Gasification of solid carbonaceous materials to obtain high BTU product gas

Abstract

A process for converting solid carbonaceous material to combustible gases which comprises: A. heating and reacting the solid carbonaceous material with reactive gases in a gasification zone to obtain said combustible gases and withdrawing the combustible gases from the gasification zone, B. methanating at least a portion of said combustible gases in a methanation zone to obtain methanated gases, C. feeding at least a portion of said methanated gases through the gasification zone thereby increasing the methane content and BTU value of said combustible gases and transferring the sensible heat from said methanated gases to said gasification zone.

Parent Case Text

RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser. No. 252,450, filed May 11, 1972, now U.S. Pat. No. 3,817,725 the disclosure of which is incorporated herein by reference. Commonly-assigned application titled "Gasification of Solid Carbonaceous Waste Material", Ser. No. 252,449, filed May 11, 1972, now U.S. Pat. No. 3,817,724 is related to this application, the disclosure of which is incorporated herein by reference.

Description

BACKGROUND OF THE INVENTION

The present invention relates to gasification of solid carbonaceous material.

Previous gasification methods for solids mostly fall into the classifications of eduction processes or partial oxidation processes.

Eduction type processes are described in U.S. Pat. No. 1,055,344, Process of Making Gas; U.S. Pat. No. 2,640,014, Oil-Shale Eduction Process and Apparatus; and U.S. Pat. No. 3,361,644, Shale Retorting Process.

According to the U.S. Pat. No. 1,055,334, process gas is obtained from coal by distillation comprising passing the gas-making material between heated walls in a finely divided condition and while separated and distributed in a substantially non-oxidizing medium and distilling the material during such passage and regulating the motion of the material and said medium by imparting thereto a vertical motion by jets of heated gas directed along the heated walls transversely to the direction of passage.

It can be noted, however, that coal distillation is not gasification. Distillation drives off volatile matter already present; gasification creates volatiles not originally present.

In the process of U.S. Pat. No. 2,640,014, part of the material left after eduction of oil from the shale feed is burned to ashes with the resulting hot gas then being used to educt gas and oil from the shale feed material.

According to U.S. Pat. No. 3,361,644, gases and oil are educted from shale by passing oxygen-free hot gas countercurrent to downflowing shale particles. The hot gas is obtained by heating a portion of the gas effluent from the eduction zone. Temperature is maintained below about 1800.degree.F. in the eduction zone to avoid clinker formation by fusion of ash constituents. The process of U.S. Pat. No. 3,361,644 is largely a pyrolysis-distillation process which is not based on a gasification reaction such as reacting H.sub.2 O with carbonaceous material.

Partial oxidation processes are described, for example, in U.S. Pat. Nos. 1,977,684; 2,592,377; 2,657,986; 2,633,417; 2,727,812; 2,987,387; 2,657,124; and 3,025,149.

These processes all involve injection of oxygen into the reaction zone. Also, steam is usually injected into the reaction zone.

U.S. Pat. No. 2,660,521 is directed to partial oxidation of gases with expansion of the resultant hot gases through a turbine and thereby generating power and obtaining a cooled synthesis gas. Temperature in the reaction zone is controlled by carbon dioxide recycle. The carbon dioxide is a diluent and also lowers temperature by virtue of the endothermic reaction of carbon dioxide with hydrocarbon to form carbon monoxide and hydrogen.

Production of hydrogen and other combustible gases from waste substances produced in the manufacture of paper from wood chips and the like has been discussed in the literature, as, for example, in U.S. Pat. No. 3,317,292. In the manufacture of paper, wood chips are digested, for example, with an aqueous calcium sulfide liquid thereby forming calcium lignin sulfonate waste product in solution, leaving wood pulp behind. As disclosed in U.S. Pat. No. 3,317,292, the waste substances containing ligninderived organic components can be converted to a gas mixture comprising hydrogen by contacting the waste material with steam in a reaction zone at an elevated temperature at least of the order of several hundred degrees centigrade.

Both U.S. Pat. No. 1,777,449, titled Process of Producing Gas from Garbage, and U.S. Pat. No. 3,471,275, titled Method of Disposal of Refuse, are directed to producing combustible gases in an externally heated retort rather than by introducing hot gases directly into the retort vessel.

