GB2145732A - Process for making aqueous transportable fuel slurry from carbonaceous materials - Google Patents

Abstract

A process for converting moist carbonaceous materials into an aqueous slurry suitable for pipeline transport and subsequent combustion in furnace burners such as power boilers and the like. In accordance with the process, a moist carbonaceous feed material such as peat, brown coal, lignite, subbituminous coal, waste agricultural cellulosic materials and the like is subjected to an autoclaving treatment forming a thermally upgraded solid reaction product which is comminuted, if necessary, to a desired particle size and thereafter is slurried with a controlled amount of the recovered condensate derived from the process including the condensible organic constituents producing a fuel-grade slurry of enhanced heating value and stability. This slurry can be concentrated by centrifugation and solvent extraction before being burned.

Description

SPECIFICATION Process for making aqueous transportable fuel slurry from carbonaceous materials The present invention is broadly applicable to the processing of carbonaceous materials containing appreciable quantities of moisture employing an autoclaving step by which a solid upgraded reaction product is produced of a substantially lower residual moisture content which is comminuted, if necessary, to the desire particle size and admixed with a controlled amount of the recovered moisture liberated during the autoclaving step, preferably, in combination with at least a portion of the recovered volatile organic constituents forming an aqueous fuel slurry of enhanced heating value.

The shortages and rising prices of conventional energy sources such as petroleum and natural gas have provided impetus in deveioping alternative energy sources in plentiful supply such as peat, brown coal, lignite, subbituminous coal, waste cellulosic materials including sawdust, bark, wood scrap, branches and chips derived from lumbering and sawmill operations as well as various agricultural waste materials such as cotton plant stalks, nut shells, corn husks and the like. Such alternative carbonaceous materials, unfortunately, are inefficlient for direct use as a high energy fuel for a variety of reasons including the relatively high moisture and oxygen contents thereof and their physical form.In view of the foregoing, a variety of processes have been proposed for converting such carbonaceous materials into a form in which their heating value on a moisture-free basis is substantially enhanced, in which they are stable and resistant to weathering during shipment and storage and in which the upgraded fuel product can more readily be adapted for use in conventional furnace burning equipment.

Typical of such prior art processes are those such as described in United States Patent No.4,052,168 by which lignitic-type coals are chemically restructured through a controlled thermal treatment providing an upgraded carbonaceous product which is stable and resistant to weathering as well as being of increased heating value approaching that of bituminous coal; U.S. Patent No.4,127,391 in which waste bituminous fines derived from conventional coal washing and cleaning operations are treated to provide solid agglomerated coke-like products suitable for direct use as a solid fuel; U.S. Patent No.

4,129,420 in which naturally occurring cellulosic materials such as peat as well as waste cellulosic materials are upgraded by a controlled thermal restructuring process to produce solid carbonaceous or coke-like products suitable for use as a solid fuel either by itself or in admixture with conventional fuel oils; and co-pending United States PatentApplication Serial No. 449,421,filed December 13,1982 by Edward Koppelman et al for "IMPROVED APPAR ATUS FOR THERMAL TREATMENT OF ORGANIC CARBONACEOUS MATERIAL" which discloses both an apparatus and process for thermal pretreatment and treatment of carbonaceous material such as peat to provide an upgraded solid fuel product.An improved apparatus and process for achieving such an upgrading of carbonaceous materials is further described in United States Patent No.4,126,519 which is assigned to the same assignee as the present invention. The teachings of the aforementioned United States patents and pending United States patent application are incorporated herein by reference to the extent they are relevant to the present invention.

