Method for high efficiency for producing fuel gas for power generation

Abstract

A method is provided for operating a coal gasifier, preferably an oxygen-blown two zone-coal gasifier, without the need for the addition of water and producing a high LHV-content coal product fuel gas from coal. A first supply of coal, preferably dry coal, first supply of carbon dioxide, and a supply of oxygen are passed into a high pressure combustion vessel wherein the coal reacts with the oxygen and carbon dioxide to form an exothermic zone producing a combustion temperature in excess of the melting point of the ash in the coal. This in turn produces a melted ash and a combustion product gas comprising carbon monoxide. The melted ash is drained or otherwise removed for disposal and the combustion product gas subsequently is passed into an entrained flow reaction vessel. A second supply of dry coal and a second supply of carbon dioxide are injected into the entrained flow reaction vessel wherein the coal thermally reacts with the combustion product gas and the carbon dioxide and produces a product fuel gas. The resultant product fuel gas comprises more moles of carbon monoxide than moles of carbon in the first and second supplies of coal. The product fuel gas may exhibit a gas exit temperature at a value below the melting point of the coal ash but no lower than about 1400° Kelvin, and it may be passed to a turbine having closed loop internal cooling thereby recovering energy and lowering the product gas pressure and temperature prior to potential heat exchange with incoming gasifier feed gases.

Description

CROSS-REFERENCE

This application is a continuation-in-part of U.S. patent application Ser. No. 12/381,949 filed Mar. 17, 2009; which in turn claims the benefit of U.S. Provisional Patent Application No. 61/070,375 filed Mar. 20, 2008.

FIELD OF THE INVENTION

The present invention relates to a method for maximizing the heating value of syngas produced by gasification of coal; thus improving the thermal efficiency of a two-stage gasifier to supply gasified coal. The loss of coal Lower Heating Value to water-latent heat is essentially eliminated. In particular, the present invention comprises a method for maximizing the conversion of coal heating values to carbon monoxide-rich fuel gas.

BACKGROUND OF THE INVENTION

Brief Description of the Related Art

With energy usage directly related to economic growth, there has been a steady increase in the need for increased energy supplies. In the U.S., coal is abundant and low in cost. Unfortunately, conventional coal-fired steam plants, which are a major source of electrical power, are inefficient and pollute the air. Thus, integrated coal gasification combined cycle (“IGCC”) systems have been developed which can achieve significantly improved emissions control in comparison to conventional steam plants. Such processes allow cleanup of sulfur and other impurities before combustion.

In the present state of the art, a gasifier provides a fuel gas comprising carbon monoxide and hydrogen produced by reaction of water with carbon. The loss of the heat of vaporization from water is significant as is the loss from water addition used in older coal gasification designs.

Regardless of the process, carbon represents the major heating value of the coal. Gasification means partial oxidation to produce either carbon monoxide and heat or carbon monoxide and hydrogen. Gasifiers use either air or high purity oxygen to combust at least a portion of the coal. And may consist of either a single or two stage design.

For every mole of hydrogen produced by reaction of water with carbon, approximately 15% of the Lower Heating Value (LHV) energy is lost. The result is a syngas having a reduced LHV, i.e. work potential, as compared to the original coal.

Even with high water vaporization losses, IGCC systems still are more efficient than steam plants in which combustion of coal releases all the heating value of the coal. Still, better gasifiers are needed to improve efficiency and thus reduce costs.

SUMMARY OF THE INVENTION

The present invention is a method of gasifying coal to produce a high LHV-content coal product fuel gas. This product fuel gas comprises carbon monoxide derived from combustion of the carbon of the feed coal plus additional carbon monoxide derived from the reaction of carbon dioxide with coal carbon. Thus, essentially all the LHV is available in the product fuel gas. A two-zone system is used to reduce the amount of oxygen needed for high temperature combustion. It also minimizes the temperature of the product fuel gas before particulate removal.

Methods for gasifying coal are commonly referred to as producing a “fuel gas” or a “syngas”. Applicant refers to the product of gasifying coal as having produced a “product fuel gas” for the description provided herein. In the present invention, water is avoided to conserve the work potential of the high LHV-content coal in the product fuel gas. Thus, it is advantageous for the coal to be dry and therefore to avoid adding water to the coal before use. “Dry coal” in the embodiments of the present invention comprises a supply of coal without a deliberate addition of water. Added water will cause losses in product fuel gas LHV. This will reduce the efficiency benefit that would have accrued from using no added water. In general, efficiency losses in the method of the present invention resulting from the latent heat of water preferably are limited to no more than about ten percent, and even less than about three percent, rather than the higher efficiency losses realized in conventional coal gasification. Accordingly, it is beneficial to dry the coal to the maximum extent possible if the energy and cost of doing so is less than the improvement in total product fuel gas LHV. For example, drying the coal may be accomplished by blowing the coal with nitrogen, preferably at heated or elevated temperatures for an increased drying effect.

