FR2497221A1 - Partial oxidn. of carbonaceous fuels with increased carbon conversion - by sepn. of particulates from quench water and recycling the high carbon content portion - Google Patents

Abstract

A process is claimed for the partial oxidation, in the fly ash mode, of solid, ash-contg. carbonaceous fuels, where an aq. suspension of particulates is obtained by passing the product gas, contg. low-C ash particles and high-C unconverted fuel particles, from the partial oxidation zone through a water-contg. quench chamber. The improvement comprises (a) screening the suspension to remove large particles, (b) sepg. the screened suspension in a continuous centrifuge operating at 10-1000 G into (i) a high-C cake and (ii) a dilute aq. suspension contg. low-C particles, and (c) recycling the high-C cake with fresh feed to the partial oxidation zone. High overall C conversion is obtained and the amt. of waste material to be disposed of is minimised.

Description

 The present invention relates to the gasification of solid fuels. More specifically, it relates to the separation of ash and non-gasified solid fuel from the products of gasification or partial combustion and to the return to service of non-gasified solid fuel in the gasification zone where it is subjected to partial oxidation with fresh addition of solid fuel.

Usually, during the gasification of solid fuel such as coal or coke, the fuel is subjected to partial oxidation with an oxidizing agent such as air, oxygen-enriched air or practically pure oxygen, a degree of purity greater than 95%, in a gasification zone comprising the production of a gas mainly consisting of carbon monoxide and hydrogen and also containing small amounts of C02, CH4,
H2S and COS. As not enough oxygen is introduced into the gasification zone to effect a complete oxidation of carbon, of the solid fuel, part of the solid fuel passes through the gasification zone without being transformed into carbon monoxide. If a liquid hydrocarbon is subjected to partial oxidation, the unprocessed carbon appears in the gaseous product in the form of fine soot particles, while if a solid fuel is subjected to partial oxidation, the unprocessed carbon appears in the product in the form of fines solid fuel particles. In addition, depending on the type of starting solid fuel, ash appears in different quantities in the combustion products.

 To cool the hot products of partial oxidation leaving the gas production area usually at a temperature above 9800C and to remove the entrained ash and solid fuel particles which are entrained, the hot gas is generally contacted a sprinkling medium, preferably water, in a sprinkling zone where the gas is cooled and where the entrained particles are sent into the sprinkling medium. The larger, low-carbon ash particles rest on and are removed from the bottom of the spray area, but the smaller particles form a suspension in the spray medium. To control the concentration of the solid substance in the sprinkling medium, a part is removed continuously periodically and is replaced by a fresh sprinkling medium. For economic and ecological reasons, it is advisable to separate the substance with low content. carbon from the high carbon substance and reuse the sprinkler water and unburnt fuel.

 As indicated above, when a liquid hydrocarbon is sent to the gas combustion zone, the unprocessed carbon is present in the form of fine particles of soot which have a microscopic size whereas if the product sent to the zone of gas production is solid fuel, unprocessed carbon comes in the form of small solid fuel particles. The soot formed during the gasification of the liquid hydrocarbon can be recovered from the suspension in the irrigation water by mixing it with a liquid hydrocarbon, as described in the U.S. Patent. 2,992,906 published July 18, 1961 in the name of F.E. Guptill, Jr. and in the U.S. Patent.

3,917,569 published on November 4, 1975 to GN Richter,
WL Slater, ET Child and JC Ahlborn. Unfortunately, unprocessed solid fuel particles have no affinity for liquid hydrocarbons as is the case for soot particles formed during the partial combustion of a liquid fuel and the separation technique that is used to recover soot is not suitable for recovering particles of unprocessed solid fuel from sprinkler water.

 Generally, two methods of gasification of solid fuel are used, one is carried out in the presence of fly ash and the other in the presence of mud. During operation of the first type, the gas generator is maintained at a temperature at which the ash remains practically solid while in the second type the temperature of the gas generator is high enough for the ash to be in the molten state. Usually when the operation is carried out in the presence of fly ash, the temperature is in the lowest operating range and the transformation of carbon is low. Unprocessed fuel and ash particles appear in the product of the synthesis gas. in the form of finely divided fly ash. The economic implementation of this type of process requires that a certain part of the unprocessed carbon is recovered and put back in circuit towards the gas generator, to obtain a more complete use of the carbon of the fuel.

