AU2002307073A1 - Black water recycle circulation loop use with a gasifier - Google Patents

Description

BLACK WATER RECYCLE CIRCULATION LOOP USE WITH A GASIFIER

BACKGROUND OF THE INVENTION

The process and advantages of gasifying hydrocarbonaceous material into synthesis gas are generally known in the industry. In high temperature gasification processes, synthesis gas may be produced from combustible organic fuels, such as coal, residual petroleum, wood, tar sand, shale oil, and municipal, agriculture or industrial waste. Prior to the gasification step, these combustible organic fuels are commonly mixed with water to form slurry or emulsion. The solid combustible organic fuels, in slurry form, are then reacted with a reactive oxygen-containing gas, such as air or oxygen, and a moderator such as water or steam in a gasification reactor to obtain synthesis gas.

In the reaction zone of a gasification reactor, the combustible organic fuel is contacted with a free-oxygen containing gas, optionally in the presence of a temperature moderator such as steam or water. In the reaction zone, the contents will commonly reach temperatures in the range of about 1,700° F (930° C) to* about 3,000° F (1650° C), and more typically in the range of about 2,000° F (1100° C) to about 2,800° F (1540° C). Pressure will typically be in the range of about 1 atmosphere (100 KPa) to about 250 atmospheres (25,000 KPa), and more typically in the range of about 15 atmospheres (1500 Kpa) to about 150 atmospheres (1500 KPa).

In a typical gasification process, the synthesis gas will substantially comprise hydrogen, carbon monoxide, and lessor quantities of impurities, such as water, carbon dioxide, hydrogen sulfide, carbonyl sulfide, ammonia, and nitrogen. The synthesis gas is commonly treated to remove or significantly reduce the quantity of impurities before being utilized in downstream processes.

An example of one such use is integrated gasification combined cycle (IGCC) power generation systems. Such systems are used throughout the world to generate power from the gasification of a fuel source. In such systems, a raw synthesis gas (or syngas) syngas stream, comprising H , CO, CO , and H O, is produced by the partial oxidation reaction of a hydrocarbonaceous fuel with a free-oxygen containing gas, typically in the presence of a temperature moderator such as steam water, in a quench gasification reactor.

The quenching process generates wastewater that must be treated for a wide range of contaminates including solids, water soluble compounds and partially water soluble compounds often referred to as "black water". Conventional treatment methods for the treatment of black water are expensive and thus most wastewater treatment systems are designed to store and treat no more than one day's generation of waste water. Thus any significant repair to the waste water treatment system causes the entire gasification process to be shut down. For this reason there remains a need for systems that allow the continued operation of a gasification system while maintenance is being performed on the waste water treatment system.

SUMMARY OF THE INVENTION The present invention is generally directed to handling the raw syngas scrubbing wastewaters, or black water, from a quench gasification reactor. More particularly, the present invention describes recirculation of the black water in a recirculation loop from which at least a portion of the moderator used in the gasifier may be taken. The use of a recirculation loop for the black water permits the operation of the gasifier in situations such as when a source of high pressure steam is inadequate. The present invention also permits the service and maintenance of the black water treatment system while permitting the continued operation of the gasifier.

A further aspect of this invention is treating the wastewater from a quench gasification reactor by recycling the wastewater to a location upstream of the gasifier. In this embodiment, the wastewater is recycled back to the gasification stage, where the black water and any carbon content is mixed with liquid or pulverized solid combustible organic materials to form slurry. The slurry is then fed to the gasifier where it is reacted with oxygen and optional additional steam at high temperatures and pressures so as to convert any carbon contained in the wastewater, along with the combustible organic fuel, into synthesis gas.

These and other features of the present invention are more fully set forth in the following description of illustrative embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The description is presented with reference to the accompanying drawings in which: FIG. 1 illustrates an embodiment of the invention in schematic form. It particularly shows the recycle of the black water back to the gasifier.

FIG. 2 illustrates an embodiment of the invention in schematic form. It particularly shows the recirculation of the black water back within a loop that can be isolated from the gasifier. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

- The following terms and phrases are used herein and are intended to have the following meaning:

"Combustible organic fuel" is defined as any combustible organic material such as coal, residual petroleum, wood, tar sand, shale oil, and municipal, agriculture or industrial waste. The scope of this definition is to include any combustible organic fuel that can be used in a gasification process to produce synthesis gas.