In accordance with the processes disclosed in the aforementioned United States patents and pending patent application, a moist carbonaceous feed material is subjected to an autoclaving treatment at a controlled elevated temperature under a controlled pressure for a period of time to effect a vaporization of substantially all of the moisture content and at least a portion of the volatile organic constituents therein to produce a gaseous phase and to further effect a controlled thermal restructuring of the chemical structure and composition of the carbonaceous material. The resultant reaction product is stable and is of an increased heating value approaching that of bituminous coal.A continuing problem associated with the use of such alternative energy sources such as naturally occurring bituminous coal and the upgraded reaction product as described in the aforementioned patents is that the geographical location of the source is generally disposed at a great distance from the point of ultimate use thereby presenting serious problems in the transportation, handling and storage. One proposed solution to this problem has been to form an aqueous slurry of bituminous coal in a finely particulated form enabling transportation thereof by pipeline to the point of ultimate use. The formation of such an aqueous slurry usually containing from about 50 percent to about 70 percent solids requires substantial quantities of local water which in many instances is in short supply imposing a prohibitive burden on the local surface and ground water supply.Afurther problem arises in the removal of substantially all of the water from the aqueous slurry at the point of destination requiring expensive centrifuge equipment and the associated expenses in the maintenance and operation of such equipment. The removal of substantially all of the water content has been found necessary to provide a net fuel product of substantially high heating value and eliminating the loss of energy that would otherwise be necessary for effecting a vaporization of the water content during the combustion process. Frequently, the extracted water contains contaminants leached from the coal during pipeline transit further requiring expensive waste treatment faciiities to render it environmentally acceptable.The foregoing factors and considerations have seriously detracted from a more widespread use of plentiful supplies of alternative energy sources such as subbituminous coal because of the economic and environmental aspects involved.

The process of the present invention overcomes many of the foregoing problems and disadvantages by providing an upgraded aqueous fuel slurry produced substantially from the original feed material as mined which is of relatively high heating value and can readily be burned in the form of a slurry using conventional burner equipment providing an economical fuel which is readily transportable by pipeline without anv of the aforementioned environmental disadvantages.

Summary of the invention The benefits and advantages of the present invention are achieved by a process in which a moist carbonaceous feed material is subjected to an autoclaving treatment at an elevated temperature and pressure for a controlled time period to effect a vaporization of substantially all of the moisture content therein and a vaporization of at least a portion of the volatile organic constituents therein while simultaneously effecting a controlled thermal restructuring of the chemical structure of the material as well as a change in the chemical composition thereof.The resultant reaction product of upgraded structure and heating value is comminuted, if necessary, to a desired fine particle size and the vaporized moisture incorporating condensibie organics therein is admixed with the comminuted reaction product in appropriate proportions to provide a pumpable slurry suitable for pipeline transportation. The condensed liquid phase containing the condensible organic constituents is usually employed to provide a slurry containing from about 50 percent up to about 70 percent solids. The quantity of the liquid condensate will vary depending upon the specific type of moist carbonaceous feed material employed.

In those instances in which the liquid phase is in excess of that required to form the desired aqueous fuel slurry, a concentration of the liquid condensate can be effected such as by evaporation employing by-product heat from the process reactor with a retention of substantially all of the organic constituents in the concentrated liquid residue. In some instances in which the feed material is of relatively low moisture content, only minimal quantities of local fresh water are required to provide the deficiency in liquid to produce a desired slurry. It is also contemplated that other liquids miscible with water such as alcohol can be added to form a slurry of the desired flow characteristics.

At the point of destination, the slurry can be further concentrated, as may be desired, to achieve optimum combustion characteristics by extraction of a portion of the water present which can be further treated such as by solvent extraction to remove the organic constituents therein which in turn are recombined with the slurry. The presence of the organic constituents in the aqueous phase contributes substantially to the heating value of the slurry offsetting the energy required for vaporizing the water during the combustion process.

It will be appreciated from the foregoing, that the process of the present invention produces a high heating value aqueous fuel slurry substantially entirely from the original as-mined moist feed material obviating the need of any significant amount of local fresh water; which slurry is in a form eminently suitable for transportation by pipeline; which slurry can be further concentrated, if desired, at the point of destination by the extraction of a small proportion of the water content thereof employing economical extraction techniques; and wherein the organic constituents in the aqueous phase of the slurry further contribute toward the heating value thereof while at the same time obviating the need for any water waste treatment facilities.

Additional benefits and advantages of the present invention will become apparent upon a reading of the Description of the Preferred Embodiments taken in conjunction with the accompanying drawing and specific examples provided.

