Process for Producing CO.sub.2, and CO.sub.2 So Produced

The present invention relates to a process for producing CO.sub.2, and to CO.sub.2 so produced.

It has recently been demonstrated that CO.sub.2 is useful for reducing the viscosity of crude oil and can be used to recover additional crude oil from oil-bearing formations which have been depleted by primary and secondary recovery techniques. The amounts of CO.sub.2 required for this purposes could be very large since a substantial proportion of gas injected into an oil well is irrecoverable. One scheme for providing CO.sub.2 for enhanced crude oil recovery proposes the recovery of CO.sub.2 from a subterranean source of CO.sub.2 by means of a well, and then piping the thus recovered CO.sub.2 at a high pressure for several hundred miles to an oil-field. It goes without saying that this scheme is very costly.

CO.sub.2 is becoming widely used in horticulture to promote the growth of plants in greenhouses and like enclosures.

CO.sub.2 can be obtained from the flue gas of a conventional boiler, but its concentration of up to 14 vol % would necessitate the use of a suitable process to concentrate the CO.sub.2. At the present time, only wet gas-scrubbing processes can satisfactorily recover large volumes of CO.sub.2, but wet scrubbing processes are notoriously energy-inefficient.

The present invention provides a process, and an installation for performing the process, for making CO.sub.2, the process comprising the steps of:

(a) passing a fuel to a dense-phase fluidized fuel-conversion bed containing particles comprising reactive CaSO.sub.4 and particles comprising reactive CaO, the bed being fluidized by passing into the bottom thereof a fluidizing gas which is substantially free of inert diluents but which contains a mediating moiety and/or a source thereof for mediating the transfer of oxygen from CaSO.sub.4 to fuel whereby the fuel is partially oxidized to products including CO and CO.sub.2, and some CaSO.sub.4 is reduced to CaS substantially without the liberation of sulfur moieties, the temperature of the fuel conversion bed being no greater than the maximum temperature at which CaO will react with CO.sub.2 to form CaCO.sub.3 ;

(b) causing particles including particles comprising CaSO.sub.4, particles comprising CaS and particles comprising CaCO.sub.3 to pass to a fluidized decarbonation bed (preferably a dense phase bed) which is fluidized by passing into the bottom thereof at least some of the partially oxidized gas products from the fuel conversion bed, which gas products include a mediating moiety and/or a source thereof for mediating the transfer of oxygen from CaSO.sub.4 to CO whereby to oxidize CO to CO.sub.2 and to reduce CaSO.sub.4 to CaS, the decarbonation bed being operated at a temperature above the decarbonation temperature of CaCO.sub.3 whereby CaCO.sub.3 is decomposed to CaO and CO.sub.2 thereby increasing the amount of CO.sub.2 in the gas passing through the decarbonation bed; and

(c) recovering a gas containing CO.sub.2 from the top of the decarbonation bed.

Preferably, particles including particles containing reactive CaS, are caused to pass to a fluidized oxidation bed (preferably a dense phase bed) in which the particles are fluidized by passing into the bottom of the bed a gas containing molecular oxygen whereby at least some reactive CaS is oxidized to reactive CaSO.sub.4 with the liberation of heat which maintains the oxidation bed at a temperature above that of the fuel-conversion bed. Preferably, CaS-containing particles are passed to the oxidation bed from the decarbonation bed.

The oxidation bed is preferably operated under such conditions that the gas leaving the top of the bed is substantially free of sulfur moiety.

Particles containing reactive CaSO.sub.4 are preferably caused to pass to the decarbonation bed and/or to the fuel conversion bed.

The oxidation bed is operated at a temperature exceeding the temperature of the decarbonation bed when the operating pressures of the oxidation and decarbonation beds are substantially equal.

Preferably, the fluidizing gas passed into the bottom of the fuel conversion bed comprises at least part of the partially oxidized gas products produced by the operation of the fuel conversion bed.