The present invention comprises a method of operating a coal gasifier, preferably an oxygen-blown two zone-coal gasifier, without the need for the addition of water. A first supply of coal, preferably dry coal, and first supply of carbon dioxide are injected into a high pressure combustion vessel. A supply of oxygen for combustion of the coal is passed to the vessel. The coal reacts with the oxygen and carbon dioxide to form an exothermic zone producing a combustion temperature in excess of the melting point of the ash in the coal. This in turn produces a melted ash and a combustion product gas comprising carbon monoxide. The melted ash is drained or otherwise removed for disposal.

The combustion product gas subsequently is passed into an entrained flow reaction vessel. A second supply of coal, preferably dry coal, and a second supply of carbon dioxide are injected into the entrained flow reaction vessel. The coal thermally reacts with the combustion product gas and the injected carbon dioxide and produces a product fuel gas. The resultant product fuel gas comprises more moles of carbon monoxide than moles of carbon in the first and second supplies of coal. Preferably, the product fuel gas exhibits a gas exit temperature at a value below the melting point of the coal ash but no lower than about 1400° Kelvin. The product fuel gas may be passed to a turbine having closed loop internal cooling thereby recovering energy and lowering the product gas pressure and temperature prior to potential heat exchange with incoming gasifier feed gases.

Preferably, the carbon dioxide partial pressure does not exceed supercritical pressure and therefore the carbon dioxide does not achieve supercritical properties. The operating pressures of the gasifier preferably range from 20-100 atmospheres, with carbon dioxide partial pressure less than about 50 atmospheres which is subcritical pressure for carbon dioxide.

In a preferred method of the present invention, a portion of the coal charge, typically at least about half but preferably less than about eighty percent of the total coal charge, and carbon dioxide is injected under pressure into a high pressure combustion vessel comprising an oxidation zone or exothermic zone. The coal charge refers to the total amount of coal fed to the gasifier per unit time. A supply of oxygen is passed to the vessel for the combustion of the coal. The reaction produces a combustion temperature in excess of the melting point of the coal ash. Ash from the coal fed to a downstream endothermic zone may be fed to the oxidation zone.

Preferably, the coal should be dried. The amount of oxygen used preferably is less than about eighty five percent of that required to convert the entire coal charge carbon to carbon monoxide; but at least about sixty-five percent. The preferred amount of oxygen provided to react with the coal charge depends on the inerts content of the coal. In general, an increased amount of oxygen may be required to elevate the combustion temperature to melt the increased inerts content of the coal. Carbon dioxide is preferably added with the coal wherein the supply of carbon dioxide is limited in order to maintain the temperature above the coal ash melting point. The resultant melted ash or slag is removed and cooled in a zone isolated from the oxidation zone.

The combustion product gases from the oxidation zone are passed to an endothermic reaction zone for mixing and reaction with the balance of the coal charge along with added carbon dioxide. At least the stoichiometric amount of carbon dioxide relative to the balance of the coal charge passed to the endothermic zone is preferred. Preferably, an entrained flow reaction vessel such as an entrained flow reactor is used. The balance of the coal and carbon dioxide is injected into the entrained flow reaction vessel for mixing and endothermic reaction with the combustion product gas wherein the coal thermally reacts with the combustion product gas and the injected carbon dioxide. If needed, a minor amount of oxygen may be added to maintain the temperature above about 1400° Kelvin. The endothermic zone product is a product fuel gas comprising primarily carbon monoxide and hydrogen from the coal and represents a high percentage of the LHV heating value of the coal charge.

In contrast to current IGCC syngas technology, this approach substitutes carbon dioxide for water and produces additional carbon monoxide from reaction of coal with carbon dioxide. In prior art gasification reactions, where CO₂ is derived from coal carbon, the product gas contains 1 mole of CO per mole of coal carbon fed, as seen in Equations 1 and 2. If water is fed to the process, then some of the heating value of the coal is lost, as shown in Eqn. 3.

C+O₂→CO₂  (Eqn. 1)

C+H₂→CO+H₂  (Eqn. 2)

CO+H₂O→CO₂+H₂  (Eqn. 3)

In contrast, when CO₂ is fed separately into the process as in this invention, the product gas contains more moles of CO per mole of coal carbon fed, i.e. 2/1 as shown in Eqn. 4. Moreover, if water is not fed into the process, none of the carbon heating value is lost.