 The object of the present invention is therefore to separate the ash from the unprocessed solid fuel produced during the gasification of the solid fuel in the form of fly ash.

The invention also aims to obtain a high transformation of the carbon of the solid fuel.

The object of the invention is also to reduce the quantity of waste which must be eliminated. Other objects will become apparent to specialists on reading the following description.

 According to the invention, there is provided a partial fuel oxidation process comprising the separation of particles having a high carbon content from an aqueous suspension of particles having a low carbon content produced by partial oxidation in the state fly ash, carbon fuel containing solid ash in a partial oxidation zone, and the passage of the resulting ash particles containing gas and unprocessed solid fuel in water to form the suspension, a process which consists in sieving the suspension to remove large particles of substance, in subjecting the sieved suspension to continuous centrifugation, at an acceleration force of between 10 and 1000 times the acceleration of gravity, so as to obtain a part of cake containing high carbon particles and a suspension in dilute water of low carbon particles, and to recycle the cake entrified with a fresh feed product in the partial oxidation zone.

 The starting material of the process according to the invention consists of solid carbon fuel containing ash-forming products, such as coal, sub-bitumi-lignite coal, petroleum coke, organic waste and similar products.

 The solid fuel in finely divided form, Sroy preferably so as to have a particle size having a maximum sectional dimension not greater than 0.65 cm and even more preferably crushed so that at least 90% passes through a sieve, the openings of which have a diameter of 1.168 mm, is introduced into the gas production zone where it is brought into refraction with water vapor and oxygen. Oxygen can be in the form of air, oxygen-enriched air, or virtually pure oxygen. The finely divided fuel can be introduced into the gas producing area in the form of a slurry in water or oil or a suspension in a gaseous or vapor medium such as water vapor, C02 and their mixtures.

In the gas production area, the solid fuel is subjected to partial oxidation at a temperature between 9800C and at least 270C below the initial deformation temperature of the ash which, depending on the type of solid fuel for supply , can be between 1090 and 1370au. The pressure in the gas production zone can be between 2.7 and 205 bars or even be higher and preferably between 4 and 170 bars. Oxygen can be introduced into the gasification zone with an atomic oxygen to carbon ratio between approximately 0.7 and 1.1 and preferably between 0.8 and 1.0. If the solid fuel is introduced into the gasification zone in the form of a slurry in water, the slurry will preferably contain not more than 50% by weight since a water content greater than this value affects the thermal efficiency of the reaction. Good results are obtained by using a water / fuel weight ratio of between 0.3 and 1 and preferably between 0.4 and 0.9.

 According to a preferred embodiment of the invention, the hot gaseous product containing entrained particles of ash and of unprocessed solid fuel is directed towards the gas chamber Zification by an opening situated at the bottom of this chamber and in a watering chamber containing water. The hot gases are introduced by passing through a dispersing device such as a dip tube provided with a toothed lower end, beneath the surface of the irrigation water and thereby cooled. In this process, the entrained particles are humidified with water and are led to the suspension in the sprinkling medium. In order to carry out the elimination of certain denser solid particles, which have a high ash content and which do not remain in suspension in the irrigation water, despite the agitation provided by the introduction of gaseous products under its surface, an outlet is provided at the bottom of the irrigation chamber for the elimination of these particles. denser and larger. Preferably, such particles are periodically removed by a lockable hopper connected to an outlet at the bottom of the chamber so that there is no loss of pressure during removal.

 The remaining unprocessed ash and fuel particles remain suspended in the sprinkler medium. In fact, there are only a few particles present which are purely carbon or purely ash, so it is more precise to classify them according to whether they have a high carbon content or a low carbon content. carbon. In this regard, particles having a carbon content of more than 33% by weight are considered to have a high carbon content, while particles having a carbon content of less than 33% by weight are considered to have a content low carbon.

 The separation of the high carbon particles from the low carbon particles is effected by gravity. It has been found, surprisingly, that although the particles with high carbon content are less dense than the particles with low carbon content, when they are subjected to the action of gravity they are the particles with the highest density. low carbon content which is retained in the aqueous phase while the carbon-rich particles are deposited. This phenomenon seems to be specific to the particles formed during the gasification of solid fuel as fly ash, since the particles produced by gasification of the fuel solid in the mud state behaved as expected, that is, the densest low carbon particles are deposited faster than high carbon and low ash particles.

