Method for separating CO2 from waste gases, converting it to CH4 and storing same

[0002] Beside the methane emanating from technical installations and by a natural way from the soil, carbon dioxide is the most important greenhouse gas. It destructs the ozone belt which is protecting life on earth from to intensive ultraviolet radiation.

[0003] Since more than a decade technicians and scientists are seeking for methods and means in order to eliminate the noxious CO₂from waste gases and to store it in the form of an insoluble chemical compound or outside of the atmosphere of the earth. There are technically feasible proposals (partly tested in pilot plants) in order to separate the CO₂from combustion or other waste gases, to concentrate it (by liquefing), to use it industrially or to store it underground (in aquifers, deep sea or to stimulate poorly producing oil reservoirs). One of the practical ways to concentrate the CO₂consists of reintroducing the combustion CO₂by a secondary cycle in the combustion process after having made the combustion-air nitrogen-free by catalytic and/or cryogenic separation. Instead of the use in neighbouring chemical plants, the binding of CO₂on sea weed as well as the bacterial fermentation of the CO₂in special reactors have been technically examined in pilot plants. These latter processes need either large storing volumes for the CO₂resp. CH₄produced or an absolutely constant operation of the CO₂producing installations, because these binding resp. fermentation processes are not flexible. Specially thermal power plants demand such flexibility in order to cover daily, weekly and seasonal peaks.

[0004] The present invention permits such flexible operation. The invention comprises the pumping of the separated, purified, liquefied and dryed CO₂in a pipeline to a nearby aquifer, a natural gas reservoir or an oil bearing structure and injecting the CO₂into it and treating the structures in the first two cases previously with bacteria cultures (and ev. suitable substrate). In the third case the methanogenic bacteria and the substrate will be already naturally present so that the conversion of the CO₂to CH₄may immediately begin. For that purpose the following conditions have to be naturally present or artificially created (see bibliography in the annex):

[0005] 1. There has to be enough H₂for reduction of the CO₂(or as electron donor) in the structure (underground reservoir), either

[0006] in free form produced by metamorphic rock in the depth, or

[0007] by bacterial direct transmission from organic H₂containing substrate, or

[0008] by bacterial splitting off of the pore water, or

[0009] by artificial feeding of H₂in pure form of ammonia (NH₃) which forms in the structure (underground reservoir), in contact with the pore water, partly ammoniumhydroxide NH₄OH and with CO₂urea (H₂N)₂CO which, by bacterial aided hydrolyzing, splits off free H₂, or

[0010] e.g. by artificial feeding of formiates NaHCO₂or Ca(HCO₂)₂.

[0011] 2. The sulfate content of the sediment in the structure (underground reservoir) has to be either

[0012] enough poor that the sulfate no more can selectively bind the H₂necessary for the CO₂reduction, and/or

[0013] no H₂S or other sulfide may act as methanogenic poison, or

[0014] the sulfate competing with the CO₂reduction may be artificially blocked by an inhibitor (e.g. NA-molybdate or fluorlactate), or

[0015] by feeding H₂in to the structure (underground reservoir) enough long before the CO₂injection, so that the sulfate will be sufficiently reduced.

[0016] 3. There must be present in the sediment a methanogenic bacterial population large enough (e.g. methanococcus, methanobacterium formicicum, methanobacterium thermoautotrophicum, methanosarcina barkerii, or other photosynthetic bacteria) which is able,

[0017] to split off free H₂from the pore water or from H₂S, and

[0018] to reduce the CO₂to CH₄sufficiently,

[0019] eventually by addition of catalysators like e.g. palladium,

[0020] adding, if necessary, the needed bacterial population and/or the substrate artificially.

[0021] The main condition to realize the CO₂conversion is a drilling (in a aquifer, a natural gas reservoir or an oil bearing formation) across the gas or oil containing structure till in the aquatic phase thereunder, with cores from the interesting stratums and analyzing these cores in the laboratory.

[0022] The main aim is finding (or not) the necessary bacteria, the substrate whereon they live, the chemical composition of all sediment parts, the presence of H₂, sulfate as well as the absence of Na—AL-silicates. With these results it will be further established under which conditions the found (or added) bacteria and the substrate are able to produce CH₄from CO₂and how much.