Brief description of the drawing Figure lisa schematic flow diagram of the operating steps performed on a feed material to produce an aqueous slurry suitable for pipeline transport; and Figure 2 is a schematic flow diagram of a sequence of steps to which the aqueous fuel slurry is subjected to provide a slurry suitable for combustion.

Description of the preferred embodiments The sequence of steps typically employed in producing a pipeline slurry from a moist carbonaceous feed material is schematically illustrated in the flow diagram comprising Figure 1 ofthedraw- ing. As shown, a feed material such as coal in a storage hopper 10 is transferred via conduit 12 into a reactor section 14 in which it is heated to an elevated temperature under controlled pressure for a period of time to effect the desired vaporization of moisture and organic volatile constituents and the desired magnitude ofthermal restructuring ofthe chemical structure of the feed material.The noncondensible gases evolved in the reaction section are removed through a conduit 16 and can be further processed, as desired, or can be employed as a source of energy for heating the reactor section. The condensible gases including entrained water and chemically combined water in the feed material liberated during the reaction as well as volatile organic constituents and those produced by the thermal cracking and/or degradation of the carbonaceous material are suitably transferred through a conduit 18 to a condenser 20 and the liquid condensate recovered is transferred through a conduit 22 to a mixer 24 for forming the pipeline slurry. The solid reaction product produced in the reactor which is substantially low in residual moisture content is transferred through a conduit 26 into a grinder 28 in which it is comminuted to the desired particle size whereafter it is transferred through a conduit 30 into the mixer 24. It is also contemplated that under certain conditions, it is more economical to effect a comminution of the reaction product bywetgrinding in which eventthe grinding operation is performed after the liquid condensate and reaction product are combined in the mixer 24.

The reactor section 14 may further include a pretreatment section for effecting a partial dewatering of the feed material prior to the high temperature autoclaving step whereby liquid water is recovered and transferred through a conduit 32 to the mixer for forming the slurry. It is also contemplated, depend ing upon the particular moisture content of the feed material employed and the conditions under which the autoclaving reaction is performed, that any excess of water produced beyond that required for forming the slurry can be removed by an evaporator 32 and the resultant waste water concentrate incorporating organic constituents therein is transferred to the mixer.The mixer 24 is equipped with highshear agitation to form a substantially uniform disperson of the finely particulated upgraded reaction product in the recovered liquid phase from the reactor forming a slurry normally containing from about 40 percent up to about 70 percent by weight solids, and preferably, from about 50 percent to about 55 percent solids. The slurry is transferred through a conduit 34 to a holding tank 36 equipped with agitation or directly into a transport pipeline.

In the processing of feed materials of relatively low initial moisture content, it is also contemplated that the substantially dry reaction product produced can be divided into two separate fractions such that one fraction is admixed with the liquid condensate recovered forming a pumpable slurry of the desired characteristics. The other solid fraction can be handled as a separate solid fuel product. Under such conditions in which inadequate liquid condensate is available for forming a pumpable slurry of the entire solid reaction product, it is also contemplated that additional liquid can be added to achieve the desired flow characteristics of the slurry. Any liquid miscible with water can be used for this purpose including surplus liquid condensate from another reactor or other organic liquids such as alcohol, for example.

The pipeline transportable slurry upon arriving at its destination can be employed directly as a fuel slurry, and preferably, is concentrated to remove a portion of the water therefrom to enhance the heating value and combustion characteristics thereof. Generally, the quantity of water necessary to provide a slurry satisfactory for pipeline transportation is somewhat higher than that required to provide an optimum fuel grade slurry. Generally speaking, fuel grade slurries adapted to be burned in conventional burner equipment requiring only minor modifications to accommodate the aqueous slurry, contain water in amounts up to about 30 percent by weight with amounts of about 20 percent to about 30 percent by weight being typical depending upon the specific burner equipment employed.A typical sequence for effecting a concentration of the pipeline slurry to produce a fuel grade slurry is schematically illustrated in the flow diagram comprising Figure 2 of the drawing. As shown, the pipeline slurry is transferred from a receiving terminal 38 through a conduit 40 into a liquid removal device such as a centrifuge 42 in which the desired fraction of liquid is extracted through a conduit 44 providing a concentrated slurry withdrawn through a conduit 46. The extracted liquid 44 contains a portion of the valuable organic constituents and is preferably subjected to an extraction operation such as a solvent extractor 48 to remove substantially all of the organic constituents therefrom. The balance of the extracted liquid comprising essentially water is removed through a conduit 50.The extracted water removed through the conduit 50 may contain small residual amounts of organic constituents therein which can be further removed such as by an activated charcoal treatment and the purified water can readily be treated such as by ponding or conventional aeration to enable it to be harmlessly discharged to waste.