U.S. Pat. No. 2,840,462 relates to the production of high BTU-content gas from carbonaceous solid fuels such as coal. This patent basically teaches separation and removal of the methane formed in the gasification and hydrogenation of carbonaceous solids. Hydrogen is formed for the hydrogenation reaction using the classical water-gas shift reaction followed by a CO.sub.2 removal step. The methane formed is not recycled through the gasifier.

SUMMARY

According to the present invention a process is provided for converting solid carbonaceous material to combustible gases which comprises:

a. heating and reacting the solid carbonaceous material with reactive gases in a gasification zone to obtain said combustible gases and withdrawing the combustible gases from the gasification zone,

b. methanating at least a portion of said combustible gases in a methanation zone to obtain methanated gases, and

c. feeding at least a portion of said methanated gases through the gasification zone thereby increasing the methane content and BTU value of said combustible gases and transferring the sensible heat from said methanated gases to said gasification zone.

The present invention, among other features, is based on the combination of solids gasification with the methanation and recycling of a portion of the combustible gas effluent from the gasification zone. Also, temperature control in the gasification zone is preferably obtained by partially or wholly burning a second portion of the effluent gases from the gasification zone and then recycling the hot gases thus obtained to the gasification zone. It is critical in the process of the present invention that gasification reactions are carried out in the gasification zone to generate hydrogen and carbon oxides from solid hydrocarbonaceous matter in the gasification zone and also to generate hydrogen from the H.sub.2 O which is added to the reaction zone via the recycled hot gases. It is also preferred that the gasification zone be essentially free of molecular oxygen so as to substantially avoid any combustion in the gasification zone of the recycled gas or solid carbonaceous material feed. The present invention is also based on a critically important third feature or concept which is that the present process is extremely attractive for the conversion of solid carbonaceous material, particularly coal, to high BTU gas.

The term "high BTU gas" is used herein to mean gas having a BTU value of at least 100 BTU per cubic foot and preferably above 300 BTU per cubic foot. With CO.sub.2 removal from the product gas of the present invention the BTU content would be closer to 300 BTU/ft.sup.3 ; and with both CO.sub.2 removal from the product gas and use of purified oxygen (instead of air) for the burning of said recycled second portion, the BTU content would be about 400-800 BTU/ft.sup.3. Preferably the process is operated without any CO.sub.2 removal stages.

The term "solid carbonaceous material" is used herein to connote carbonaceous fuels such as coal, oil shale, and tar sands. A particularly preferred feed is coal including anthracite, bituminous and subbituminous coals and lignite.

The term "methanation" is used herein to mean conversion of carbon oxides and H.sub.2 to methane by reaction of the carbon oxides with H.sub.2. For example:

Co + 3h.sub.2 .fwdarw.ch.sub.4 + h.sub.2 o

co.sub.2 + 4h.sub.2 .fwdarw.ch.sub.4 + 2h.sub.2 o

the methanation reaction is exothermic, and this exothermic heat of reaction is integrated into the process of the present invention to supply a portion of the heat required for the endothermic reactions in the gasification zone. In the process of the present invention the temperatures in the gasification zone are highly advantageously controlled by means of adjusting the temperature and/or amount of methanated gases recycled to the gasification zone and preferably also by means of adjusting external to the gasification zone the temperature and/or amount of a second hot recycle gas stream to the gasification zone.

Thus, the temperatures in the gasification zone can be controlled pursuant to the present invention by controlling both the amount of each hot recycle gas stream and the temperature of each recycle gas. Usually all four of these variables will be adjusted to an optimum level for the process of the present invention so as to achieve desired reaction rates of solids in the gasification zone at minimum energy requirement for recycle gas compression, but yet to maintain temperatures in the gasification zone below about 2000.degree.F.

One of the particular advantages of the present invention is that the recycle of methanated gases allows an added means for temperature control in the gasification zone. Usually the main means of heat input and temperature control in the gasification zone is by the second hot recycle gas stream.