[0023] By adding other substrates (e.g. acetate, methanol, methylamine, dihydronicotinamide, dihydro-5-diazaflavine, 2-mercaptoethane-sulfonic acid and like) it will be established if the CO₂reduction to CH₄is intensified resp. accelerated. With a given substrate the addition of CO₂, in the presence of H₂, increases the CH₄production normally by more than one magnitude. The same is the result when H₂is added in presence of CO₂. Consequently, both gases have to be present together for ensuring an optimal CH₄production. In this connection it may be noted that the 4 H atoms needed for the CH₄do not originate from the added H₂gas (or NH₃), but from the pore water. The H₂gas serves only as electron donor. All research work hitherto existing shows that bacteria and substrate are only a transition station for the H₂necessary for CO₂reduction. Consequently, during the laboratory experiments the consumption of bacteria and substrate,as well naturally present as also artificially added matter, has to be established. Concerning the practical use of the invention, a consumption of bacteria and/or substrate to high could dictate an economic limit.

[0024] As soon as the different questions are resolved by the laboratory experiments, the same tests are following under in situ conditions of pressure and temperature (still in the laboratory). This clarifying by steps conducts to the optimum method which is than tryed out in a pilot test 1:1 after a second drilling (observation drilling) in relative vicinity to the first drilling. If there is a flow direction of the pore water, the testing has to be made on the low side and the CO₂addition on the high side of the flow. An increasing CH₄content signifies a positive conversion, which is confirmed by an also increasing H₂content.

[0025] This is specially the case with the addition of ammonia gas instead of pure H₂. The basic chemical transformation according to the formula CO₂+4 H₂=CH₄+2 H₂O becomes with ammonia in molecular writing 2 CO₂+6 NH₃=2 CH₄+3 N₂+4 H₂O+H₂, what means that the transformation yields a H₂excess which passes over in the next step, so that, in constant operation, will be an equilibrium CO₂: NH₃of approx. 1:2,7. But this will only be the case if the H₂procurement is not aided by bacterial assistance. Such bacterial assistance liberates practically always supplemental H₂from organic substrates or from pore water, eventually supported by H₂from metamorphic rock, what reduces the CO₂/NH₃proportion from 2,7 to 1 and less.

[0026] The CO₂reduction produces in every case additional pore water. The use of NH₃as H₂donor has the special disadvantage of producing the double quantity of pore water compared to the use of pure H₂and that 40% of the NH₃gas remain as N₂(ballast) in the structure and appear occasionally as pollution in the produced CH₄and demand a separation (purification). Economically it could be better to inject pure H₂under high pressure as emulsion in the liquid CO₂(with or without emulgators). At the maximum possible proportion of volume CO₂:H₂of 1:4 the emulsion becomes foamy and at smaller H₂requirements and higher pressure the H₂is broken up in small gas bubbles in the liquid CO₂.

[0027] The liquid medium CO₂covers than the inner surface of the steel tube and not the H₂gas and forms a protection layer having itself a soluted proportion H₂:CO₂(volume) of approx. 1:100 (at the normally present conditions of pressure and temperature) and protects in such manner the steel from brittleness by direct hydrogen attack. For this protection it is essential that there is a turbulent flow in order to maintain the CO₂/H₂emulsion during injection in the structure. Moreover the structure temperature (approx. 80° C.) is not high enough to really cause hydrogen brittleness (over 300° C.). Because of the difficulty to control long pipelines, the H₂should be produced in the field at the injection point. But there is no danger for an auto-reduction of the CO₂to CH₄within the CO₂/H₂mix in the injection tube at the existing temperatures.

[0028] If the CO₂conversion takes place in a aquifer, the addition of H₂, bacteria and/or substrate is unavoidable, but may be avoidable in a natural gas reservoir created by gas migration and is most probably not necessary in oil and gas bearing (generating) formations. The eventual additions beside the CO₂(NH₃, H₂, bacteria, substrates, catalysators, inhibitors) shall not pollute the groundwater of aquifers being situated above the deep lying brack waters. For these latter the pollution aspect is of minor significance.

[0029] The sole drawing is a schematic showing of the process according to the present invention.