"Process water" is defined as any water used to make the slurry other than the wastewater from a hydrocarbon synthesis reactor. The scope of the term "process water" is to include any composition of materials in which water is the predominate component.

"Slurry" is defined as the combination of solid combustible organic fuel and water, where the water is process water or wastewater from a hydrocarbon synthesis reactor. See US Patents 4,887,383, 4,722,740, 4,477,259, and 4,242,098 describing some of the multitude of processes known in the art to produce slurry. The entire disclosures of the above referenced patents are hereby incorporated by reference and relied upon.

"Gasifying" or "gasification" is defined as the process in which various carbonaceous fuels may be converted to synthesis gas by partial oxidation at an elevated reaction temperature and pressure. In the typical gasification process, the carbonaceous fuel is contacted with a free- oxygen containing gas, such as air or oxygen, optionally in the presence of a temperature moderator such as steam. In the reaction zone, the contents will commonly reach temperatures in the range of about 1,700° F (930° C) to about 3,000° F (1650° C), and more typically in the range of about 2,000° F (1100° C) to about 2,800° F (1540° C). Pressure will typically be in the range of about 1 atmosphere (100 Kpa) to about 250 atmospheres (25,000 KPa), and more typically in the range of about 15 atmospheres (1500 Kpa) to about 150 atmospheres (1500 KPa). See US Patent 3,945,942 describing a partial oxidation burner assembly. See US Patent 5,656,044 describing a method and an apparatus for the gasification of organic materials. See also US Patents 5,435,940, 5,345,756, 4,851 ,013, and 4,159,238 describing a few of the many gasification processes known in the prior art. The entire disclosures of the above referenced patents are hereby incorporated by reference and relied upon. "Synthesis gas" (used interchangeably with the term "syngas") is defined as a gaseous mixture consisting substantially of hydrogen and carbon monoxide, with lessor quantities of impurities present such as water, carbon dioxide, hydrogen sulfide, carbonyl sulfide, ammonia, and nitrogen. It is within the scope of this definition to include any synthesis gas that been treated to remove or reduce the quantity of any of the impurities, so long as the primary components are hydrogen and carbon monoxide. In the present invention, carbonaceous fuel is first obtained and prepared for feeding to a gasification reactor. Carbonaceous fuel is any solid, liquid, or gaseous combustible organic material that can be used as feedstock to a gasification process for produce synthesis gas production. The feedstock for a gasification process is usually a hydrocarbonaceous material, that is, one or more materials, generally organic, which provide a source of hydrogen and carbon for the gasification reaction. The hydrocarbonaceous material can be in a gaseous, liquid or solid state, or in a combination as desired, for example, a solid-liquid composition in a fluidized state.

The feed preparation step may not be necessary, given the composition and physical nature of the feedstock. Generally, solid carbonaceous fuels will need to be liquefied with oil or water prior to feeding to the gasifier. Liquid and gaseous carbonaceous fuels may be suitable for direct feed to the gasifier, but can be pre-treated for removal of any impurities that might be present in the feed.

The term liquid hydrocarbonaceous fuel as used herein to describe various suitable feedstocks is intended to include pumpable liquid hydrocarbon materials and pumpable liquid slurries of solid carbonaceous materials, and mixtures thereof. For example, pumpable aqueous slurries of solid carbonaceous fuels are suitable feedstocks. In fact, substantially any combustible carbon-containing liquid organic material, or slurries thereof may be included within the definition of the term "liquid hydrocarbonaceous." For example, there are:

(1) pumpable slurries of solid carbonaceous fuels, such as coal, particulate carbon, petroleum coke, concentrated sewer sludge, and mixtures thereof, in a vaporizable liquid carrier, such as water, liquid CO₂, liquid hydrocarbon fuel, and mixtures thereof;

(2) suitable liquid hydrocarbon fuel feedstocks to the gasifier, is intended to include various materials, such as liquefied petroleum gas, petroleum distillates and residua, gasoline, naphtha, kerosine, crude petroleum, asphalt, gas oil, residual oil, tar sand oil and shale oil, coal derived oil, aromatic hydrocarbons (such as benzene, toluene, xylene fractions), coal tar, cycle gas oil from fluid-catalytic-cracking operations, furfural extract of coker gas oil, and mixtures thereof; (3) also included within the definition of the term liquid hydrocarbonaceous are oxygenated hydrocarbonaceous organic materials including carbohydrates, cellulosic materials, aldehydes, organic acids, alcohols, ketones, oxygenated fuel oil, waste liquids and by-products from chemical processes containing oxygenated hydrocarbonaceous organic materials, and mixtures thereof.

