GB1596723A

GB1596723A - Reducing gas generation

Info

Publication number: GB1596723A
Application number: GB826878A
Authority: GB
Original assignee: Ralph M Parsons Co
Current assignee: Parsons Government Services Inc
Priority date: 1977-03-10
Filing date: 1978-03-02
Publication date: 1981-08-26
Also published as: JPS53110991A; DE2809760A1; CA1090574A

Description

(54) REDUCING GAS GENERATION (71) We, THE RALPH M. PARSONS COMPANY, a corporation organised and existing under the laws of the state of Nevada, United States of America, residing at 100 West Walnut Street, Pasadena, California 91124, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the generation of reducing gas e.g. with the aid of flue gases.

The invention is related to the disclosure of our co-pending patent application No. 52711/76 (Serial No. 1578149), to which reference may be had.

In our co-pending application there is provided a method' and apparatus to improve the recovery of fuel burning power generators and the like while minimizing emissions containing sulfur and nitrogen to the atmosphere.

In the process as described, after combustion of a carbonaceous fuel such as coal, a secondary hydrocarbon, such as methane which is capable of forming a reducing gas, is introduced into the combustion chamber to scavenge excess oxygen and create a reducing atmosphere for the subsequent reduction of the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and/or ammonia, with nitrogen formation being favoured.

The gas is then passed through the remaining sections of the boiler and to a catalytic conversion zone containing a catalyst capable of converting the oxides of sulfur to hydrogen sulfide by reaction with the hydrogen present in the reducing gas, and or converting the oxides of nitrogen by reaction with the reactants present to form inert nitrogen and/or ammonia at a temperature from about 300 to about 8000F.

Many other operations exist where there is a requirement for a reducing gas comprising hydrogen and/or carbon monoxide reductants. Examples include the regeneration of SO2 absorbants, and the reduction in external streams of the oxides of sulfur and nitrogen. Usually such reducing gases are prepared by reacting steam plus hydrocarbons at high temperatures (1200--18000F) over a catalyst.The principal chemical reactions which take place using methane as a typical hydrocarbon are: CH4+H2OoC0+3H2 (1) CO+H2O < CO2+H2 (2) Another method is to burn a hydrocarbon with a substoichiometric amount of air at elevated temperatures (2000--37000F). The principal chemical reactions taking place in this process are: CH4±21O2CO+2I12 (3) CH4+202 < CO2+2H2O (4) In both cases a relatively expensive hydrocarbon fuel must be used as the sole fuel and/or as the process material. In the commercial application via the steam methane reforming of (1) and (2) above, a fairly expensive furnace must be employed with high alloy tubes to contain the catalyst and to withstand high furnace temperatures. The cost of such equipment is becoming prohibitively expensive.

The present invention aims to provide a more economical route to the production of a reducing gas stream containing carbon monoxide.

According to the present invention there is provided a method of producing a reducing gas by thermal conversion of a hydrocarbon fuel, wherein a high temperature gaseous effluent by-product, containing an oxidant capable of reacting with the fuel, is extracted from a reaction zone of a process plant at a temperature sufficiently high to provide the heat input necessary to promote and sustain thermal reactions between the fuel and oxidant, and a carbon-containing fuel in an amount which provides the sum of 5 to 100 mols of carbon and hydrogen per 100 mols of extracted gas is intermingled with the-gas to establish adiabatic thermal conversion of the fuel in the presence of the oxidant, a gaseous oxidant as hereinafter defined being provided to the extent necessary to supply sufficient oxidant to the thermal conversion system to convert the carbon in the fuel to carbon monoxide but insufficient to convert all the carbon to carbon dioxide.

whereby resulting from the thermal conversion is formed a high temperature reducing gas consisting of carbon monoxide or a mixture of carbon monoxide and hydrogen.

The nature of the process plant is immaterial so long as it is possible to extract therefrom a hot enough waste or by-product gas which contains oxidant capable of entering into thermal conversion reaction with the fuel. Plant from which such gas may be obtained includes power-generating combustion furnaces, boilers, smelters, roasters, and calcining kilns.

The invention also provides a method for the production of a reducing gas from a carbon-containing fuel which comprises: (a) extracting from a reacton zone in a process plant a high temperature gas at a temperature from 2000 to 3800"F, said gas containing an oxidant capable of reacting with the fuel; (b) intermingling with the extracted high temperature gas a carbon-containing fuel in an amount which provides the sum of from 5 to 100 mols of carbon and H2 per 100 mols of extracted gas; and (c) adiabatically thermally converting the fuel in the presence of oxidant which comprises oxygen, steam or mixtures thereof and present in a quantity sufficient to convert all of the carbon in the fuel to carbon monoxide, but insufficient to convert all of the carbon in the fuel to carbon dioxide, to form a resultant high temperature reducing gas consisting of carbon monoxide, or a mixture of carbon monoxide and hydrogen.