In instances wherein the process is operated with a fuel which contains sulfur, sulfur moieties from the fuel are preferably fixed by reaction with CaO in the fuel conversion bed to form CaS.

In order to avoid a progressive increase in the inventory of chemically fixed sulfur, the process may comprise the further step of passing particles containing CaS to a fluidized regenerator bed, passing a gas containing molecular oxygen into the bottom of the regenerator bed, and maintaining the operating conditions of the regenerator bed such that CaS is converted to reactive CaO with the liberation of sulfur moiety.

Preferably, particles containing reactive CaO are returned directly and/or indirectly to the fuel conversion bed for use in fixing further amounts of sulfur moiety from the sulfur-containing fuel.

Particles are preferably passed from the decarbonation bed to the fuel conversion bed to maintain the inventory of reactive materials in the fuel conversion bed.

The invention is now further described with reference to a non-limitative example thereof and with reference to the accompanying drawing which is a flow sheet showing the principal features of an installation for performing the process of the invention.

In the drawing, a dense phase fluidized fuel conversion bed 21 of particles comprising reactive CaSO.sub.4 and CaO is contained in vessel 1. A fuel, which may be a solid fuel (e.g. coal, lignite, oil shale) and/or a fluid fuel (e.g. natural gas, crude oil or one or more fractions thereof) is passed into the fuel conversion bed 21. A gas containing a mediator to mediate the transfer of oxygen from CaSO.sub.4 (which is reduced to CaS) to the fuel is passed into the bottom of the fuel conversion bed 21 and the fuel is partially oxidized to produce a mixture of pyrolysates and partial oxidation products including (inter alia) tars and other hydrocarbons, CO, H.sub.2 and CO.sub.2, the proportions of CO and CO.sub.2 being determined by the operating conditions in bed 21 and kinetic considerations, as will be appreciated by those skilled in the art. The bed 21 is maintained at conditions of pressure and temperature such that a significant proportion of the CO.sub.2 produced reacts exothermically with CaO to form CaCO.sub.3 in the bed. The removal of CO.sub.2 by reaction with CaO promotes the production of further quantities of CO.sub.2 in the bed 21 in accordance with well known chemical and thermodynamic considerations in relation to the oxidation of carbon to CO and CO.sub.2. At about atmospheric pressure, the bed 21 may be operated at a temperature of 700.degree. to 750.degree. C., e.g. about 730.degree. C. Sulfur from the fuel reacts with CaO to form CaS. Thus, the gas leaving the top of the fuel conversion bed 21 at the temperature of the bed 21 comprises condensible tars and hydrocarbons, H.sub.2, water vapour and carbon oxides. The hot gas is passed through a heat recovery device 4 which may be part of a boiler installation and wherein the gas is cooled to a temperature at which tars and hydrocarbons condense. The combustible part of the condensate is recovered (not shown) and is preferably employed as fuel which is passed to the fuel conversion bed 21 via a suitable conduit (not shown). The cooled gas exiting device 4 comprises CO.sub.2, CO and H.sub.2 and passes through a particulates-removing system 5 and cool, clean gas is circulated by a fan 8 into line 30. Some of the gas from line 30 passes into the bottom of the bed 21 where it serves to fluidize the particles in the bed. The amount of gas passing into the bed 21 is determined by the setting of the valve 10. A supplement of steam, which may be produced in the boiler installation, may be added to the gas to improve fuel conversion by the water gas reaction and also to improve fluidization. The amount of steam added is regulated by valve 12.

The hydrogen in the bed 21 tends to be the main mediator for the transfer of oxygen from CaSO.sub.4 to the fuel and substantially none of the CaSO.sub.4 is decomposed to liberate sulfur moiety in the presence of hydrogen and/or other mediator(s), particularly at lower operating temperatures.

It will be seen from the foregoing that the gas passed into the fuel conversion bed 21 is substantially free of inert diluents, such as nitrogen, and the gas product is also free of inert diluents except for minor amounts of nitrogen (and other inert diluents) which may have been present in the fuel.

