July 15, 2008

Options for reducing a coal-fired plant’s carbon footprint, Part II

Dr. Justin Zachary, Bechtel Power Corp.

Renewable options abound

Renewable power technologies have matured and are capable of replacing significant amounts of oil-, gas-, and coal-fired generation. In particular, wind energy has become the clear leader (ignoring existing hydroelectric projects) of large-scale renewable power generation. There’s now more than 20 GW installed capability, 5,224 MW of that brought on-line in 2007 alone. Solar photovoltaic, and in particular concentrated solar thermal (tower and trough) technologies, are being deployed in “utility-scale” projects in the 100- MW to 400-MW range.

It’s only natural to consider renewable resources either in stand-alone or hybrid configuration as another CO₂ reduction technology. The dramatic impact of how much CO₂ can be displaced by using renewables is illustrated in Table 3. For each MW of solar or wind power used to offset new fossil-fired generation, approximately 1 ton per hour of CO₂ emissions is avoided.

Table 3. CO2 emissions of various energy conversion processes. Source: Bechtel Power Corp.

One particular form of renewable energy technology integrated with a convention power system is the hybrid plant. In a typical hybrid configuration, the steam generated by a solar power tower is supplied to a nearby coal-fired facility, rather than being used in a dedicated steam turbine to produce electricity. Such arrangements are particularly suitable for states where the solar energy is abundant and space is available. A modern Solar Tower, such as the SEGS plants built in the California desert in the 1980s, can produce steam at 2,400 psia and 1,000F conditions. In addition to the CO₂ credit for the solar heat input, significant cost savings can be obtained in a hybrid arrangement. A stand-alone solar plant will require its own steam turbine and heat sink, whereas in the hybrid case these components are part of the main coal-fired facility (Figure 3).

3. Hybrid solar and coal-fired plant schematic. A hybrid plant, where 150 MW thermal (10% of the total plant heat input) is provided by a solar trough system is illustrated. The superheated steam at 865 psia and 650F is fed back into the cold reheat line of the coal-fired plant. Source: Bechtel Power Corp.

No silver bullet

A variety of alternatives for reducing CO₂ emissions from power plants have been described. A plausible scenario for achieving future CO₂ emissions reductions, or to demonstrate to permitting authorities that meaningful provisions are designed into the plant, may require a developer to implement several alternatives in concert.

For example, Figure 4 shows where the emissions of a coal-fired plant must be reduced from 2,071 lb CO₂/MWh to the level of a natural gas-fired combined-cycle plant (1,200 lb CO₂/MWh). In this case, from the available “tool box,” the following alternatives have being selected:

• Future CO₂ capture for 20% of the flue gases stream. Cofiring with 10% biomass.
• Cycle efficiency improvement by 2 percentage points.
• Development of an on-site 100-MW wind farm.
• Buying CO₂ credits at $20/CO₂ ton.

4. Team approach to gas control. Here is how one set of options can reduce coal-fired plant CO₂ emissions to the levels of a gas-fired combined-cycle plant. Source: Bechtel Power Corp.

Depending on the site conditions and the plant economics, different combinations can be created. The main point of the selection process is to choose alternatives that are considered “proven demonstrated technologies.” Because implementation of a CO₂ reduction plan will occur after a plant is complete, the second consideration in the selection process is to identify options that are the least intrusive to the plant in terms of additional equipment, control, and the balance-of-plant’s ability to support the new additions.

—Dr. Justin Zachary (jzachary@bechtel.com) is senior principal engineer for Bechtel Power Corp., an ASME fellow, and a POWER contributing editor.