Emissionless Oxyfuel Combustion Process and a Combustion System Using Such a Process

Abstract

A method of combusting carbonaceous fuel in a combustion system. The combustion system includes a source of oxygen and a furnace. The method includes the steps of (a) feeding fuel and combustion gas including oxygen and recycling gas into the furnace for combusting the fuel with the oxygen and producing exhaust gas that includes CO2, water and excess oxygen as its main components, (b) conducting the exhaust gas discharged from the furnace into a scrubber so as to remove pollutants from the exhaust gas, (c) dividing the exhaust gas into a first exhaust gas stream and a second exhaust gas stream, and conducting the second exhaust gas stream as a recycling gas stream into the furnace, (d) conducting the first exhaust gas stream into a CO2 purification and capturing unit (CPU) to produce one or more condensate streams, a purified liquid CO2 stream and a vent gas stream that includes remaining CO2, (e) discharging the purified liquid CO2 stream from the combustion system, (f) conducting the vent gas stream into an adsorption unit so as to adsorb compounds, including remaining CO2, from the vent gas stream to an adsorbing material and to produce a pass-through gas stream, and (g) regenerating the adsorbing material to produce a desorbed gas stream that includes at least a portion of the adsorbed compounds, and conducting at least a portion of the desorbed gas stream into the furnace.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to an emissionless oxyfuel combustion process and a combustion system using such a process. More particularly, the present invention relates to an oxyfuel combustion process and combustion system with an emission recycling loop.[0003]2. Description of the Related Art[0004]The exhaust gases of a conventional power station typically contain from about 4% (by volume) to about 14% carbon dioxide (CO2). It is commonly believed that this CO2 represents a significant factor in increasing the greenhouse effect and global warming. Therefore, there is a clear need for efficient methods of capturing CO2 from exhaust gases so as to produce a concentrated stream of pressurized CO2 that can readily be trans-ported to a safe storage site or to a further application. CO2 has been captured from gas streams by means of four main technologies: absorption, in which CO2 is selectively absorbed into liquid solve...

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Benefits of technology

[0018]An advantage of the present invention is that it is not necessary to aim for a sharp cut of the adsorbed molecule species at the adsorption unit, but a portion of the N2 and O2 in the vent gas stream may also be adsorbed in the adsorbing material. When practically all CO2 and some of the N2 and O2 of the vent gas stream are adsorbed in the adsorption unit, practically all pollutants which may remain in the vent gas stream, especially NOx and CO molecules, are typically also adsorbed in the adsorbing material. Thus, when the desorbed gas stream is conducted back to the furnace, also some N2 and O2 and all remaining pollutants, especially NOx and CO molecules, are recycled back to the furnace.

[0019]As described above, the combustion system forms two closed loops: direct recycling of the second portion of the exhaust gas and the recycling of the compounds desorbed from the adsorption unit, without releasing any emissions to the atmosphere. As the pollutant compounds are recycled back to the furnace, a portion of the compounds, especially CO, SO3 and NOx molecules, as well as possible VOC-compounds (volatile organic compounds), are destructed in the furnace. The destructing of the pollutant compounds is important because otherwise the pollutant levels in the exhaust gas might increase to relatively high values before a balance is reached between the forming and capturing of the pollutants.

[0020]Due to the pollutant recycling, conventional NOx reducing processes of the combustion system function very effectively even without high NOx removal efficiency during a single cycle. Therefore, it is possible to use relatively low NH3 / NOx ratio in a conventional NOx removing process. According to a preferred embodiment of the present invention, the combustion process does not include any dedicated NOx reduction process, i.e., for example, a selective catalytic NOx reduction (SCR) or selective non-catalytic NOx reduction (SNCR) process. Correspondingly, according to a preferred embodiment of the present invention, the combustion system does not comprise any dedicated means for NOx reduction, i.e., for example, a NOx catalyst or means for selective non-catalytic NOx reduction. Reason for this is that any NOx in the vent gas from the CPU is efficiently adsorbed in the adsorption unit, and the NOx is desorbed and recycled back to the furnace. The recycled NOx is efficiently reduced to N2 in the furnace, which N2 will then eventually be separated by the adsorption unit to the pass through gas stream which is released to the atmosphere, either directly or via the ASU. On the other hand, in order to avoid dissolving sulphur in the exhaust gas into the liquid CO2 stream discharged from the CPU unit, the combustion system advantageously comprises an efficient SOx capturing process, like a scrubber, in the system.

[0021]When the adsorption unit adsorbs all the CO2 of the vent gas stream, it adsorbs in practice also some N2 and O2. At least when some zeolites are used for efficient adsorption of CO2, a considerable amount of N2 may also be adsorbed. In this case, the pass through gas stream may have a relatively high O2 concentration, and it forms a high quality feed gas to the ASU. The pass through gas stream fed from the adsorption unit to the ASU consists, in addition to N2 and O2, also some Ar. The ASU removes the N2 and separates O2 to be conducted to the furnace. The ASU usually leaves a portion of the Ar in the product gas stream, and thus some accumulation of Ar in the circulating gas may result. In case Ar accumulation becomes a problem, it is possible to release at least a portion of the pass through gas to the atmosphere, or to modify the ASU to improve the Ar separation.

[0022]According to a most preferred embodiment of the present invention, the adsorbing material and / or operation conditions of the adsorption unit are selected in such a way that, simultaneously when CO2 is efficiently adsorbed, also clearly more O2 than N2 is adsorbed in the adsorbing material. Suitable material may be, for example, activated carbon when used at suitable operating conditions. When using such an adsorption process, the pass through gas comprises mainly N2 and only a relatively small amount of O2. Thus, the pass through gas can especially in these conditions advantageously be released to the atmosphere instead of being used as a feed gas of the ASU.

[0023]In case the adsorption unit operates at a low temperature, the pass through gas is, before being released to the atmosphere, advantageously used as a coolant in the CPU and / or a cryogenic ASU. In some cases it is also advantageous to use at least a portion of the desorbed gas stream as a coolant of the inlet gas streams of one or more of the CPU and ASU, before it is conducted back to the furnace.

Problems solved by technology

However, application thereof considerably decreases the total efficiency of the combustion system.

Another difficulty is that, in order to minimize contamination of the solvent by impurities, effective measures are necessary for cleaning the exhaust gas, for example, from sulphur and nitrogen oxides.

A drawback in this process is that even if relatively high pressures and low temperatures are used, the uncondensed gas stream may comprise a considerable amount of CO2, which may then be released to the atmosphere together with the vent gases of the ASU.

A drawback in this process is that the uncondensed gas stream may comprise a considerable amount of N2 and Ar, which will be recycled to the furnace.

A drawback of this process is that the CO2 stream released from the TSA unit may comprise a considerable amount of impurities, such as N2 and O2, which will then remain as impurities in the combined liquid CO2 stream.

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