Char and other solid carbonaceous fuel material in the bed tends to have a relatively high ratio of aerodynamic drag to weight, and there may be a tendency for char materials to be more concentrated at the top regions of the bed 21 than elsewhere. A baffle 14 extends from above the top level of the bed 21 to below the top level (but not to the bottom of the bed) to prevent or at least reduce the passage of solid fuel material into the space 32 defined between the baffle and the wall of the vessel 1, and particles pass from the space 32 into a transfer line 33 which extends from a top region of the bed 21 in space 32 to a bottom region of a fluidized decarbonation bed 34 by any means known in the art (e.g. pneumatic transfer using a suitable gas). The bed 34 is contained in a vessel 2, and the bed is fluidized by passing into the bottom thereof clean product gas from line 30 at a rate determined by valve 11.

The bed 34 is operated at decarbonation conditions, and for operation at about atmospheric pressure, a temperature of about 850.degree. to 900.degree. C. is suitable. The gas entering from line 30 contains CO and H.sub.2 which react with CaSO.sub.4 to reduce the latter to CaS to produce CO.sub.2 and H.sub.2 O. The CO.sub.2 thus produced is in addition to CO.sub.2 produced by the decarbonation of CaCO.sub.3 obtained by the carbonation of CaO in bed 21, and the gas leaving the top of the decarbonation bed 34 at a temperature approximately equal to the bed temperature contains CO.sub.2, H.sub.2 O and is substantially free of inert diluents, such as nitrogen. The gas will be substantially free of sulfur moiety since at the operating temperature of bed 34, the amounts of sulfur moiety which could be liberated even under oxidizing conditions would be very small (e.g., of the order of about 100 parts per million), and the tendency for even these small amounts of sulfur moiety to be liberated is reduced or substantially eliminated by the presence of H.sub.2 (or other oxygen-transfer mediator) in bed 34.

Heat is recovered from the hot gas leaving vessel 2 by a heat recovery device 6 which may be part of the boiler installation, and the gas leaving device 6 is substantially pure CO.sub.2 together with some H.sub.2 O which may be removed in any convenient manner, if desired.

The particles of the decarbontion bed will include CaS resulting from the reduction of CaSO.sub.4, CaO resulting from the decomposition of CaCO.sub.3, CaS resulting from the capture or fixing of sulfur from the fuel (if the fuel contains sulfur, as it usually will) in the fuel conversion bed 21 and unreacted CaO from bed 21. Particles are passed from a top region of the decarbonation bed 34 to a bottom region of the fuel conversion bed 21 via a transfer line 35. The particles thus transferred are substantially at the temperature of the bed 34 and add their sensible heats to the bed thereby helping to maintain the temperature of bed 21.

The reactions in the fuel conversion bed 21 deplete the availability of CaSO.sub.4 to maintain the partial oxidation of fuel. In order to restore the availability of CaSO.sub.4, particles are passed from a top region of the decarbonation bed 34 to a bottom region of an oxidation bed 36 via a transfer line 37.

The oxidation bed 36 is contained in a vessel 3 and is fluidized by passing a gas containing molecular oxygen into the bottom of the bed 36. A suitable gas is air (which may be preheated by heat exchange (not shown) with one or more of the gas streams leaving any one of the fluidized beds and/or by passage through heating coils in one or more of the vessels). The air (or other gas containing molecular oxygen) is passed into the oxidizing bed by a fan 9 at a rate determined by a suitable valve system (not shown). GaS is oxidized to CaSO.sub.4, and this oxidation being strongly exothermic maintains the temperature of the oxidation bed 36 at 950.degree. to 1100.degree. C., e.g. about 1000.degree. C. The air is supplied in excess of the stoichiometric requirement so that the oxygen-depleted gas leaving the top of the bed 36 has a small content of oxygen (e.g. up to 5 vol %, preferably 1 to 2 vol %). The provision of excess oxygen suppresses the oxidation of CaS to CaO and SO.sub.2 so that the hot gas leaving the top of bed 36 is substantially free of sulfur moiety. The hot gas leaving the oxidation bed 36 at approximately the temperature thereof is passed through a heat recovery device 7 which may be part of the boiler installation. The oxygen-depleted gas, which will be predominantly nitrogen, may be recovered for use as an inerting or blanketing gas or for other use in which nitrogen is employed (e.g. ammonia manufacture and/or for use in the recovery of crude oil from oil-bearing rock formations either alone or in combination with the CO.sub.2 available from the decarbonation bed 34.