[0030] The enclosed drawing shows schematically and as an example a modern installation according to the invention, applied on a anticlinal structure. The drawings considers only process phases. In the whole process N₂is considered as ballast. It is separated from the combustion air catalytically or cryogenically in phase 1, being afterwards vented directly to the atmosphere or e.g. in a NH₃production plant 16. The remaining O₂, CO₂from air, noble gases and some pollutants are burnt in the following combustion plant 2 (gas turbine, gas motor, boiler and like). In the separation plant 3 the combustion products H₂O and CO₂as well as N₂(remaining N₂or total N₂in the case of absence of phase 1) are separated from each other. The catalysator plant 4 reduces resp. oxidizes the oxides NOx, SO₂, CO to N₂, elemental sulfur, oxygen and CO₂; feeding practically pure CO₂in the compressor 5. Here the CO₂is liquefied, cooled down (not shown in the drawing) and is stored in the reservoir 6. Eventually here accumulating water may be drained off in order to be reconducted in the process phase 16 or to the environment. Such reconduction is also possible for CO₂from the reservoir 6 to the burning installation 2 (if there is air separation in phase 1) in order to create a combustion in a O₂/CO₂atmosphere.

[0031] If there is no or not enough H₂present in the storage and conversion structure 13, 14 to ensure complete reduction of the CO₂, the process is equipped with an ammonia plant 16. The nitrogen N₂for the production of NH₃comes from the phase 1, the necessary H₂from fresh water and the stripped off O₂will be sent in the combustion process in phase 2.

[0032] CO₂and NH₃are pumped on identical high pressure in the compressors 7 resp. 17 and flow as a mix along the pipeline 8 to the storage field 13. If during this transport urea (H₂N)₂CO should be formed from the CO₂/NH₃mix, it is only of interest in view to the also forming H₂O as byproduct, because this water would form together with the CO₂the acid H₂CO₃which could attack the pipeline by internal corrosion. At the same moment the NH₃would form in the presence of H₂O the basic NH₄OH able to neutralize—at least partly—the dangerous H₂CO₃. It is indicated to check for internal corrosion and to add eventually a corrosion inhibitor. Another possibility is the batchwise separated transport of the liquid CO₂and NH₃as usual in oil and product pipelines.

[0033] According to the idea of the present invention, the storage field (aquifer, natural gas reservoir, oil generating rock structure) serves as conversion reactor for the CO₂: The injection drillings 11 for the CO₂, the H₂(or NH₃) are arranged around the extraction drilling 10 for the CH₄in such a manner that there will be a central flowing. In the drawing the 4 injection drillings 11 are unrolled to one plane, but in reality they are arranged in a square around the extraction drilling 10. The injection drillings 11 reach in the pore water containing part 14 of the anticlinale, because the reduction of the CO₂to CH₄takes place in this part. In order to take the cores for the laboratory experiments mentioned further above, the drilling 10 reaches initially the pore water part 14, is later cemented and than perforated. The CH₄produced from the CO₂is first stored in the water part 14 and—with increasing volume—comes in contact with the gaseous part 13 and forms so a compact gas phase. During the in situ tests in scale 1:1 mentioned above the drilling 10 reaches in the water part 14. From the injection drillings 11 the produced CH₄is driven by a small overpressure to the extraction drilling 10. This technique is also used in the case of CO₂pressuring (stimulation) of poorly producing oil fields. This known technique also provides for a rich experience concerning the compatibility of the rock and CO₂.

[0034] Each injection drilling 11 has a nozzle 12 for the injection of separate additions of bacteria, substrates, inhibitors and/or catalysators. The extraction drilling 10 is joined at his head to the gas pipeline 9 which in her turn is joined to the combustion plant 2. Under the aquifer 14 with the gas reservoir 13 there is the rock basis 15, furnishing eventually enough H₂diffusing from metamorphic rock in the depth to the top.

[0035] The operation of the field installations is very similar to that of a natural gas storing field with injection and extraction drillings. The volumes of the consumed CO₂and CH₄formed from it being theoretically equal (appart from small losses), the pressure in the storage structure 13 (or conversion reactor 13) remains stable for a long time because there is a practically closed circuit CO₂—CH₄. The only supplemental feeding quantities are the H₂donor (e.g. NH₃), catalysators, inhibitors and eventually bacteria and substrates. Natural gas becomes in this way a renewable energy.

[0036] In order to preserve clearness, the following installations are omitted from the drawing: pumps and compressors only for feeding and circulation purposes between the process phases, coolers, separators, compensation tanks and regulator parts.

[0037] In view to completeness the drawing shows also industries 18 in vicinity to the power plant 2, which can use as well CH₄as CO₂and other byproducts of the whole process. This guarantees an optimal utilization combined with high flexibility.