Gaseous hydrocarbonaceous fuels that may be burned in the partial oxidation gasifier alone or along with the liquid hydrocarbonaceous fuel includes vaporized liquid natural gas, refinery off-gas, -C₄ hydrocarbonaceous gases, and waste carbon-containing gases from chemical processes. After the feed preparation step, if used, the carbonaceous fuel is sent to a gasification reactor, or gasifier. In the gasifier, the carbonaceous fuel is reacted with a reactive free oxygen- containing gas. The term free-oxygen containing gas as used herein means air, oxygen-enriched air i.e. greater than 21 mole % O₂, and substantially pure oxygen, i.e. greater than about 90% mole oxygen (the remainder usually comprising N₂ and rare gases). Substantially pure oxygen is preferred, such as that that is produced by an air separation unit (ASU). The partial oxidation of the hydrocarbonaceous material, is completed, advantageously in the presence of a temperature control moderator such as steam, in a gasification zone to obtain hot synthesis gas, or syngas. Syngas and synthesis gas can and are used interchangeably throughout this specification.

The need for a temperature moderator to control the temperature in the reaction zone of the gas generator depends in general on the carbon-to-hydrogen ratios of the feedstock and the oxygen content of the oxidant stream. A temperature moderator is commonly used with liquid hydrocarbon fuels with substantially pure oxygen. Water or steam is the preferred temperature moderator. Steam may be introduced as a temperature moderator in admixture with either or both reactant streams. Alternatively, the temperature moderator may be introduced into the reaction zone of the gas generator by way of a separate conduit in the feed injector. Other temperature moderators include CO -rich gas, nitrogen, and recycled synthesis gas.

A gasification reactor generally comprises a reaction zone, made up of a vertical cylindrically shaped steel pressure vessel lined with refractory, and a quench drum, such as shown in U.S. Pat. No. 2,809,104, which is incorporated herein by reference. A feed injector, such as shown in U.S. Pat. No. 2,928,460, which is incorporated herein by reference, may be used to introduce the feed streams into the reaction zone. In the reaction zone of a gasifier, the contents will commonly reach temperatures in the range of about 1,700° F (927° C) to 3,000° F (1649° C), and more typically in the range of about 2,000° F (1093° C) to 2,800° F (1538° C). Pressure will typically be in the range of about 1 atmospheres (101 kPa) to about 250 atmospheres (25331 kPa), and more typically in the range of about 15 atmospheres (1520 kPa) to about 150 atmospheres (15,199 kPa), and even more typically in the range of about 60 atmospheres (6080 kPa) to about 80 atmospheres (8106 kPa). See US Patent 3,945,942 describing a partial oxidation feed injector assembly. See US Patent 5,656,044 describing a method and an apparatus for the gasification of organic materials. See also US Patents 5,435,940, 4,851,013, and 4,159,238 describing a few of the many gasification processes known in the prior art. The entire disclosures of the above referenced patents are hereby incorporated by reference and relied upon.

The hot gasification process product synthesis gas, or syngas, comprises carbon monoxide and hydrogen. Other materials often found in the synthesis gas include hydrogen sulfide, carbon dioxide, ammonia, cyanides, and particulates in the form of carbon and trace metals. The extent of the contaminants in the feed is determined by the type of feed and the particular gasification process utilized as well as the operating conditions.