C+CO₂(feed)→2CO  (Eqn. 4)

This method avoids the consequent loss of LHV energy that is entailed in H₂ production from water. Thus, more energy is available for use downstream in an energy production process or power generation apparatus such as a gas turbine or a fuel cell.

The increased work potential of the product fuel gas can provide fuel-to-power efficiencies significantly higher, e.g. five to ten percent or more, than conventional IGCC designs. Losses to latent heat of water are not inherent as in conventional systems. Combustion of the product fuel gas with pure oxygen produces carbon dioxide which is readily recovered from the exhaust by removal of water (such as from combustion of hydrogen in the coal). Moreover, molecular hydrogen from the coal may recovered by permeation through a hydrogen permeable membrane. Typically, operating pressures are in excess of twenty or thirty atmospheres, and pressures of a hundred atmospheres offer advantages, such as by expansion in a turbine to a pressure suitable for gas turbine fuel gas. Note that the volume of gas available of gasifier fuel gas product is almost double the amount of oxygen and carbon dioxide that must be compressed to supply the high pressure gasifier.

In a method of the present invention, coal, oxygen, and carbon dioxide are fed to a two-zone oxygen-blown gasifier operating at a high oxidation zone temperature. The gasifier produces an endothermic zone product fuel gas comprising at least about twenty percent to as much as forty percent or more moles of carbon monoxide than moles of carbon in the feed coal. A catalyst such as potassium carbonate may be used in the endothermic zone. However, endothermic zone temperature must be maintained high enough for thermal reactions, i.e., at least above about 1400° Kelvin.

To capture impurities in the ash, operating temperature in the exothermic (oxidation) zone must be sufficiently above the ash melting point, typically 100° F. or more above the ash melting point, so that molten ash can be removed and quenched. An isolated water pool quench may be used and forms a glassy frit and encapsulating ash toxics, a composition commonly referred to as slag. The produced steam may be used to preheat the feed oxygen or carbon dioxide- or to heat nitrogen for coal drying. This recovers a portion of the slag heat.

Alternatively, the steam produced may be fed to a steam turbine. In proposed systems, mercury may be sequestered underground with the product CO₂ rather than collected on an adsorbent creating a hazardous waste for disposal. Sulfur can be recovered from the exhaust. Conventional mercury and sulfur recovery systems may be used.

In one embodiment of the present invention, the product fuel gas is cooled such as by expansion in a turbine to a pressure suitable for gas turbine combustion with the turbine exhaust is further cooled by heat exchange with the incoming oxygen and/or carbon dioxide feeds

Alternately dilution of the product fuel gas with recycled carbon dioxide eliminates the need for a high temperature (high cost) heat exchanger. Heat exchange between the CO₂ feed and the turbine exhaust in order to raise the temperature of the CO₂ being input to the gasifier also eliminates the need for a high temperature (high cost) heat exchanger. Product fuel gas may also be cooled by expansion as in a turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic of a gasifier system in accordance with the present invention showing the use of a turbine for expansion of the product fuel gas for energy recovery and cooling prior to sulfur removal.

DETAILED DESCRIPTION OF THE INVENTION

In one example of an embodiment of the present invention, a dry coal having an analysis of 0.37 moles of hydrogen per mole of carbon is fed to a two-zone lagging gasifier. Carbon dioxide is fed to the exothermic and endothermic zones in accordance with the present invention; however, no water is fed to either zone. Molten ash is removed and quenched in an isolated water bath. Product fuel gas exits the gasifier at a temperature above 1400° K. Typically, the product fuel gas contains less than about thirty percent unconverted carbon dioxide. Combustion of the carbon monoxide-rich gas with oxygen allows ready capture of carbon dioxide.

FIG. 1 represents a simplified schematic diagram of an oxygen-blown two-stage gasifier system 10. As shown, coal 12 is passed to a dryer 14, thereby producing a dry coal 16, which in turn is fed into the two-stage gasifier system 10. Dry coal 16, carbon dioxide 18, and oxygen 22 are fed to the oxidation or exothermic zone 20 of the gasifier system 10. Additional dry coal 16 and carbon dioxide 18 are fed to the endothermic zone 24 of the gasifier system 10 to react with combustion product gas 32 from the exothermic zone 20.