The organic constituents recovered in the extractor 48 are transferred through a conduit 52 to a mixer 54 in which the concentrated slurry and recovered organic phase are recombined to form a fuel-grade slurry of a liquid concentration less than that of the pipeline slurry. The fuel-grade slurry can be transferred to a holding tank 56 equipped with agitation for subsequent combustion in a burner apparatus. It is also contemplated that in some applications, it may be advantageous to add additional organic liquids, such as alcohol, for example, to improve the combustion characteristics of the fuel slurry. Such additionai liquids can be conveniently introduced in the mixer 54 of Figure 2.A further specific description of the flow diagrams provided in Figures 1 and 2 of the drawings will subsequently be provided by way of the specific examples including a material and energy balance for the processing of a subbituminous coal.

The carbonaceous feed material, as previously set forth, may comprise any one or mixtures of mate rials such as lignitic-type coals, cellulosic materials such as peat, waste cellulosic materials including sawdust, bark, wood scrap, branches and chips derived from lumbering and sawmill operations, various agricultural waste materials such as cotton plant stalks, nut shells, corn husks or the like, as well as subbituminous coals. The foregoing feed materials will vary in their initial moisture content including entrained moisture as well as chemically combined water.In view of the foregoing, the particular conditions to which the feed material is subjected in the reactor section will vary depending upon the specific characteristics of the feed material and its initial moisture content to provide optimum efficiency and to provide a fuel-grade slurry of the desired characteristics.

When lignitic-type coal is employed as the feed material, process conditions such as described in United States Patent 4,052,168 are preferably employed for producing an upgraded, stable and weather-resistant reaction product. Similarly, when cellulosic materials are employed as the feed, conditions as described in United States Patent No.

4,129,420 are preferably employed. When cellulosic materials such as peat containing a relatively high percentage of moisture up to as high as about 92 percent are employed as the feed material, it is preferred to employ the process and apparatus as described in pending United States PatentApplication 449,421, filed December 13, 1982 to provide for a pretreatment and a partial dewatering of the feed material in the reactor section before subjecting the dewatered feed material to the wet pyrolysis treatment in the reactor section. Briefly stated, the dewatering pretreatment may comprise preheating the entering feed material to a temperature generally up to about 500"F and subjecting the preheated feed material to pressure whereby moisture in liquid form is extracted and is withdrawn through a conduit such as the conduit 32 as illustrated in Figure 1.In the case of peat having an ir homing moisture content of from about 50 up to about 70 percent by weight or greater, a dewatering can be effected to provide a residual moisture level of from about 15 to about 30 percent by weight, and preferably less than about 15 percent by weight. In the case of all such carbonaceous feed materials, it is desirable to retain a small percentage of moisture in the feed stock entering the subsequent reaction chamber to enhance the thermal pyrolysis reaction with amounts of about 5 to about 15 percent by weight being preferred. It is also usually preferred to comminute or shred the incoming feed material into particles of a size that facilitatestheirtransport and heat transfer through the several reaction stages.

The feed material is reacted in a reaction chamber provided in the reaction section by heating the material to a temperature generally within a range of about 400" up to about 1200 F, and preferably from about 5000 to about 1 0000F under pressures ranging from about 300 to about 3000 psi, and preferably from about 600 to about 1500 psi. The specific temperature and pressure employed is established depending upon the specific type of feed material being processed and the desired characteristics of the reaction product produced. The reaction can be carried out in a continuous manner such as described in the aforementioned United States patents and pending U.S. patent application, or alternatively, can be carried out in a batch-wise autoclave.The time necessary to effect the appropriate thermal restructuring and upgrading of the feed material is interrelated to the temperature and pressure conditions employed and the characteristics of the product desired. A portion of the dwell time of the material in the reactor is consumed by the vaporization of the residual moisture content therein as well as the volatilization of organic constituents whereaf terafurtherheating of the feed occurs effecting a mild wet pyrolysis treatment. In order to provide optimum efficiency, the various gaseous products produced are transferred in a countercurrent manner relative to the direction of travel of the feed material as more fully described in the aforementioned United States patents and pending application, the teachings of which are incorporated herein by reference.