Also according to the invention, there is provided a method for the generation of a reducing gas from a carbon-containing fuel, which comprises: (a) combusting carbonaceous material in a combustion zone of a boiler in the presence of excess air to form a high temperature flue gas comprising the oxides of carbon, steam and uncombined oxygen; (b) extracting a portion of the flue gas from the combustion zone, the extracted flue gas having a temperature from 2000 to 38000F; (c) intermingling with the extracted flue gas a carbon-containing fuel in an amount which provides the sum of from 5 to 100 mols of carbon and H2 per 100 mols of extracted flue gas; and (d) adiabatically thermally converting the fuel in the presence of oxidant comprising steam, oxygen or mixtures thereof and present in a quantity sufficient to convert all of the carbon present in the fuel to carbon monoxide, but insufficient to convert all carbon in the hydrocarbon to carbon dioxide, to form a resultant high temperature reducing gas comprising carbon monoxide, or a mixture of carbon monoxide and hydrogen.

In practising this invention the high temperature gas can be generated in a furnace reaction zone, and can be the flue or combustion gases generated in a furnace or present in the convection zones of the boiler. The thermal conversion system comprising the extracted gas, the fuel and the oxidant is allowed to adiabatically react to the resultant high temperature to produce a reducing gas stream comprising carbon monoxide or carbon monoxide and hydrogen.

The extracted gas is normally provided at a temperature of 2000"F to 3800"F.

The amount of carbon containing fuel added is proportional to extracted gas temperature and will be added to provide a range from 5 to 100 mols of carbon plus H2 inclusive, preferably from 15 to 50 mols per 100 mols of extracted gas. The use of high purity oxygen as oxidant can permit higher than normal hydrocarbon additions by maintaining the net reaction products at a high temperature and avoiding nitrogen dilution. While conversions primarily occur thermally, a catalyst can be employed to speed the reaction, especially where the extracted gas is at a temperature below about 2000"F with the normal precautions of preventing poisoning or masking of the catalyst, being taken.

The invention will now be described in more detail by way of example with reference to the sole accompanying drawing, which illustrates exemplary plant useful in the practice of the invention.

With reference to the Drawing, in a power generator 10, the boiler 12 is supplied with a primary fuel, normally a sulfur bearing carbonaceous fuel such as pulverized sulfur bearing coal or sulfur bearing hydrocarbon gas or liquid, through line 14 along with preheated air from duct 16 through conduit 18 to combustion section 20. Carbon values are completely consumed due to the addition of excess air, usually 1% to 25 /ó and preferably 10 to 20% in excess of that required to convert the carbonaceous fuel to carbon dioxide and heat. The amount of excess air introduced depends on the nature of the carbonaceous fuel. As little as 1% excess air can be employed for gaseous to liquid fuels with at least 10% excess air being employed for normally solid fuels.

In addition to combustion zone 20, boiler 12 normally contains a radiant boiler section, a convection boiler section, and a high temperature economizer and may be followed by electrostatic precipitator 22 to remove fly ash. Other means to remove ash can also be employed, for instance, cyclone and bag filters. The air required for the combustion is blown into air preheater 24, and passes by duct 16 to the combustion zone, by conduit 18 normally at temperatures from 500 to 6000F.

A portion of the combustion products, rather than being utilized in transferring their heat by convection and radiation to boiler feed water, are removed by line 26 from the high temperature section of the boiler. The combustion gases will normally range from 2000 to 38000F depending on the point of extraction.

The extracted gases which contain the carbon oxides and unconsumed oxygen pass to reactor 30 where there is introduced a carbon-containing fuel (H.C.) as well as a gaseous oxidant for the fuel to the extent oxidant required for conversion of the fuel to reducing gas is not provided in the form of unconsumed oxygen in the extracted combustion gases.

The fuel employed may be any carbon containing reactants capable of reaction in the gas phase at the temperatures provided without appreciable formation of soot. Exemplary of such fuels include hydrocarbons, methane, ethane, propane, the butanes, and atomized or vaporized liquids with natural gas preferred for reason of economy. Finely divided solid fuels such as coal and char may also be used.