 The suspension in water of the unprocessed solid fuel particles is preferably sieved through a 1.168 mm mesh opening sieve to remove all large particles of substance and then it is pumped to a centrifuge operating continuously where the carbon-rich cake is separated from a suspension of denser, low-carbon particles.

 A centrifugal accumulating force of between 10 and 1000 and preferably between 100 and 600 times the acceleration of gravity is applied to the suspension. The cake obtained is recirculated to the partial oxidation zone.

 The following example is given by way of illustration without limitation. The gasification operation is carried out in the form of fly ash.

EXAMPLE I
The starting material is Lake coal
Da Smet giving the following analysis result
TABLE I
Carbon 51.82% by weight
Hydrogen 3.92% by weight
Nitrogen 0.68% by weight
Sulfur 1.20% by weight
Oxygen 15.90% by weight
Ash 26.48% by weight
The coal, ground in such a way that it passes 100% through the sieve having a mesh opening diameter of 1.168 mm, is transformed into mud with an equal weight of water and with pure oxygen in a oxygen: carbon atomic ratio of 0.859, is introduced into a gasifier which is maintained at a temperature of 9800C and a pressure of 23.8 bars.

The gaseous product containing entrained fly ash is sprinkled in water to form a cooled gaseous product and a suspension of fly ash in the sprinkler water. The gaseous product has the following composition on a% molar basis:
TABLE 2
CO 11.68
H2 18.79
C02 20.10
H20 45.59
CH4 3.26
N2 0.23
H25 0.34
COS 0.01
The suspension containing 14.9% by weight of solid products containing 22.75% by weight of carbon and 77.25% by weight of ash is introduced after sieving into a centrifuge with continuous operation (model LB 224 manufactured by Bird Machine Co Inc. of
South Walpole, Mass.) Where it is subjected to a centrifugal force of acceleration equal to 570 times the acceleration of gravity. This treatment provides a centrifuged cake containing 66.80% by weight of solid consisting of 66.8% by weight of carbon and 36.2% by weight of ash and an aqueous suspension containing 12% by weight of solid products containing 9, 01% by weight of carbon and 90.99% by weight of ash.

 The above treatment corresponds to a transformation of 95% carbon. However, with the recirculation of the centrifuged cake and its introduction with a frank feed into the gasifier, the total transformation into carbon is increased up to 94.9%.

EXAMPLE IN
In this example, a sample of the suspension from Example I is subjected to a flotation test in a mixture of halogenated hydrocarbons having a density of 1.3.

 After standing for two hours, the floating part is separated and the two parts are filtered and dried. The carbon content of the depositing part is 83% by weight while that of the floating part is 30.5% by weight.

 This example shows that, even under ambient conditions, the less dense high carbon particles are deposited before the denser high ash particles.

 Although the examples indicate that the invention applies to the recirculation of particles formed by the spraying of synthesis gas produced by partial oxidation of coal united with water to form mud, it does apply also for the separation of particles suspended in water, the latter having been used for the irrigation or distillation of the synthesis gas produced by partial oxidation, of other carbonaceous fuels containing ash, introduced into the zone partial oxidation in the form of a suspension in water, in hydrocarbon oil or in a medium in the form of vapor or gas.

 It is understood that many modifications and variations may be made by specialists to the above examples, without departing from the scope or the spirit of this invention.

Claims (4)

REVEAiDICATIONS  1. Method for partial oxidation of a fuel comprising the separation of particles having a high carbon content from an aqueous suspension of particles having a low carbon content produced by partial oxidation in the state of ash flywheels of a solid carbonaceous fuel containing ash in a partial oxidation zone, and the passage of the gas obtained containing entrained particles of ash and unconverted solid fuel, in water to form the suspension, process characterized in that the suspension is sieved to liner large particles of substance and subjected to continuous centrifugation at an acceleration force of between 10 and 1000 times the acceleration of gravity so as to form a portion of cake containing high carbon particles and a dilute aqueous suspension of low carbon particles, and the centrifuged cake is recirculated with fresh feed to the partial oxidation zone.  2. Method according to claim 1, characterized in that the suspension is stored in an opening mesh screen with a diameter of 1.168 mm.  3. Method according to any one of the preceding claims, characterized in that the centrIfugation force has an acceleration between 100 and 800 times the acceleration of gravity.  4. process salop 1. any of the preceding claims, characterized in that the solid carbonaceous fuel containing ash is ground in the form of particles passing through the sieve having a diantre of opening of meshes of 1s1C6 rnm before the partial oxidation.