At the completion of the reaction, the hot reaction product is transferred under a protective atmosphere and is cooled to a temperature at which it can be exposed to the atmosphere without adverse effects. Generally, the cooling can be effected by injecting water on the reaction product in a manner to form by-product steam which can be advantageously employed in preheating the incoming feed material. The cooled product is discharged from the reactor through a suitable pressure let-down device such as an extruder, for example. The upgraded solid fuel product can be subjected to a grinding or pulverizing step to provide a particle size of the solid fuel within a range in accordance with conventional practice to provide a pumpable slurry in further consideration of the specific density of the particles and the percentage of solids in the slurry.The desired particle size will also depend on whether a further grinding of the coal is to be effected at the terminal prior to combustion to better adapt the slurry to a specific burner apparatus. In such instances in which no further grinding is effected at the terminal site, then it is preferred to pulverize the coal to a particle size particularly adapted for optimum combustion in the burner apparatus to be employed.

Particle size and particle size distribution will also vary in consideration of whether any water is to be extracted from the slurry at the teminai site prior to combustion. Typically, for most applications, the grinding or pulverizing step is performed to provide a particle size range of from about 400 up to about 16 mesh (U.S. Standard Sieve Series) with a particle size range of about 325 to about 16 being usual. The particles are generally distributed over a range of sizes with the optimum size distribution for any particular situation varying in consideration of minimum void volume of a mixture of particles as well as viscosity and flow parameters in accordance with known techniques.

In order to further illustrate the process of the present invention, the following examples are provided. It will be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the present invention as herein described and as set forth in the subjoined claims.

Example 1 With reference to Figure 1 of the drawing, an aqueous slurry suitable for pipeline transit was prepared employing an as-mined subbituminous coal from Ft. Union Coal Mine of Gillette, WY. The proximate analysis of the feed coal sample was: moisture -31.0%; volatiles-31.51%; fixed carbon 32.17%; ash - 5.31%. The higher heating value of the feed material was 7776 Btu/pound.

The subbituminous feed material was subjected to a wet pyrolysis treatment in a batch-type autoclave employing a maximum pressure of 800 psig and a maximum temperature of 750"F. A period of 90 minutes was required to heat the feed material in the autoclave from room temperature (70"F) up to about 750"F. The reaction vessel was subsequently cooled and the reaction product was extracted and transferred to a grinder such as shown at 28 in Figure 1 in which it was comminuted to a particle size distribution of 20 percent less than 325,70 percent less than 100 and 100 percent less than 16 mesh by weight whereafter the particulated reaction product was transferred to a mixer to which the condensible gas phase was added forming a pipeline slurry nominally containing about 55 percent by weight solids.

In accordance with the foregoing processing sequence, a material balance on the feed material as charged and the pipeline slurry as recovered is hereinafter provided based on 1 pound of the as-mined subbituminous coal. The reaction product extracted from the autoclave comprised 0.493 pounds of upgraded solid fuel. The noncondensible gas phase comprised 0.106 pounds while 0.401 pounds of condensible liquid was recovered including water and volatile organics.

The solid reaction product upon analysis had a higher heating value of 6326.6 Btu equivalent to 12,834 Btu per pound. The noncondensible gas phase upon analysis had a higher heating value of 521 Btu which is equivalent to 478 Btu per standard cubic foot. The residual higher heating value of the liquid phase recovered was calculated by difference to be 928.4 Btu which is equivalent to 2315 Btu/ pound.

The siurry produced by combining the recovered condensible liquid phase and the comminuted solid reaction product provided a slurry mixture comprising 0.894 pounds having a higher heating value of 7255 Btu which is equivalent to 8,115 Btu per pound.