The hot raw effluent syngas stream leaving the refractory lined reaction zone of the partial oxidation gas generator at substantially the same temperature and pressure as in the reaction zone, less ordinary drop in the lines is directly introduced into a pool of water contained in the bottom of a quench drum or tank such as the one described in coassigned U.S. Pat. No. 2,896,927 which is herewith incorporated by reference. The quench drum is located below the reaction zone of the gas generator, and the stream of raw syngas which it receives carries with it substantially all of the ash and/or slag and the particulate carbon soot leaving the reaction zone of the gas generator. The turbulent condition in the quench drum, caused by large volumes of gases bubbling up through the water helps the water to scrub much of the solids from the effluent gas. Large quantities of steam are generated within the quench vessel and saturate the gas stream. The stream of raw gas is cooled in the quench drum and leaves at a temperature in the range of about 350°F to 600°F (about 175°C to 315°C), such as about 450°F to 550°F (about 230°C to 290°C), and a pressure in the range of about 500 to 2500 psia, such as about 1000 psia. Thus a significant amount of waste water contaminated with solids, particulate carbon soot and other water soluble and insoluble materials is generated by the quench process being carried out in the quench drum. In order to prevent the plugging of downstream catalyst beds and/or the contaminating of liquid-solvent absorbents that may be used in subsequent gas purification steps, the cooled and partially cleaned syngas stream leaving the quench drum is further cleaned by contact with hot scrubbing water in another gas cleaning zone. This gas cleaning zone may include a conventional orifice such as shown and described in coassigned U.S. Pat. No. 3,524,630 which is incorporated herein by reference and conventional venturi scrubbers and sprays, along with a gas scrubbing chamber such as shown and described in coassigned U.S. Pat. No. 3,232,727, which is incorporated herein by reference. In the gas scrubbing chamber, the stream of raw syngas is scrubbed with scrubbing water comprising hot return condensate and make-up water as described herein. For example, in one embodiment the gas stream leaving the quench tank associated with the gasifier is scrubbed and intimately contacted with scrubbing water e.g. in a venturi scrubber. However, the use of a venturi scrubber in the gas cleaning zone is optional. The syngas passes into and up through a pool of gas scrubbing water contained in the bottom of a gas scrubbing chamber. The scrubbed gas is then passed up through a packed section or trays 'in the upper portion of the scrubbing chamber where it is contacted by condensate i.e. scrubbing water flowing in a downward direction.

The syngas can optionally be subjected to further cooling and cleaning operations involving a scrubbing technique wherein the syngas is introduced into a scrubber and contacted with a water spray which further cools the syngas and removes particulates and ionic constituents from the synthesis gas. The initially cooled gas is then treated to desulfurize the gas prior to utilization of the synthesis gas.

Syngas can be utilized as a fuel gas for power generation or for the synthesis of hydrocarbons in a Fischer-Tropsch operation or for use as a feedstock gas to many other different chemical process. One of ordinary skill in the art should appreciate and understand the value and use of syngas in the petrochemical industry.

In employing the syngas generation and cleaning procedure described above, the amount of solid particles in the scrubbed syngas stream is reduced to very low level such as less than about 3 parts per million (ppm), and preferably less than about 1 ppm. However, this also generates a considerable amount of waste water that is contaminated with solids, hydrocarbons and other various materials and often is referred to as "black water". Due to clean water regulations, such water must be treated prior to release. Conventional treatment methods such as clarification, bioreactor treatment, filtration, centrifugation, chemical treatment and other such techniques significantly add to the cost of operation. In addition, many plants only have the capacity to store one day or less of this waste water before the gasification reactor must be shut down. Thus anything but minor maintenance on the waste water treatment system causes the entire system to be shut down.

The present invention resolves the above problems by using a recycle loop in which the black water is passed though a recycle loop. It is further contemplated that at least a portion of the moderator water can be drawn from the recycle loop of the black water. In such embodiments the use of the black water as a source of moderator can substantially reduce the gasifier' s requirements for high pressure steam as a moderator.

