GB2134921A

GB2134921A - High temperature pyrolysis process

Info

Publication number: GB2134921A
Application number: GB08403603A
Authority: GB
Inventors: Francis Thomas Matthews; David Gordon Turek
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1983-02-14
Filing date: 1984-02-10
Publication date: 1984-08-22
Also published as: JPS59155492A; GB8403603D0; DE3405213A1

Abstract

A carbonaceous alkali-containing fuel (10) is heated in a vessel (12) driving off gaseous hydrocarbons and alkalis. A portion of the product gas (28) is mixed with an oxidant (32) and reacted (38) substoichiometrically to form a hot gas (14). The hot gas (14) is routed to the vessel (12) to provide heat for the pyrolysis process. The remaining gas (30) is cooled (46) to condense and remove the alkalis (50). The alkali-free gas (52) is reheated slightly (54) and emerges from the final particulate removal step (56) as a clean, alkali-free fuel gas (58) suitable for use in a high temperature gas turbine or the like. <IMAGE>

Description

SPECIFICATION High temperature pyrolysis process Field of the invention This invention pertains to a pyrolysis process for producing a gas from a solid or liquid fuel, and more particularly, to a high temperature pyrolysis process for producing a clean, alkali-free gas.

Background of the invention Recent changes in the world energy market have placed conflicting demands on the producers and users of electric power. On the one hand, the general increase in the cost of all forms of energy has resulted in a demand for increasingly efficient electric generating plants.

On the other hand, the specific increase in the cost of refined petroleum products relative to other less clean forms of energy, such as coal, has created a large incentive for producers to switch from petroleum to these less expensive energy sources.

The conflict between these two incentives becomes most apparent in the case of the high temperature gas turbine combined cycle electric generating plant. These plants combine the high temperature generating efficiency of a gas turbine with the low temperature heat recover efficiency of a steam boiler to provide an overall plant efficiency in the range of 5 to 10 percent higher than that of an ordinary steam Rankine cycle generating plant. Combined cycle plants of this type are not new in the art and have been operated successfully on refined petroleum fuels in the past. The efficiency of these plants is directly related to the gas turbine inlet temperature, with higher turbine inlet temperatures resulting in higher overall electric generation efficiencies.Increased inlet temperatures have palced increased performance demands upon the gas turbine vanes, blades, and other internal parts. In order to obtain a suitable performance life for a gas turbine with an inlet temperature in excess of 2,000F (1 093C), it is necessary that the incoming hot gas be substantially free of all particulate matter and any alkali containing comounds to prevent the respective erosion and corrosion of the turbine internals.

The demand that the gas turbine inlet gas be of such high purity has heretofore substantially eliminated the use of an alternate fuel for a high temperature gas turbine combined cycle generating plant. These alternate fuels, such as coal, heavy oil, tar, sands, wood chips, etc., contain inert compounds which may remain in the flue gas after the combustion process. As the inlet to the gas turbine generator is generally in excess of five atmospheres pressure, the removal of these inert compounds must take place at a high temperature and pressure. Such high temperature and pressure cleanup systems are costly and complex, and have not received wide acceptance in the electric generation industry. The direct combustion of these alternative fuels has therefore found use only in low inlet temperature, low efficiency gas turbine generators.

One possible method which has been proposed to allow the use of an alternative fuel, such as coal, in a high temperature gas turbine combined cycle generating plant involves the gasification of the alternative fuel by means of partially combusting the solid or liquid fuel to produce a fuel gas. The fuel gas is then cooled to a temperature suitable for the removal of any particulate matter therein and burned in a pressurized combustor upstream of the gas turbine inlet. Several such systems, such as the well known Texaco process, are undergoing construction of testing at this time.

Many of the alternative fuels discussed above, such as coal, heavy oil, and wood chips, consist of a volatile portion, a fixed carbon portion, and an inert, or ash, portion. The volatile portion of these fuels contains hydrocarbons, water, and other compounds easily driven off when the fuel is heated. The gas evolved during the devolatilization, or pyrolysis, of these fuels generally has a heating value in the range of 500 to 800 BTU/scfm (4450 to 7119 kcal/scm) and, if suitably cleaned of any impurities which may be present, would make an excellent fuel for a high temperature gas turbine.