The present example clearly substantiates the benefits of the process of the present invention in enabling upgrading of a moist bituminous coal mined in Wyoming to form an upgraded slurry suitable for transport through a pipeline without requiring any local surface or well water for producing the slurry.

Example 2 With reference to Figure 2 of the drawing, a pipeline slurry such as produced in accordance with Example 1 is converted into a fuel-grade slurry suitable for combustion in a burner apparatus. In accordance with the arrangement as illustrated in Figure 2, a pipeline slurry such as derived from Example 1 is subjected to centrifuging or other liquid extraction operation to remove only a portion of the liquid phase from the slurry. It should be pointed out, that when subjecting such a slurry to centrifugation, it is relatively simple and economical to remove only a portion of the liquid although it is very difficult and expensive to remove all of the liquid.The liquid phase removed is subjected to a solvent extraction treatment to extract all of the organic content thereof and the organic fraction along with the concentrated slurry is recombined in a mixer 54 such as shown in Figure 2 to produce the fuel-grade slurry.

In accordance with a material balance of the foregoing process, 1 pound of pipeline slurry received from Example 1 of a higher heating value of 8115 Btu per pound is centrifuged to remove 0.280 pounds of the liquid phase. The liquid phase is subjected to solvent extraction producing 0.130 pounds of water which is discarded and 0.106 pounds of organic liquids which is transferred to the mixing unit. The concentrated slurry and organic liquids are recombined to produce a fuel-grade slurry containing 70 percent by weight solids and comprises 0.786 pounds of the original 1 pound pipeline slurry received having a higher heating value of 7983 Btu which is equivalent to 10,156 Btu per pound. It will be apparent that the dissolved organics in the liquid phase contribute substantially to the energy content of the aqueous fuel-grade slurry.

Claims (11)

1. A process for converting moist carbonaceous materials into a useful aqueous fuel slurry comprising the steps of charging a moist carbonaceous feed material into an autoclave, heating the feed material to an elevated temperature and undersuperatmos- pheric pressure for a period of time sufficient to convert at least a portion of the moisture and the volatile organic constituents therein into a gaseous phase and to effect a partial thermal restructuring of the chemical structure and a change in the chemical composition of the feed material to produce a solid reaction product, comminuting the reaction product to a desired particle size, recovering at least a portion of the gaseous phase as a liquid condensate, admixing the liquid condensate with the comminuted solid reaction product to form an aqueous fuel slurry comprising the particulated solid reaction product dispersed in an aqueous solution containing combustible organic constituents dissolved and dispersed therein.

2. The process as defined in claim 1 including the further step of transporting the aqueous fuel slurry through a pipeline to a location remote from the site of the process.

3. The process as defined in claim 1 including the further step of introducing the aqueous fuel slurry into a burner apparatus and effecting a combustion thereof.

4. The process as defined in claim 1 including the further step of controlling the quantity of solid reaction product and liquid condensate to provide an aqueous slurry containing about 40 percent to about 70 percent by weight solids.

5. The process as defined in claim 1 including the further step of controlling the quantity of solid reaction product and liquid condensate to provide an aqueous slurry containing about 50 percent to about 60 percent solids.

6. The process as defined in claim 1 including the further step of admixing additional fresh water with the liquid condensate to provide an aqueous slurry containing the desired concentration of solids.

7. The process as defined in claim 1 including the further step of admixing additional liquid with the liquid condensate to provide an aqueous slurry containing the desired concentration of solids.

8. The process as defined in claim 7 in which said additional liquid comprises alcohol.

9. The process as defined in claim 2 including the further step of extracting a portion of the liquid phase of the aqueous slurry forming a slurry concentrate, extracting substantially all of the organic constituents in the extracted liquid phase, and thereafter admixing the slurry concentrate and extracted liquid organic constituents to form a fuelgrade slurry.

10. The process as defined in claim 9 including the further step of admixing an additional liquid comprising an organic material with the slurry concentrate and extracted liquid organic constituents.

11. A process for converting moist carbonaceous materials into a useful aqueous fuel slurry substantially as herebefore described with reference to the accompanying drawings.