C02 scrubbing

Figure 11.6 Basic idea of pre-combustion C02 capture based on the ICCC process.

Flue gas: H20

Figure 11.6 Basic idea of pre-combustion C02 capture based on the ICCC process.

The fuel gas mixture now consists of C02 and H2, with the hydrogen stemming from gasification and the CO shift reaction. In a second process step, the C02 is now washed out of the fuel gas. Pure hydrogen finally reaches the gas turbine, so that the waste gas no longer contains C02.

Because the C02 is captured before (complete) combustion, this technology is also referred to as pre-combustion (Figure 11.6).

11.3.3.2 Technological Implementation

The IGCC process without C02 capture was implemented in the 1990s in four demonstration plants (Buggenum/Netherlands/250MW, Wabash River/ USA/250 MW, Polk/USA/250 MW, Puertolano/Spain/300 MW). After that time, a series of projects was started, only one of which was ultimately implemented. For these projects, most of which were launched in the USA, the IGCC process was an interesting option, because it mitigates air pollutant emissions. What is more, in the USA, unlike Europe, conventional steam power plant engineering was not so far developed, the hope in the USA being that efficiency advantages could be obtained with IGCC. However, IGCC is much more complex and has higher investment costs, so that it has not been able to gain traction to date, although the picture is changing thanks to the new carbon capture remit. Carbon capture in the IGCC involves lower additional outlays than in the case of the other two concepts, so that, proceeding from the higher outlays for the basic power plant process, IGCC with carbon capture overall is a competitive and promising proposition.

For the gasification of coal with oxygen, entrained- flow gasifiers are usually envisaged. The gasification temperature is about 1500 °C. For lignite, with its higher share of volatile matter compared to hard coal, too, a gasifier was developed on the basis of fluidized- bed technology which works at lower temperatures of below 1000 °C. Gasification is under pressure, with the pressure in the IGCC process being determined by the inlet pressure of the gas turbine. Depending on the gas turbine, the pressure can be in excess of 40 bar. Converting the coal into a fuel gas or synthesis gas in autothermal gasification is on the basis of numerous equilibrium reactions running in parallel. The most important are (Scheme 11.5 ):

Scheme 11.5 Important equilibrium reactions in gasification; (a) Boudouard equilibrium; (b) Water gas reaction; (c) Shift reaction.

The energy necessary for the gasification process is partially added by combusting the coal.

Downstream of the gasifier, the raw gas must be cooled, so that the subsequent cleaning and conversion steps can be implemented. The greatest efficiency is achieved if the heat of the raw gas is used in a heat exchanger to generate steam which produces electricity in a turbine. Such a heat exchanger is prone to fouling, however, since the raw gas carries with it the ash of the coal which is sticky or liquid at high temperatures. One alternative for avoiding the fouling problems is quenching with water, although this involves losing much of the exergy in the raw gas. Quenching in the IGCC with carbon capture has one more merit, however: the CO shift reaction necessary for carbon capture needs water vapor as reaction partner, and this must be added to the raw gas. With the quenching action, this step is performed simultaneously.

Once the temperature is lowered, the process steps of raw gas treatment follow. These comprise desulfurization, CO shift reaction and CO2 scrubbing. In gasification, H2S emerges instead of the SO2/SO3 that forms during combustion in conventional steam power plants. For the sequence of the process steps, there are two alternatives, each having a different impact on energy requirements and on the efficiency of carbon capture. One factor is that H2S and CO2 scrubbing are marked by low temperatures, which-depending on the scrubbing solution deployed-can be well below 0 °C, whereas the CO shift reaction runs at a temperature of about 300 °C. If the CO shift reactor is located downstream of the H2S scrubbing system (sweet shift), several temperature leaps must be performed, which entails exergy losses. However, there is also the option of locating the CO2 shift reactor upstream of the H2S scrubbing system (sour shift), so that the ups and downs of the temperature profile in the process are avoided. One drawback, however, is that the choice of catalyst is restricted for the CO shift where H2S is present. Accordingly, converting CO into H2 and CO2 is less complete, so that some of the CO remains in the fuel gas and is emitted as CO2 after combustion in the gas turbine. Consequently, the degree of CO2 capture is lower.

The CO2 in the IGCC is ultimately separated using a scrubber. In this respect, there are fundamental differences compared with post- combustion CO2 scrubbing. In the IGCC, the fuel gas is under pressure. Moreover, the CO2 concentration in the fuel gas is much higher than in the flue gas downstream of a conventional steam power plant. Under such marginal conditions, physical scrubbing processes can be used that get along with less energy. The most common scrubbing substances are methanol (Rectisol), dimethyl ether polyethylene glycol (Selexol) and N-methyl-2-pyrrolidone (Purisol).

A fuel gas finally reaches the gas turbine that consists of virtually pure hydrogen. Hydrogen has substantially different combustion properties than the natural gas which is usually input in gas turbines:

• hydrogen spreads flames eight times faster;

• the stochiometric combustion temperature is 150 °C higher;

• the volumetric flow is three times greater, with the same energy flow.

These properties necessitate various measures in the combustion area. Hydrogen is diluted with nitrogen, which is generated from air separation parallel to oxygen extraction. Diffusion burners are used, since the combustion process is not at present completely controlled with the modern pre-mix burners. However, diffusion burners produce too much NOx, so that a denox system must be installed downstream of the gas turbine.

Basically, the principle of pre-combustion CO2 capture can be used for natural gas as well. Here, the gas must be re-formed. However, the conversion into CO and H2 leads to a high exergy loss. This being so, post-combustion CO2 scrubbing is more suitable for gas-based power plants.

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