A pyrolysis process for producing a fuel gas is much simpler than any of the gasification processes discussed above due to the decreased system conversion requirements. It is the conversion of the fixed carbon portion of a fuel such as coal into a gas which requires the bulk of the equipment and process complications, In general, however, total gasification processes have received much more favorable attention in recent years than the simple pyrolysis process for two reasons: first, a pyrolysis process leaves behind a large amount of the fuel chemical heating value as a fixed carbon, or char, which is too valuable to throw away;sand second, a high efficiency pyrolysis process takes place in excess of 11 OOF (593C), the temperature at which any alkali compounds within the feed fuel would be devolved along with the volatile portion of the fuel.

The advent of fluidized bed technology has provided a means for efficiently utilizing the chemical heat energy available in the char left over from the pyrolysis process, but the problem of alkali and other particulate matter in the generated gas still remains. Although any combined cycle generating plant using a pyrolysis process to produce fuel gas for the gas turbine generator would suffer an efficiency penalty in the sense that a part of the feed fuel chemical heat energy would be bypassing the high temperature gas turbine generation cycle, the simplicity and reduced cost of such a system could well compensate for this operating penalty.

What is needed is a method for pyrolyzing a solid or liquid fuel at high temperature which results in a clean, alkali-free fuel gas suitable for use in a high temperature gas turbine.

Summary of the invention The process according to the present invention pyrolyses the feed fuel at a high temperature (over 11 00F) to give a high volatile yield while still delivering a clean, alkali-free fuel gas suitable for use in a high temperature gas turbine.

The alkali-containing feed fuel is mixed with a high temperature gas to heat the fuel to a temperature in the range of 1 1001 800F (593-982C), causing devolatilization of the feed fuel. The high temperature gas is a non-oxidizing or reducing gas formed by recycling a portion of the raw gas produced by pyrolysis of the feed fuel.

This recycled raw gas, in effect a dirty fuel gas, is mixed with a substoichiometric amount of oxidant gas and reacted in a combustion zone to produce the high temperature non-oxidizing gas described above.

The remaining raw gas is cooled to a temperature below that at which any gaseous alkali products therein condense. The condensed alkali compounds along with any condensed hydrocarbon matter, are then removed from the gas stream, and the cooled gas is passed through a mechanical or electrical filter to remove any remaining particulates or liquid matter.

The clean gas thus prepared contains little or no particulate matter or alkali compounds that would be injurious to a high temperature gas turbine. The cooled gas may be optionally reheated prior to removing the particulate matter in order to avoid condensation or plugging within the collector.

Brief descriptoin of the drawing The drawing figure shows a schematic representation of the process according to the present invention.

Description of the preferred embodiment Referring now to the appended drawing figure, solid or liquid fuel is shown being fed 10 into the pyrolysis vessel 12. The fuel used in the process according to the present invention would preferably be one which is unsuitable for direct combustion in a gas turbine generator for reasons of particulate carryover or alkali content, as well as containing a significant volatile fraction which would produce a suitable amount of gas in a high temperatuie pyrolosys process. An exemplary list of such fuels would contain coal, oil shale, heavy oil, wood chips, waste hydrocarbons, biomass and other carbonaceous fuels.

The fuel is mixed with a hot gas 14 within the pyrolysis vessel 12 in a turbulent fluidized bed 1 6.

A turbulent fluidized bed provides a high rate of gas-fuel contact resulting in rapid and uniform heat-up of the input fuel 10. As is common with fluidized bed reactors, the hot gas 14 is distributed beneath the fluidized bed 1 6 by a perforated plate 1 8.

The temperature within the fluidized bed 1 6 is in excess of 11 OOF, preferably in the range of approximately 1 80OF. This preferred temperature provides a high amount of volatile yield without resulting in excessive breakdown of any higher hydrocarbons which might be present in the volatile content of the feed fuel, and without exceeding the fusion temperature of any inert material present in the feed fuel.

The devolved gas resulting from the pyrolysis process as well as the fluidizing gas 14 passes through the internal cyclone 21, exits the upper surface of the fluidized bed 20, and passes out of the pyrolysis vessel 12 by means of duct 22. This raw gas stream 24 is split 26 into a recycle portion 28 and a product portion 30. The recycle portion is conducted to the throat portion of a gas venturi 32. Pressurized oxidant 34 enters the inlet of the venturi 32 at a velocity suitable to create a reduced pressure region in the throat of the venturi 32. This reduced pressure region induces the flow of the recycle portion 28 into a venturi and creates a mixture of the recycle portion 28 and the pressurized oxidant 34 in a region of elevated pressure 36 downstream of the venturi 32.