As used herein, the term "gaseous oxidant" means any gas including free oxygen, such as air, or any gas containing oxygen in a combined form which is capable of reacting with the fuel under the reaction conditions to form carbon monoxide, and includes steam, or a mixture of any such gases.

The extracted high temperature gas serves as a medium to initiate and promote the thermal conversion of the hydrocarbons to hydrogen and carbon monoxide in a stoichiometric deficiency of the oxidant. The principal reactions to occur are (1) to (4) above with reaction (1), for instance, being generalized, depending on the hydrocarbon to: m CnHm+n[o]*nCO+H2 (5) 2 [o]represents the oxygen of the gaseous oxidant.

In addition, carbon dioxide to the extent present may reduce to carbon monoxide increasing conversion efficiency based on the hydrocarbon fed to, under certain circumstances, over 100%.

The amount of carbon containing fuel fed will provide a total equivalent of from 5 to 100, preferably from 15 to 50 mols of carbon and H2 as present in the feed per 100 mols of extracted gas. For example, a mole of methane is equivalent to one mole of carbon and two moles of H2. The amount added is dependent on and proportional to temperature of the extracted gas. Effective conversions of over 100No are realized for hydrocarbon feeds of 10 mols or more per 100 mols of flue gas. Reaction is carried out adiabatically to a new but somewhat reduced temperature. High purity oxygen can be used to avoid contributing nitrogen to the product and may not become involved in CO2 production.

The product reducing gas contains appreciable hydrogen and/or carbon monoxide. The latter forms hydrogen on reaction with water via a water-gas shift reaction.

To the extent the products of the combustion are not extracted, they are used for power generation and exhausted to stack 32.

The advantages of the process include the use as heating fuel source of inexpensive coal or residual fuel oil used to fire the boiler. No heat transfer surfaces of expensive metals are required and no catalysts that may be susceptible to poisoning or subject to carbon deposition, are essential, and accordingly the process is inherently cheaper, more flexible and more rugged. If, however, more rapid reaction rates, particularly at temperatures below 2000"F are desired, a catalyst may be employed but then, the usual precautions must be observed to guard against poisoning or masking the catalyst.

Another advantage of this invention is that heat from the combustion gases can be transferred most efficiently and economically to produce steam at the high temperature and the gases extracted after cooling, at the desired temperature for the efficient promotion of reactions (1) or (3). Reaction (4) serves only to increase the temperature of the reaction mix.

Example 1 To an extracted flue gas mixture shown below there is added natural gas (shown as CH4) as follows in Table I.

TABLE I Flue Gas CH4 Stream Lb. mols/hr. Lb. mols/hr.

CH4 5.0 CO2 14.45 H2O 8.58 O2 2.70 SO2 0.27 N2 74.00 100.00 Pressure PSIA 14.7 14.7 Temperature 0F 2500 60 The adiabatic reaction temperature achieved is 2367"F and the equilibrium composition of the product gases is shown in Table II.

TABLE II Lb. mols/hr.

CO 9.42 COS 0.01 CO2 10.02 H2 4.57 H2O 13.85 H2S 0.16 N2 74.00 SO2 0.05 S2 0.02 S 0.01 Total 112.11 Example 2 With the flue gas mixture as in Example 1, there is added 10 mols of CH4. The adiabatic reaction temperature reached is 1742"F and the product gas consisted of the following components shown in Table III.

TABLE III Mols/hr.

CO 16.77 COS 0.01 CO2 7.67 H2 17.02 H2O 11.30 H2S 0.26 N2 74.00 Total 127.03 While in Example 1, 9.42 mols of CO and 4.57 mols of H2 were formed for a total of 13.99 mols or 93.3% conversion efficiency based on reaction (3) above, in this Example 43.79 mols of CO+H2 were formed from 10 mols of CH4 or 146% conversion efficiency based on reaction (3). This is due to the reduction of part of the CO2 to CO.

Example 3 The flue gas of Example 1 is extracted at 35000F instead of 2500"F. There is added 15 mols/hr of CH4 instead of 5 mols/hr. There is achieved a mix temperature above 2000"F. The mix is far removed from the conditions for carbon formation.

Table IV shows the reactant gas composition, product gas composition and operating conditions.

TABLE IV Reactants Product Gas Mols/hr. Mols/hr. Mols/hr.

CO 25.26 COS .01 CO2 14.45 4.18 H2 28.53 H2O 8.58 9.79 H2S .26 N2 74.0 74.00 O2 2.7 SO2 0.27 CH4 15.0 100.0 15.0 142.03 Pressure PSIA 14.7 14.7 14.7 Temperature 0F 3500 60 2025 In this Example, the yield of H2+CO was 53.79 mols/hr. or 120% yield efficiency based on reaction (3) above.

