Method of and system for generating power by oxyfuel combustion

Abstract

A method of generating power by oxyfuel combustion. Carbonaceous fuel and oxidant gas are fed into a furnace. In a first operating mode, the oxidant gas includes a stream of substantially pure oxygen conveyed from an oxygen supply for combusting the fuel with the oxygen to produce exhaust gas including mainly carbon dioxide and water. The exhaust gas is discharged from the furnace and is divided into a recycling portion and an end portion. The recycling portion is recycled to the furnace. Heat is transferred from the end portion to the stream of substantially pure oxygen by circulating a liquid heat transfer medium in a passage between an exhaust gas cooler and an oxygen heater.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a method of and a system for generating power by oxyfuel combustion. The invention especially relates to a flexi-burn or dual-firing combustion system, i.e., a system which can be easily switched between the modes of oxyfuel combustion and combustion with air.[0003]2. Description of the Related Art[0004]Oxyfuel combustion is one of the methods suggested for removing CO2 from the combustion gases of a power generating boiler, such as a pulverized coal (PC) boiler or a circulating fluidized bed (CFB) boiler. Oxyfuel combustion is based on combusting carbonaceous fuel with substantially pure oxygen, typically of about 95% purity, so as to have carbon dioxide and water as the main components of the exhaust gas discharged from the boiler. Thereby, the carbon dioxide can be captured relatively easily, without having to separate it from a gas stream having nitrogen as its main component, such a...

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Benefits of technology

[0013]The process of transferring heat from the exhaust gas to the oxidant gas improves the efficiency of the boiler and the process as a whole. The present invention differs from the prior art solution shown in U.S. Pat. No. 6,202,574, for example, in that the pure oxygen stream is heated by low grade heat obtained from the end portion of the exhaust gas, which is to be discharged from the system, and not from the exhaust gas upstream of the take off point of the recycled gas. Thereby, the thermal efficiency of the process is improved. While the exhaust gas cooler is at the low temperature portion of the exhaust gas channel, it may cool below the acid condensation temperature during the heat transfer. Therefore, the exhaust gas cooler is advantageously of a corrosion resistant type, such as a plastic gas cooler.

[0025]The method advantageously comprises, in the second operating mode, a step of transferring heat from the exhaust gas to the stream of air by circulating a liquid heat transfer medium in a passage between an exhaust gas cooler and an air heater. Because the exhaust gas flow rate is, in the second operating mode, typically much higher than the flow rate of the end portion of the exhaust gas in the first operating mode, advantageously, at least one other channel with an exhaust gas cooler may be formed parallel to the exhaust gas channel portion comprising the exhaust gas cooler used in the oxycombustion mode. Thereby, the heat transfer medium is circulated in the air-firing mode between the air heater and at least two parallel exhaust gas coolers. Thus, the air stream, which has a clearly higher flow rate than does the oxygen stream in the oxycombustion mode, can be efficiently heated by the exhaust gas.

Problems solved by technology

Generating power by oxyfuel combustion is more complicated than conventional combustion by air, because of the need of an oxygen supply, for example, a cryogenic or membrane based air separation unit (ASU), where oxygen is separated from other components of air.

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