The mixture of raw gas 28 and pressurized oxidant 34 flows from the region of increased pressure 36 into a combustion zone 38. This combustion zone is preferably included within a combustor 40 shown in the drawing figure. The hot gas 14 generated by the combustion reaction is used to fluidize the fuel 10 in the pyrolysis vessel 12 as discussed previously. The flow rates of the recycle portion 28 and the pressurized oxidant 34 are controlled according to two parameters. First there must be sufficient recycle raw gas 28 to provide sufficient heat energy to maintain the fluidized bed 1 6 at the desired temperature. Although the pyrolysis vessel 12 as well as all the ducting in the recycle gas loop is insulated 42, the pyrolysis process which takes place within the fluidized bed 1 6 is endothermic and requires a continuous heat source.The specific temperature of the hot fluidizing gas 14 is determined by the design of the particular process according to the materials and fuel sources involved.

For a fluidized bed operating in the neighborhood of 1 800F, the temperature of the hot fluidizing gas 14 would preferably be in the range of 2,000 to 2,300F (1093 to 1260C). This temperature range would supply sufficient heat energy to maintain the fluid bed temperature at the desired level without requiring extensive use of exotic or other refractory materials and to rapidly heat the feed fuel to the desired temperature, thus increasing the yield of pyrolysis products.

The second constraint upon gas and oxidant flow rates is the requirement that the hot fluidizing gas 14 be non-oxidizing or reducing.

The use of a non-oxidizing gas as a fluidizing gas for the fluidized bed 1 6 prevents any oxidation reactions from taking place within the pyrolysis vessel 12. Any such oxidation reactions would result in a diminished chemical heating value of the gas generated during pyrolysis process and could possibly result in softening and agglomeration of the inert ash particles within the fluidized bed 1 6. The hot fluidizing gas 14 is maintained in its non-oxidizing state by regulating the flow of pressurized oxidant 34 so as to maintain the combustion zone 38 under substoichiometric conditions. The exact stoichiometry, while always less than 1.00, is also determined by the particular nature of the process and feed fuel.The split of the raw gas 24 is shown in the drawing as being regulated by dampers 44, however any of a number of means well known in the art would be suitable for this purpose.

The remainder of the raw gas not recycied 30 is conducted to a condensor 46 which reduces the temperature of this product portion 30 below the alkali condensation temperature, approximately 11 00F. The cooling of the gas results in the condensation 48 of any alkali compounds as well as certain tars and oils which may be present within the product gas stream 30.

This condensed material is collected 50 within the condensation vessel 46 and removed from the gas stream.

The alkali-free gas 52 is next optionally routed through a gas reheater 54 which serves to slightly elevate the temperature of the gas prior to entering the final particulate removal device 56.

This slight elevation of temperature serves to re evaporate any condensate still present within the cooled gas 52, therefore reducing the possibility of plugging in the particulate removal means 56.

Although final particulate removal may be accomplished by any of a number of well known collector systems, a dry removal system such as the depicted baghouse 56 or an electrostatic precipitation system (not shown) is preferred. The particulate matter removed 60 is collected beneath the baghouse and routed for disposal or energy recovery. The clean, alkali-free product gas 58 is now suitable for use in the combustor of a high temperature gas turbine or any other application requiring a stringently clean gas.

Process perspective The above-disclosed process is obviously only a portion of a complete combined cycle electric generation plant. It is therefore felt to be important at this time to briefly discusss the overall configuration and nature of such a plant in order to fully describe the scope of the disclosed process. Additionally it should also be noted that no mention has been made of sulfur removal within the disclosed pyrolysis process, and in fact, none is contemplated within the process according to the present invention. Sulfur removal will take place in the remainder of the combined cycle generating plant as discussed below.

As disclosed in the preceding section, the pyrolysis process according to the present invention produces a clean, alkali-free gas suitable for use as a fuel in a high temperature gas turbine. In the case in which the fuel 10 contains any sulfur bearing compounds, the clean gas 58 will most likely contain sulfur in the form of hydrogen sulfide, H2S. This hydrogen sulfide will become an oxide of sulfur such as SO2 or SO3 upon combustion within the gas turbine combustor (not shown). Sulfur bearing compounds may also be present within the fluidized bed 16, the condensed alkali and hydrocarbons 50, or the particulate 60.

