ~ewton.
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 ~ewton.
By briquetting of the Australian brown coal itself (without
applying a heat treatment at all) briquettes were ob-t&ined with
a crushing strength of only 4.0 ~ewton. By briquetting of the
first-stage product of the two-stage heat treatment the crushing
strength of the briquettes formed was 6.o ~ewton and 7.5 ~ewton
respectively when a pH = 8 and a pX = 3 was applied.
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