In the process of the present invention, the advantageous services which are afforded by the integrated methanation step include the following:

1. Increased BTU value of product gas by raising CH.sub.4 content of product gas

2. Provide heat input source for gasification reaction

3. Provide temperature control for gasification reaction.

To reduce the amount of reversal of the methanation reaction when the recycled methanated gases are introduced to the gasification reactor, it is preferable to introduce the methanated gases to about the middle of the reactor where the temperature is about 800.degree.-1200.degree.F. versus about 1600.degree.-2000.degree.F. at the bottom and 200.degree.-600.degree.F. at the top. Thus, according to a preferred embodiment of the present invention wherein the gasification zone comprises an upright cylindrical gasifier reactor, the methanated gases are introduced at a central position along the length of said gasifier, feed material is introduced at an upper position along the length of said gasifier and heating gases are introduced at a lower position along the length of said gasifier.

The gases obtained from the gasification zone are preferably cooled and cleansed by a water quence. This water in part finds its way back to the gasification zone via that portion of the gases which are recycled to supply heat and reactive gases to the gasification zone. Additional water is supplied to the gasification zone from the solids fed to the gasification zone, as the solids fed invariably will have at least some moisture content. Additional water and carbon dioxide are produced by partial burning of the recycle gas. Thus, there will be H.sub.2 O and CO.sub.2 present in the gasification zone for conversion of carbon and hydrocarbons into hydrogen and carbon monoxide according to well known reactions such as

C + H.sub.2 O.fwdarw.H.sub.2 + CO and C + CO.sub.2 .fwdarw. 2CO

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic process flow diagram illustrating a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWING

Referring more particularly to the drawing, a solid carbonaceous material, such as coal, is introduced via line 1 to gasification zone 2. Carbonaceous solid waste materials are also suitable as feed materials as discussed in our copending application, Ser. No. 252,450, filed May 11, 1972.

The solids can be considered to undergo a series of steps or reactions in the gasification zone including, for example, H.sub.2 O distillation, oil distillation, pyrolysis, and gasification reactions such as previously mentioned. In the present invention there must be gasification reactions in the gasification zone.

Ashes are withdrawn from the bottom of the gasification zone as indicated via line 3.

A gaseous stream is withdrawn from the gasification zone as indicated by line 4. This gas stream is quenched and scrubbed by water to remove particles introduced to quench drum 6 via line 7 and recycle line 8. Makeup water and preferably an alkali carbonate catalyst is introduced via line 5. The water serves to quench the gasifier effluent, and the hot gasifier effluent gases serve to heat water for efficient subsequent use in the gasifier. The heated water is ultimately recycled via lines 9 and 10 to gasifier 2.

The water preferably carries alkali carbonate catalyst to the gasifier to catalyze the reaction of the solids in gasifier 2 as is further described in U.S. Pat. No. 3,759,677 in regard to the gasification of solid waste material.

Heavy sludge material and water is withdrawn from quench drum 6 via line 9 and passed to settler 11 via line 9. Oily sludge material, water, and preferably alkali catalyst is passed to gasifier 2 via line 10 from the settler. A clarified oil stream is withdrawn from an intermediate point from settler 11 via line 12.

The hot gases withdrawn from gasifier 2 via line 4 are introduced to the quench drum at a temperature of about 600.degree.F., and the water scrub in the quench drum cools the gases to a temperature of 350.degree.F. before they are removed via line 13.

The gaseous material in line 13 is then split into two streams; a first stream 14 for recycle back to the gasification zone, and a second stream 15 which can be referred to as a product gas stream.

Preferably a portion of the recycle gases from quench drum 6 are passed to the recycle combustor via recycle compressor 22 and then line 23. Compressed air or pressurized oxygen is introduced to recycle combustor 24 via line 20. Recycle combustor 24 is preferably an in-line burner device. The partially combusted recycle gas is withdrawn from recycle combustor 24 usually at a temperature of about 2000.degree.F. and passed via line 25 to the bottom of gasification zone 2 to supply reactive gases and temperature control as well as heat for gasification zone 2.

In the present invention it is critical to methanate a portion of the recycle gases. Thus recycle gas is passed via lines 23 and 27 to methanation zone 28. In the methanation zone methane is formed by reacting carbon oxides with hydrogen. The carbon oxides and hydrogen are present in the recycle gas due to gasification reactions in gasifier 2. Preferably the methanation reaction is carried out catalytically over a methanation catalyst. Such catalysts are known in the art and typically contain nickel.

If the methanation catalyst is sulfur sensitive, sulfur removal should be effected before the recycle gases are introduced to the methanator. Sulfur compounds can be removed by many known methods, such as by use of a bed of solid particles containing zinc oxide.

