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Pre-combustion

Pre-combustion CO2 capture refers to the removal of the carbon from a fuel before combustion, so that the combustion generates no CO2. In hydro-carbon fuels (such as fossil fuels), carbon can be separated from hydrogen as CO2, and the hydrogen can then be used directly as a fuel in a boiler, furnace or turbine. Burning hydrogen emits no CO2 and the primary exhaust gas from hydrogen combustion is water.

Several technologies for producing hydrogen from fossil fuels are commercially available. In principle, hydro-carbon reformation to hydrogen and CO2 can produce a pure stream of CO2 and hydrogen for storage and combustion respectively. In practice, further purification of the CO2 stream may be necessary before storage, depending on the fuel, the reforming process and the separation process used.

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Producing hydrogen from natural gas

A steam methane reformer in Quebec, Canada (Source: GCM Consultants)

Steam reforming of methane (natural gas) or light hydro-carbons is the dominant technology for producing hydrogen today. The production of hydrogen happens in two separate steps. First, desulfurized natural gas is reformed to syngas, a mixture of hydrogen and carbon monoxide (CO). The syngas is passed into a shift reactor, where the CO reacts with steam to form CO2 and more hydrogen.

In the reforming stage, the fuel is mixed with steam and heated to 800-900°C over a catalyst (nickel). In industrial reforming, heat for this process is generated by burning some of the fuel, which produces a small separate CO2 emission source. This may be eliminated by using replacing the fuel with electricity or hydrogen. The reaction is summarized by:

CH4(methane) + H2O(steam) → CO + 3 H2 (syngas)

The water gas shift reaction takes place in one or two phases, reacting the syngas with steam over a catalyst (iron-chromium or copper), to form CO2 and more hydrogen. In a typical two phase shift reactor, the first stage is a high temperature shift at 400 to 550°C, followed by a low temperature shift at 180 to 350°C. The shift reaction can be summarized by:

CO + H2O → CO2 + H2

The resulting CO2/H2 mixture will have a CO2 concentration of 15-60%. The gases are separated, producing a pure hydrogen stream that can be used to fuel a power plant, furnace or can be used as a transport fuel or for other purposes. The CO2 stream can be compressed and transported to a storage site.

In principle, hydro-carbon reformation to hydrogen and CO2 can produce a pure stream of CO2 and hydrogen for storage and combustion respectively. In practice, the CO2 stream will be contaminated to some degree by unreformed hydro-carbons and carbon monoxide, and contaminants from the fuel and from air that enters into the process stream, such as nitrogen. Depending on the fuel, the exact reformation and separation processes in use and the specific requirements of stream purity for transport and storage, extra steps may be necessary to further purify the CO2 stream. However, as the waste gas will have a very high CO2 concentration and comparatively high pressure, separation will be far more energy efficient than in typical post-combustion setups.

Gasification of coal, oil or biomass

A process similar to the steam methane reforming reaction can be used to produce hydrogen from coal, oil and biomass. The process begins with the gasification of the fuel. This creates syngas, which passes into a gas water shift reactor, and from this point and the following steps are identical to the methane reformation process.

Non-gaseous fuels can be reformed to syngas by reaction with oxygen and steam at high temperature (up to 1350°C) and pressure. The oxygen can be supplied either as air or as pure oxygen. The latter is preferable for CO2 capture, as this generates a higher CO2 partial pressure. All the heat required in the process can usually be supplied by the partial combustion of the fuel, so no external heat is needed.

When O2 is used as the oxidizer, energy is consumed in the separation of oxygen from air. However, the absence of nitrogen from the air in the syngas greatly reduces the cost of separation.

Using hydrogen as fuel in power plants

Illustration of a pre-combustion CO2 capture plant (Source: Vattenfall)

As hydrogen burns at a higher temperatures than methane and other hydro-carbons, some plant modifications may be necessary to use hydrogen as a fuel in a power plant. The higher temperature of combustion increases the level of NOx produced in generation process, which is highly undesirable. Also, temperatures may be so high as to cause melting or deforming steel. Turbine blades are particularly vulnerable to extreme temperatures.

To avoid this, hydrogen is mixed with nitrogen or steam before combustion to reduce hydrogen concentration. When O2 is used as the oxidizer in a gasification process, the nitrogen that is separated to produce oxygen may be added to the hydrogen after separation. Turbine modifications may also be necessary to accommodate the difference in density of the hydrogen combustion gas mix compared to a normal methane/air mix.

IGCC

Schematic of an IGCC plant (Source: Siemens)

Integrated gasification combined cycle (IGCC) power plants are a relatively new coal power plant design where the coal is gasified before combustion. This design has several advantages over ordinary pulverized coal power plants. Most significantly, the gasification of coal makes it possible to use the a gas turbine/steam turbine combined cycle which is considerably more efficient than the single steam cycle available in pulverized coal designs.

In exsisting IGCC designs, coal is reformed to syngas in a gasification process, and the syngas is used directly in a gas turbine. As carbon is removed from the fuel as CO2 in the gasification process, CO2 emissions from the power generation are considerably lower than with firing coal directly. However, in the IGCC plants in use today, the generated CO2 is released into the atmosphere, thus achieving no emission reduction.

As CO2 is emitted in an almost pure stream, capturing this emission is relatively easy and efficient, even when capture is retro-fitted onto existing plants.

Exsisting IGCC plants are not full pre-combustion designs, as they are fired with syngas which produces the same emissions as combustion of natural gas. Future plants may be fitted with a water gas shift reactor and hydrogen separation and be designed to use hydrogen as fuel in stead of syngas.