Environmental Protection

McIlvaine: Address CO2 Capture for Power Plants Now

Capturing carbon dioxide and sequestering it underground is an attractive longer term option for greenhouse gas reduction, but decisions regarding it cannot be postponed.

Many of the decisions power plants are making today will affect their carbon dioxide capture costs. Therefore, capture has to be considered an urgent subject for analysis, according to McIlvaine's just published study, "CO2 Decisions."

A coal-fired power plant which co-fires 20 percent biomass and captures 90 percent of the carbon dioxide emissions will be a net reducer of carbon dioxide. If every plant were to achieve this capability, the U.S. power plant emissions of carbon dioxide would drop from 1.7 billion tons/yr to a minus 170 million tons/yr. This combination would achieve more than switching to other renewables such as wind and solar.

Co-firing biomass in coal-fired boilers has a number of advantages over separate dedicated biomass combustors. Co-firing can improve mercury capture and nitrogen oxide reduction in coal-fired boilers. The energy efficiency is higher than with dedicated biomass. It also lends itself to co-generation. An onsite cellulosic ethanol plant can provide the biomass to the coal-fired power plant. The coal-fired power plant provides the waste steam to the ethanol plant. Carbon dioxide capture for dedicated biomass plants would be expensive.

The flue gas entering the carbon dioxide capture system has to be very clean. Utilities are presently engaged in selecting air pollution control equipment to retrofit to existing plants. They need to consider either purchasing more efficient systems initially or designing the retrofit for an inexpensive later upgrade to the purity level needed for carbon dioxide capture.

A new, ultra-super critical coal-fired power plant will create 30 percent less carbon dioxide than an old coal-fired power plant, so upgrading the present plant to the high pressures and temperatures needed for most efficient electricity generation is another consideration.

There are modifications in the heat exchangers, combustion systems, coal handling, and other equipment that will increase efficiency and reduce the amount of carbon dioxide to be captured. Optimization software is another important tool in increasing plant efficiency.

One option is to replace the old coal-fired boiler with a new super critical unit now. The utility can agree to emit 30 percent less carbon dioxide for the first 15 years and then 80 percent less carbon dioxide for the next 15 years. This will be a win-win situation for everyone. The cost of the initial reduction will be essentially zero. The reduced cost of operating a new plant plus the 30 percent fuel savings will offset the depreciation on the new plant.

After 15 years, the plant can add the carbon dioxide capture system or close down the plant and replace it with a wind turbine or solar generator. This becomes a practical and low-cost option compared to various other plans that rely on wind and solar prior to the time they can be delivered in the quantity and at the cost required.

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