Turning now to FIG. 1 illustrated is one exemplary embodiment of the present invention in schematic form. A quench gasification reactor 2 with a quench drum 3 produces synthesis gas 4 by the partial oxidation a hydrocarbon source 6 in the presence of an oxygen rich feed stream 8 and a moderator 10. The synthesis gas is quenched in the quench drum 3 and the slag generated by the gasification reaction is collected and handled by a slag handling system (not shown). As quench water becomes contaminated with fine carbon solids, and other fine particulate materials suspended in the water, water soluble compounds and the like, it is removed from the gasifier and sent to the black water storage tank 12. As black water accumulates, it is sent to conventional water treatment facilities that may include clarification, bio-reactor treatment, filtration, centrifugation, chemical treatment and other conventional treatment processes for black water. In order to prevent the settling out of the suspended solids, the black water is passed through a recirculation loop. The recirculation loop is composed of a recycle black water pump 14 which pumps the black water through a manual block valve 16 to either a recirculation valve 18 or a auto block valve 20. The recirculation loop for the black water is completed by opening the recirculation valve 18 and allowing of the black water to return to the black water tank 12. The black water returning to the gasifier can be used as the liquid portion of the hydrocarbon slurry feed or it may be used as an additional source of moderator. In such an embodiment the black water is used as a source of moderator and the quality and quantity demands on the primary source of moderators (in this case high pressure steam) is significantly reduced. Thus, the gasifier can continue operations when the primary source of moderator (in this case high pressure steam) is incapacitated. It should also be appreciated that the gasifier can be started up on the black water if there is careful control over the timing of the auto block valve such that the amount of black water moderator is appropriate. In other situations, the amount of black water that can be recirculated back to the gasifier will be limited and thus an alternate source of fresh feed water 22 may be included upstream of the autoblock valve. If volume of the black water and steam is not sufficient or incapacitated, it is possible to start and run the gasifier using the alternate feed water source.

One of ordinary skill in the art should appreciate that the configuration illustrated in FIG. 1 is capable of operating in a manner that allows the recirculation of the black water to prevent the settling out of any suspended particles. It should also be appreciated that it is possible to pressurize the black water handling system by recirculating the black water in a closed loop system prior to the start-up of the gasifier. Thus one is able to bring the gasifier on-line under more stable conditions than is the current practice.

As shown in FIG. 1, maintenance is possible on the recycle black water pump so long as the steam source is sufficient to maintain the operation of the gasifier. This is possible because the scheme provides for double isolation of the recycle black water pump. This is in contrast with the present state of the art which requires that the gasifier be shut down.

FIG. 2 illustrates another exemplary embodiment of the invention in schematic form. It particular, FIG. 2 shows the recirculation of the black water back within a loop that can be isolated from the gasifier. It should be noted that items having the same functional role have been given the same reference number in FIG. 2 as they were given in FIG. 1.

Turning now to FIG. 2, a quench gasification reactor 2 produces synthesis gas 4 by the partial oxidation a hydrocarbon source 6 in the presence of an oxygen rich feed stream 8 and a moderator 10. The synthesis gas is quenched in the quench drum 3 and the slag generated by the gasification reaction is collected and handled by a slag handling system (not shown). As quench water becomes contaminated with fine carbon solids, and other fine particulate materials suspended in the water, water soluble compounds and the like, it is removed from the gasifier and sent to the black water storage tank 12. As black water accumulates, it is sent to conventional water treatment facilities that may include clarification, bio-reactor treatment, filtration, centrifugation, chemical treatment and other conventional treatment processes for black water. In order to prevent the settling out of the suspended solids, the black water is passed through a recirculation loop. The recirculation loop is composed of a recycle black water pump 14 which pumps the black water through a first manual block valve 16 to either a recirculation valve 18 or a second manual block valve 24. The recirculation loop for the black water is completed by opening the recirculation valve 18 and allowing at least a portion of the black water to return to the black water tank 12. Thus one can open the first manual block valve 16, close the second manual block valve 24 and open the recirculation valve 18 and establish a closed loop recirculation of the black water.

When desired, the second manual block valve can be opened thus permitting the circulation of black water to the gasifier, via the auto block valve 20. The black water returning to the gasifier can be used as the liquid portion of the hydrocarbon slurry feed or it may be used as an additional source of moderator. In such an embodiment the black water is used as a source of moderator and the quality and quantity demands on the primary source of moderators (in this case high pressure steam) will be significantly reduced. Thus, the gasifier can continue operations when the primary source of moderator (in this case high pressure steam) is incapacitated. It should also be appreciated that the gasifier can be started up on the black water if there is careful control over the timing of the auto block valve such that the amount of black water moderator is appropriate. One of skill in the art should also note that the gasifier can be operated independently of the black water system. That is to say the alternate feed water source can be used to provide the moderator water needed by the gasifier. This can be beneficial during the start up or shut down of the gasifier, or when maintenance on the black water handling system is needed. For example, as shown in Fig. 2, the recycle black water pump can be isolated from the gasifier by double isolation which would allow maintenance on the black water pump whie the gasifier is still running. It should be noted that the amount of black water that can be recirculated back to the gasifier may be limited and thus can be balanced with the alternate source of fresh feed water 22. One of skill in the art should appreciate that the ability to isolate the black water circulation system is important for both maintenance purposes and operation of the gasifier. As noted above, because the black water recirculation system can be isolated, maintenance on the system can be conducted without having to shut down the gasifier. The only practical limit to such operation is the ability to store and /or treat any generated black water. The above illustrative embodiment also permits the start up of the gasifier on an alternate feed water source and then bring on-line the black water under pressure. This is possible because of the ability to recirculate and thus pressurize the black water recirculation loop. Another advantage of the present invention is that it permits the continuous recirculation of the black water in a closed loop system. This prevents the settling out of any suspended solids in the black water during time in which recycle of the black water to the gasifier is not desired. For example, when the gasifier is not running.