Example 4 In this Example, steam is used as the gaseous oxidant. The composition of the feed and product gases and net conditions are summarized in Table V.

TABLE V Reactants Flue Gas Hydrocarbons Product Gas Mols/Hr. +Steam Mols/Hr. Mols/Hr.

CO Mols/Hr. 21.43 COS .01 CO2 14.45 8.01 H2 32.35 H2O 8.58 15.00 20.95 H2S .26 N2 74.00 74.00 O2 2.70 SO2 0.27 CH4 ~~~~~~~ 15.00 ~~~~~~~ Total 100.00 30.00 157.1 Temperature 0F 3500 212 1873 Pressure PSIA 14.7 14.7 14.7 The conversion efficiency of CH4 to CO+H2 for Example 4 is the same as for Example 3, i.e. 120% based on Equation (3).

The invention is not limited to the use of boiler flue gases but may be practised in conjunction with any process plant where a gas is available at a high temperature such as smelters, roasters, lime kilns, cement kilns, blast furnaces and magnetohydrodynamic power generating channels.

WHAT WE CLAIM IS: 1. A method of producing a reducing gas by thermal conversion of a hydrocarbon fuel, wherein a high temperature gaseous effluent by-product, containing an oxidant capable of reacting with the fuel, is extracted from a reaction zone of a process plant at a temperature sufficiently high to provide the heat input necessary to promote and sustain thermal reactions between the fuel and oxidant, and a carbon-containing fuel in an amount which provides the sum of 5 to 100 mols of carbon and hydrogen per 100 mols of extracted gas is intermingled with the gas to establish adiabatic thermal conversion of the fuel in the presence of the oxidant, a gaseous oxidant as hereinbefore defined being provided to the extent necessary to supply sufficient oxidant to the thermal conversion system to convert the carbon in the fuel to carbon monoxide but insufficient to convert all the carbon to carbon dioxide, whereby resulting from the thermal conversion is formed a high temperature reducing gas consisting of carbon monoxide or a mixture of carbon monoxide and hydrogen.

2. A method as claimed in claim 1 in which the extracted gas is at a temperature from 2000 to 38000F.

3. A method as claimed in claim 1 or claim 2, in which the fuel is added in an amount to provide the sum of from 15 to 50 mols of carbon and H2 per 100 mols of extracted gas.

4. A method as claimed in claim 1 in which the extracted gas is at a temperature below about 2000"F and the fuel conversion is conducted in the presence of a catalyst.

5. A method for the production of a reducing gas from a carbon-containing fuel which comprises: (a) extracting from a reaction zone in a process plant a high temperature gas at a temperature from 2000 to 38000F, said gas containing an oxidant capable of reacting with the fuel; (b) intermingling with the extracted high temperature gas a carbon-containing fuel in an amount which provides the sum of from 5 to 100 mols of carbon and H2 per 100 mols of extracted gas; and (c) adiabatically thermally converting the fuel in the presence of oxidant which comprises oxygen, steam or mixtures thereof and present in a quantity sufficient to convert all of the carbon in the fuel to carbon monoxide, but insufficient to convert all of the carbon in the fuel to carbon dioxide, to form a resultant high temperature reducing gas consisting of carbon monoxide, or a mixture of carbon monoxide and hydrogen.

6. A method as claimed in claim 5 in which the fuel is added in an amount to provide the sum of from 15 to 50 mols of carbon and H2 per 100 mols of extracted gas.

7. A method for the generation of a reducing gas from a carbon-containing fuel, which comprises: (a) combusting carbonaceous material in a combustion zone of a boiler in the presence of excess air to form a high temperature flue gas comprising the oxides of carbon, steam and uncombined oxygen; (b) extracting a portion of the flue gas from the combustion zone, the extracted flue gas having a temperature from 2000 to 3800"F; (c) intermingling with the extracted flue gas a carbon-containing fuel in an amount which provides the sum of from 5 to 100 mols of carbon and H2 per 100 mols of extracted flue gas; and (d) adiabatically thermally converting the fuel in the presence of oxidant comprising steam, oxygen or mixtures thereof and present in a quantity sufficient to convert all of the carbon present in the fuel to carbon monoxide, but insufficient to convert all carbon in the hydrocarbon to carbon dioxide. to form a resultant high temperature reducing gas comprising carbon monoxide, or a mixture of carbon monoxide and hydrogen.