As discussed in the background section above, it is expected that the fixed carbon remaining within the pyrolysis vessel 12 would be removed and combusted within a fluidized bed combustor (not shown) for the production of high pressure steam. Such a fluidized bed combustor would also be able to readily accept the condensed alkali, tars, or oils 50 resulting from the condensation step 46, the drain material 62 from the fluidized bed 16, as well as any particulate 60 collected 56 from the product gas 30. By introducing these streams into a fluidized bed combustor in the presence of a limestone or other calcium bearing compound, any residual sulfur remaining within these compounds would be effectively collected and placed in a form suitable for disposal.

The only remaining cleanup problem from a sulfur standpoint is the oxides of sulfur present within the gas turbine exhaust. This exhaust stream, with a typical temperature of approximately 1500F (816C) and also containing a significant amount of free oxygen, is well suited for use as the fluidizing gas for the fluid bed combustor (not shown). When used in this particular manner, not only is a waste heat recoverable from the gas turbine exhaust stream within the fluidized bed combustor and steam generator, but also the sulfur present within the gas turbine exhaust stream is absorbed by the calcium bearing compounds within the fluidized bed combustor.

The final flue gas from the gas turbine combined cycle generation plant is therefore sulfur-free and lower in oxygen content than would ordinarily be the case for a simple heat gas turbine exhaust heat recovery system. The overall effect is a recovery of a portion of the overall plant efficiency lost by passing only a portion of the feed fuel chemical heat energy through the high temperature gas turbines.

The drain from the fluidized bed combustor, consisting mainly of inert ash compounds and calcium sulfur compounds, is suitable for safe landfill or other approved disposal. The tars and oils present within the condensed matter stream 50 are completely consumed within the fluidized bed combustor. The pressurized oxidant flow 34 may consist of either oxygen or air, depending on the particular constraints of the subject design.

It should also be noted that this entire pyrolysis and combined cycle process would most preferably take place at an elevated pressure, to reduce the size of the various components within the pyrolysis-combustion-steam generation process. The pyrolysis loop would preferably operate at a pressure slightly above that of the gas turbine combustor inlet pressure, in the neighborhood of 6 to 30 atmospheres absolute, or higher. The fluidized bed combustion and steam generation process preferably operates at a pressure slightly below the gas turbine exhaust pressure, in the neighborhood of 1 to 5 atmospheres absolute.

Conclusion The high temperature pyrolysis process according to the present invention is thus seen to be an effective and efficient method for producing an alkali-free fuel gas suitable for use in a high temperature gas turbine. The process is well adapted to operate in conjunction with a combined cycle electric generating plant employing a fluidized bed steam generation process or an equivalent thereof. The foregoing discussion should not be taken in a limiting sense as any number of other uses and configurations of the process according to the present invention will be obvious to one skilled in the art upon review of the foregoing specification and the appended drawing figure.

Claims

1. A process for producing a clean, alkali-free fuel gas from an alkali-containing solid or liquid fuel, comprising the steps of: devolatilizing said fuel in a vessel by heating said fuel to a temperature in the range of 1100 to 1800F, said heating being accomplished by mixing said fuel with a high temperature reducing gas; removing any volatiles produced in the devolatilization step along with the reducing gas from the vessel as a raw gas; splitting the raw gas into a recycle portion and a product portion; partially combusting the recycle portion of the raw gas under substoichiometric conditions to form the high temperature reducing gas for heating said fuel in the devolatilization step; cooling the product portion of the raw gas below the alkali condensation temperature; removing the condensed alkali from the cooled product portion; and, removing any nongaseous components remaining in the product portion subsequent to the step of removing the condensed alkali, thereby forming said clean, alkali-free fuel gas.

2. A process according to Claim 1, wherein the mixing of said fuel with the high temperature reducing gas in the devolatilization step takes place in a fluidized bed disposed within the vessel.