It should also be appreciated that as shown in the above embodiments of the present invention, wastewater from a gasification reactor can be recirculated in a recycle loop such that at least a portion of the waste water can be sent to a gasification reactor and thus used as a moderator. By doing so, many process and economic advantages may be realized. First, and perhaps most importantly, the wastewater can be disposed of with no adverse environmental affects. The large capital and operating costs associated with traditional water treatment facilities may be drastically reduced, if not eliminated. In addition the on-site requirements for high pressure steam as the source of moderator can be substantially reduced and the reliability of gasifier operations may be enhanced. While the devices, compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the process described herein without departing from the concept and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the scope and concept of the invention.

Claims (21)

CLAIMS:

1. A process for handling black water in a storage tank comprising: a) treating a portion of the black water; b) recirculating a portion of the black water in a recirculation loop; c) using a portion of the black water in a gasification unit; wherein the recirculation loop can be isolated from the gasification unit.

2. The process of claim 1, wherein the black water is treated with a process selected from the group consisting of clarification, bioreactor treatment, filtration, centrifugation, and chemical treatment.

3. The process of claim 1, wherein the recirculation of a portion of the black water comprises pumping a portion of the black water out of the storage tank, through piping, and back to the storage tank.

4. The process of claim 1, wherein a portion of the black water is used as a moderator in the gasification unit,

5. The process of claim 1, wherein a portion of the black water is used as the liquid portion of a slurry of hydrocarbon feed.

6. The process of claim 1, wherein a stream of process water is combined with the black water before the black water is used in the gasification unit.

7. The process of claim 1, wherein the gasification unit is started up using the black water.

8. The process of claim 1, wherein the recirculation loop is isolated from the gasification unit.

9. A gasification process, comprising: a) partially oxidizing a hydrocarbon in the presence of an oxygen rich stream and a moderator, producing syngas, the partial oxidation process taking place in a gasification unit; b) quenching the syngas with quench water in a quench drum, forming black water; c) removing the black water from the quench drum; d) sending a portion the black water to a black water storage tank; e) treating a potion if the black water with treatment; and f) pumping a portion of the black water in the black water storage tank through a recirculation loop, wherein the recirculation loop can be isolated from the gasification unit.

10. The process of claim 9, wherein a portion of the black water is sent to the gasification unit.

11. The process of claim 10, wherein a portion of the black water is used as the moderator in the gasification unit.

12. The process of claim 10, wherein a portion of the black water is used as the liquid portion of a slurry of hydrocarbon feed fed to the gasification unit.

13. The process of claim 10, wherein a stream of process water is combined with the black water before the black water is used in the gasification unit.

14. The process of claim 10, wherein the black water is pressurized in the recirculation loop prior to being sent to the gasification unit.

15. The process of claim 9, wherein the portion of the black water sent to the gasification unit is combined with a stream of process water.

16. The process of claim 9, wherein the recirculation loop comprises a black water pump, a manual block valve downstream of the black water pump, and a recirculation valve allowing for flow back to the black water storage tank.

17. The process of claim 9, wherein the black water is treated with a process selected from the group consisting of clarification, bioreactor treatment, filtration, centrifugation, and chemical treatment.

18. The process of claim 9, wherein the recirculation loop is isolated from the gasification unit.

19. The process of claim 9, wherein the gasifier can be operated independently of the recirculation loop.

20. The process of claim 19, wherein a stream of process water is used as the moderator.

21. The process of claim 9, wherein the gasification unit is started up using the black water.