8. A method as claimed in claim 7 in whcih the fuel provides the sum of from 15 to 50 mols of carbon and H2 per 100 mols of flue gas.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The conversion efficiency of CH4 to CO+H2 for Example 4 is the same as for Example 3, i.e. 120% based on Equation (3).

The invention is not limited to the use of boiler flue gases but may be practised in conjunction with any process plant where a gas is available at a high temperature such as smelters, roasters, lime kilns, cement kilns, blast furnaces and magnetohydrodynamic power generating channels.

WHAT WE CLAIM IS: 1. A method of producing a reducing gas by thermal conversion of a hydrocarbon fuel, wherein a high temperature gaseous effluent by-product, containing an oxidant capable of reacting with the fuel, is extracted from a reaction zone of a process plant at a temperature sufficiently high to provide the heat input necessary to promote and sustain thermal reactions between the fuel and oxidant, and a carbon-containing fuel in an amount which provides the sum of 5 to 100 mols of carbon and hydrogen per 100 mols of extracted gas is intermingled with the gas to establish adiabatic thermal conversion of the fuel in the presence of the oxidant, a gaseous oxidant as hereinbefore defined being provided to the extent necessary to supply sufficient oxidant to the thermal conversion system to convert the carbon in the fuel to carbon monoxide but insufficient to convert all the carbon to carbon dioxide, whereby resulting from the thermal conversion is formed a high temperature reducing gas consisting of carbon monoxide or a mixture of carbon monoxide and hydrogen.
2. A method as claimed in claim 1 in which the extracted gas is at a temperature from 2000 to 38000F.
3. A method as claimed in claim 1 or claim 2, in which the fuel is added in an amount to provide the sum of from 15 to 50 mols of carbon and H2 per 100 mols of extracted gas.
4. A method as claimed in claim 1 in which the extracted gas is at a temperature below about 2000"F and the fuel conversion is conducted in the presence of a catalyst.
5. A method for the production of a reducing gas from a carbon-containing fuel which comprises: (a) extracting from a reaction zone in a process plant a high temperature gas at a temperature from 2000 to 38000F, said gas containing an oxidant capable of reacting with the fuel; (b) intermingling with the extracted high temperature gas a carbon-containing fuel in an amount which provides the sum of from 5 to 100 mols of carbon and H2 per 100 mols of extracted gas; and (c) adiabatically thermally converting the fuel in the presence of oxidant which comprises oxygen, steam or mixtures thereof and present in a quantity sufficient to convert all of the carbon in the fuel to carbon monoxide, but insufficient to convert all of the carbon in the fuel to carbon dioxide, to form a resultant high temperature reducing gas consisting of carbon monoxide, or a mixture of carbon monoxide and hydrogen.
6. A method as claimed in claim 5 in which the fuel is added in an amount to provide the sum of from 15 to 50 mols of carbon and H2 per 100 mols of extracted gas.
7. A method for the generation of a reducing gas from a carbon-containing fuel, which comprises: (a) combusting carbonaceous material in a combustion zone of a boiler in the presence of excess air to form a high temperature flue gas comprising the oxides of carbon, steam and uncombined oxygen; (b) extracting a portion of the flue gas from the combustion zone, the extracted flue gas having a temperature from 2000 to 3800"F; (c) intermingling with the extracted flue gas a carbon-containing fuel in an amount which provides the sum of from 5 to 100 mols of carbon and H2 per 100 mols of extracted flue gas; and (d) adiabatically thermally converting the fuel in the presence of oxidant comprising steam, oxygen or mixtures thereof and present in a quantity sufficient to convert all of the carbon present in the fuel to carbon monoxide, but insufficient to convert all carbon in the hydrocarbon to carbon dioxide. to form a resultant high temperature reducing gas comprising carbon monoxide, or a mixture of carbon monoxide and hydrogen.
8. A method as claimed in claim 7 in whcih the fuel provides the sum of from 15 to 50 mols of carbon and H2 per 100 mols of flue gas.
9. A method for the generation of reducing gas in accordance with any of

Examples 1 to 4 hereinbefore.
10. A method for the generation of reducing gas substantially as hereinbefore described with reference to the accompanying drawings.
11. A method for the production of hydrogen, wherein the product reducing gas obtained in practising the process as claimed in any of claims 1 to 10 is subjected to a water-gas shift reaction.
12. Reducing gas or a mixture of reducing gases when produced by the process claimed in any one of the preceding claims.