Usually the gases fed to the methanation zone will be preheated to about 500.degree.-700.degree.F., and, due to the exothermic heat of the methanation reaction, the methanated gases leave the methanator at about 650.degree.-1000.degree.F. One of the advantageous features of the present invention is the utilization of this exothermic heat of reaction in the gasification reaction zone. Thus the methanated gases are passed via lines 25 and 31 to gasifier 2.

Previously product gas stream 15 was mentioned. Stream 15 can be the entire product stream, but advantageously a portion or all of the product gases are withdrawn, after methanation, from methanation zone 28 via lines 29, 31 and 33. Gases withdrawn from the methanation step have a higher BTU content than the gases in line 15, and thus the process of the present invention has the advantage of adjustable BTU content for the product gas by, e.g., blending various portions of stream 15 with stream 33. The product high BTU gas can be used as "town gas" in residential gas burning appliances or as a fuel gas for industrial furnaces.

Heat recovery zone 32 connotes recovery of heat from product gases withdrawn from the methanation step; this heat recovery step is advantageously integrated into the process of the present invention as a heat input source for the feed gas to methanation zone 28.

Although various embodiments of the invention have been described, it is to be understood that they are meant to be illustrative only and not limiting. Certain features may be changed without departing from the spirit or scope of the present invention. It is apparent that the present invention has broad application to gasification of solids with methanation of recycle gases to the gasification zone. Accordingly, the invention is not to be construed as limited to the specific embodiments discussed, but only as defined in the appended claims or substantial equivalents of the claims.

Claims

What is claimed is:

1. A process for converting solid carbonaceous material to combustible gases which comprises:

a. heating and reacting the solid carbonaceous material with reactive gases in a gasification zone to obtain said combustible gases and withdrawing the combustible gases from the gasification zone,

b. methanating at least a portion of said combustible gases in a methanation zone to obtain methanated gases, and

c. feeding at least a portion of said methanated gases through the gasification zone thereby increasing the methane content and BTU value of said combustible gases and transferring the sensible heat from said methanated gases to said gasification zone.

2. A process in accordance with claim 1 wherein the gasification zone comprises an upright cylindrical gasifier reactor vessel and the methanated gases are introduced at a central position along the length of said gasifier, said solid carbonaceous material is introduced at an upper position along the length of said gasifier, and heating gases are introduced at a lower position along the length of said gasifier.

3. A process in accordance with claim 1, which comprises the further steps of

d. partially or wholly burning a portion of said combustible gases in a recycle gas combustion zone to obtain hot recycle gas containing essentially no molecular oxygen, and

e. passing at least a portion of the hot recycle gas into the gasification zone to supply heat and said reactive gases to the carbonaceous material.

4. A process in accordance with claim 3 wherein the temperature in the gasification zone is controlled to below about 2000.degree.F. by means of adjusting the temperature of, or amount of, said hot recycle gas to the gasification zone.

5. The process of claim 3 wherein said carbonaceous material is coal.

6. A process for converting solid carbonaceous material to combustible gases, said process being free of carbon dioxide removal stages, which comprises:

a. heating and reacting the solid carbonaceous material with reactive gases in a gasification zone to obtain said combustible gases and withdrawing the combustible gases from the gasification zone,

b. partially or wholly burning a portion of said withdrawn combustible gases in a recycle gas combustion zone to obtain hot recycle gas containing essentially no molecular oxygen,

c. passing the hot recycle gas into the gasification zone to supply heat and at least a portion of said reactive gases to the carbonaceous material.

d. methanating at least a portion of said combustible gases in a methanation zone to obtain methanated gases, and

e. feeding a portion of said methanated gases through the gasification zone thereby increasing the methane content and BTU value of said combustible gases and transferring the sensible heat from said methanated gases to said gasification zone.

7. A process in accordance with claim 6 wherein the temperature in the gasification zone is controlled to below about 2000.degree.F. by means of adjusting the temperature of or amount of said hot recycle gas to the gasification zone.

8. The process of claim 6 wherein said carbonaceous material is coal.

9. A process of claim 1 wherein a portion of the combustible gases from the gasification zone is combined with a portion of the gases from the methanation zone to form a net product combustible gas.

10. A process of claim 5 wherein a portion of the combustible gases from the gasification zone is combined with a portion of the gases from the methanation zone to form a net product combustible gas.