Preferably, oxygen 22 is supplied by an air separation plant 26 which may be a membrane separator or more typically an air liquefaction plant. Feed air 28 is typically compressed using intercooler compressors. Nitrogen 30 is made available for cooling if desired. The product fuel gas 34 comprising raw syngas is passed through a filter system 36. The higher pressure filtered product fuel gas 38 may be expanded in an optional turbine 40 for energy recovery by use of a generator 42. The expanded gas is typically at a pressure suitable for gas turbine combustion and at a reduced temperature. The lower pressure filtered product fuel gas 44 may be processed further for sulfur removal or mercury removal. Coal ash and other particulates and solids 46 retained by the filter system 36 are passed to exothermic zone 20 of the gasifier system 10 for melting into slag 48. Gasifier product fuel gas represents a high fuel-value fuel containing nearly all the Lower Heating Value energy of the original coal in the form of carbon monoxide.

Although the invention has been described in considerable detail, it will be apparent that the invention is capable of numerous modifications and variations, apparent to those skilled in the art, without departing from the spirit and scope of the invention.

Claims

1. A method of operating a coal gasifier comprising:

a) injecting a first supply of dry coal and a first supply of carbon dioxide into a high pressure combustion vessel;

b) passing to the vessel a supply of oxygen;

c) reacting the first supply of dry coal with the oxygen and the first supply of carbon dioxide to form an exothermic zone producing a combustion temperature in excess of the melting point of ash in the coal;

d) producing melted ash and a combustion product gas comprising carbon monoxide;

e) removing melted ash for disposal;

f) passing the combustion product gas into an entrained flow reaction vessel;

g) injecting a second supply of dry coal and a second supply of carbon dioxide into the entrained flow reaction vessel;

h) thermally reacting the second supply of dry coal with the combustion product gas and the second supply of carbon dioxide; and

i) producing a product fuel gas comprising more moles of carbon monoxide than moles of carbon in the first and second supplies of coal.

2. The method of claim 1 wherein the steps of thermally reacting the second supply of coal with the combustion product gas and the second supply of carbon dioxide and producing product fuel gas further comprises:

a) producing coal ash and other particulates;

b) separating the product fuel gas from the coal ash and other particulates; and

c) passing the separated coal ash and other particulates to the combustion vessel.

3. The method of claim 2 wherein the separated product fuel gas is expanded in a turbine.

4. The method of claim 3 wherein the turbine delivers an exhaust gas at a pressure greater than about twenty atmospheres.

5. The method of claim 3 wherein the turbine delivers an exhaust gas and the exhaust is further cooled by heat exchange with the supply of oxygen, the first supply of carbon dioxide, and/or the second supply of carbon dioxide.

6. The method of claim 1 wherein the product fuel gas is fed to a fuel cell.

7. The method of claim 1 wherein the product fuel gas is fed to a gas turbine.

8. The method of claim 7 wherein an exhaust produced by the gas turbine is fed to a heat recovery boiler.

9. A method of operating a gasifier comprising:

a) injecting a first supply of coal and first supply of carbon dioxide into a pressure vessel at a pressure of at least forty atmospheres;

b) passing a supply of oxygen to the vessel for combustion of the coal;

c) reacting the first supply of coal with the oxygen and the first supply of carbon dioxide to form an exothermic zone producing a combustion temperature in excess of the melting point of ash in the coal;

d) producing melted ash and a combustion product gas comprising carbon monoxide;

e) removing melted ash for disposal;

f) passing the combustion product gas into an entrained flow reactor;

g) injecting a second supply of coal and a second supply of carbon dioxide into the entrained flow reactor;

h) thermally reacting the second supply of coal with the combustion product gas and the second supply of carbon dioxide;

i) producing a product fuel gas comprising more moles of carbon monoxide than moles of carbon in the first and second supplies of coal, and having a gas exit temperature at a value below the melting point of the coal ash but no lower than about 1400 Kelvin; and

j) passing the product fuel gas to a turbine having closed loop internal cooling thereby recovering energy and lowering the product fuel gas pressure and temperature prior to heat exchange with incoming gasifier feed gases.

10. The method of claim 9 wherein the pressure is at least 100 atm.

11. The method of operating a coal gasifier comprising:

a) passing a supply of coal to a gasifier operating at a pressure in excess if forty atmospheres;

b) reacting the coal with an oxidant gas to produce a product fuel gas comprising carbon monoxide at temperature of at least about 1400° Kelvin but below the melting temperature of the coal ash;

c) separating particulate solids from the product fuel gas; and

d) passing the fuel gas to a turbine having closed loop internal cooling thereby recovering energy and lowering the product fuel gas pressure and temperature prior to heat exchange with coolant fluids.