3. A process according to Claim 1, wherein the step of partially combusting the recycle portion of the raw gas includes the steps of: passing a stream of pressurized oxidant through a venturi for creating the region of reduced pressure within the throat of said venturi and a region of elevated pressure following said venturi; admitting the recycle portion of the raw gas into the reduced pressure region of said venturi, whereby the recycle portion mixes with the oxidant and passes out of the venturi into the following region of elevated pressure; and conducting the mixture of the oxidant and the recycle portion of the raw gas from said venturi to a combustion zone wherein the oxidant and recycle portion substantially react chemicaliy to completion, thereby forming the high temperature reducing gas for heating said fuel in the devolatilization step.

4. A process according to Claim 2, wherein the step of partially combusting the recycle portion of the raw gas includes the steps of: passing ; stream of pressurized oxidant through a venturi for creating the region of reduced pressure within the throat of said venturi and a region of elevated pressure following said venturi; admitting the recycle portion of the raw gas into the reduced pressure region of said venturi, whereby the recycle portion mixes with the oxidant and passes out of the venturi into the following region of elevated pressure; and conducting the mixture of the oxidant and the recycle portion of the raw gas from said venturi to a combustion zone wherein the oxidant and recycle portion substantially react chemically to completion, thereby forming the high temperature reducing gas for heating said fuel in the devolatilization step.

5. A process according to Claim 1, further comprising the step of reheating the product portion of the raw gas following the step of removing the condensed alkali and prior to the step of removing any remaining nongaseous components in the product portion.

6. A process according to Claim 2, further comprising the step of reheating the product portion of the raw gas following the step of removing the condensed alkali and prior to the step of removing any remaining nongaseous components in the product portion.

7. A process for producing a clean, alkali-free fuel gas from a solid or liquid fuel containing alkali compounds, said fuel being suitable for use in a high temperature gas turbine these steps of: devolatiiizing said alkali-containing fuel in a fluidized bed in the presence of a non oxidizing high temperature gas, the temperature of the fluidized bed being in excess of 11 0OF for producing a raw gas containing the products of devolatilization; reacting a portion of the raw gas resulting from the devolatilization step with a substoichiometric amount of oxidant gas to produce the high temperature non-oxidizing gas of the devolatilization step, the flow rate of the recycle portion and the temperature of the non-oxidizing gas produced being regulated to control the temperature of the fluidized bed;; cooling the remaining portion of the raw gas below the alkali condensation temperature; removing any condensed alkali from the cooled gas; and removing any particulate matter, tars, and oils in the cooled gas, thereby resulting in said clean, alkali-free fuel gas.

8. A process according to Claim 1, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

9. A process according to Claim 2, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

10. A process according to Claim 3, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

11. A process according to Claim 4, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

12. A process according to Claim 5, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

13. A process according to Claim 6, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

14. A process according to Claim 7, wherein all the steps are performed at an absolute pressure in excess of 10 atmospheres.

1 5. A high temperature pyrolysis process for producing a clean, alkali-free fuel gas from coal, comprising the steps of: heating said coal in a vessel to a temperature in the range of 1100 to 1800F, the heating being accomplished in a non-oxidizing environment; separating any gaseous matter produced during the heating step into a product portion and a recycle portion; reacting the product portion with an oxidant to release heat energy for the heating step; cooling the product portion to a temperature below the condensation point of any alkali compounds present therein; separating the condensed alkali compounds from the cooled product portion; and separating any remaining solid or liquid matter from the gaseous product portion.

1 6. A process according to Claim 15, wherein the step of separating any solid or liquid matter from the gaseous product portion includes the steps of: passing the gaseous matter produced during the heating step through a high temperature gas cleanup device for removing a substantial portion of any solid matter which may be present in the produced gas; returning the solid matter removed by the high temperature gas cleanup device to the vessel; and passing the product portion through a final particulate removal device following the condensed alkali separation step for removing substantially all remaining solid or liquid matter present in the product portion.

1 7. A process according to Claim 16, further comprising the step of reheating the product portion following the condensed alkali separation step and before entering the final particulate removal device for re-evaporating any liquid compounds present in the product portion.

18. A process according to Claim 16, wherein all of the steps are performed at an absolute pressure in excess of 10 atmospheres.

1 9. A process according to Claim 17, wherein all of the steps are performed at an absolute pressure in excess of 10 atmospheres.

20. A process for producing a clean alkali-free fuel gas from an alkali-containing solid or liquid fuel, substantially as hereinbefore described with reference to the accompanying drawing.

21. A process according to Claim 1, substantially as herein described.

22. A fuel gas whenever produced by a method according to any preceding claim.