The particles in the oxidation bed 36 comprise CaSO.sub.4 and CaO some of which has been derived by decarbonation of CaCO.sub.3 in bed 34 and some of which has not participated in any chemical reaction. Particles are passed from the oxidation bed 36 either directly to the fuel-conversion bed 21 by a suitable transfer line (not shown) to maintain the concentration of reactive CaSO.sub.4 therein and to maintain the fuel conversion temperature, or particles may be passed from the oxidation bed 36 to the decarbonation bed 34 (so that particles containing CaSO.sub.4 pass therefrom to the fuel conversion bed 21), or particles may be passed to both the fuel conversion bed 21 and the decarbonation bed 34.

As shown in the drawing, particles are passed from a top region of the oxidation bed 36 to a bottom region of the decarbonation bed 34 via a transfer line 38 whereby to augment the heat input to bed 34, and to provide that CaSO.sub.4 from the oxidation bed 36 passes indirectly to the fuel conversion bed 21 by passage via the decarbonation bed 34.

Most fuels contain sulfur, and the particles include particles containing CaO to capture or fix the CaO as CaS in the fuel conversion bed 21. In order to avoid a continued increase from this source in the amount of chemically combined sulfur in the beds 21, 34 and 36, a side stream (not shown) of particles is subjected to a treatment to remove sulfur moiety. A convenient method (not illustrated) of removing sulfur is to pass particles from the decarbonating bed 34 to a regenerator wherein the particles are fluidized in a bed by air (or other gas containing molecular oxygen) under such conditions that the gas leaving the bed contains substantially no oxygen, as is described in our U.S. patent specification No. 3807090. In the substantial absence of excess oxygen, and at elevated temperatures, e.g. in the range 800.degree. to 1150.degree. C., CaS decomposes exothermically as follows:

The SO.sub.2 may be recovered as such or converted to sulfur or other sulfur products, and the hot regenerated CaO is returned to the bed system--e.g. to bed 34 and/or to bed 21 wheein it serves to maintain the operating temperature(s) and the inventory of CaO.

Inevitably, in practice, the chemical reactivity of particles declines over a period of operation and it is necessary to add fresh reactive material (e.g. CaSO.sub.4 and/or CaCO.sub.3 and/or CaO) to the bed system and to purge particles therefrom to maintain the overall reactivity of the particles in the bed system. The numerous ways in which the foregoing may be effected will not be described since they are within the competence of the technologist to which this patent specification is primarily addressed.

The installation as herein described may be regarded as or utilized as a boiler installation providing (a) useful heat which is recoverable from the heat recovery devices 4, 6 and 7 as steam for power production and/or use as process steam, and/or as preheated air (at least some of which may be the air supplied to bed 36 by the fan 9), and (b) two main off-gas streams, one being of CO.sub.2 and the other being of nitrogen containing only a small concentration of oxygen. The nitrogen-rich stream will be virtually free of NO.sub.x since the temperatures within the bed 36 are too low for NO.sub.x formation. As a further bonus, elemental sulfur may be obtained when the fuel employed contains sulfur --e.g. by the process described in our U.K. patent No. 1541431 and U.S. Pat. No. 4041141.

The invention comprises, besides the process herein described and defined, an installation for performing the process as herein described and defined .