At the heart of every coal plant is the steam generator, which converts the energy stored in coal into high-quality steam needed to drive a steam turbine generator for efficient production of electricity. The steam generator is the most critical component in coal’s energy future because it allows us to unlock the energy value of coal while determining the environmental impact or cost we pay for this value. Improving the steam generator’s environmental performance is the most important thing we can do for ourselves and the environment.
At Foster Wheeler, we have developed a very different steam generator technology called circulating fluidized bed technology (Figure 1). Unlike conventional steam generators, which burn coal in a massive high-temperature flame, CFB technology doesn’t have burners or a flame within its furnace. Instead, it utilizes fluidization technology to mix and circulate the fuel particles with limestone as they burn in a low-temperature combustion process.
1. Foster Wheeler 500-MWe supercritical circulating fluidized bed power plant. Courtesy: Foster Wheeler
The limestone captures the sulfur oxide pollutants as they are formed during the burning process, while the low burning temperature minimizes the formation of nitrogen oxide pollutants.
The fuel and limestone particles are recycled over and over back to the process, which results in high efficiency for fuel burning, capturing pollutants, and transferring the fuel’s heat energy into high-quality steam used to produce power.
The vigorous mixing, long burning time, and low-temperature combustion process allow CFBs to cleanly burn virtually any combustible material—a major advantage over the fuel limitation of conventional combustion processes. Also, unlike conventional steam generators, CFBs capture and control harmful pollutants during the burning process rather than requiring add-on pollution control equipment.
To further improve the CFB’s environmental report card, we have applied highly efficient vertical-tube, supercritical steam technology to our utility scale CFB designs. Supercritical steam technology allows more of the fuel’s energy to be transferred to the steam. This improves power plant efficiency, reducing the amount of fuel needed for electricity production and further reducing air emissions. (View a video describing Foster Wheeler’s CFB technology here.)
CFB technology offers a unique solution to reducing CO2 from the production of electricity because of its ability to burn carbon-neutral fuels like biomass. Biomass is considered carbon neutral because it absorbs and stores carbon from the atmosphere during its growth cycle through natural photosynthesis. When biomass is burned, it releases that carbon back to the atmosphere, resulting in nearly no net carbon emission to the atmosphere.
However, due to the world’s limited and undeveloped biomass supply chain, biomass power plants are practically limited to about 25 to 50 MWe in size. This small scale, coupled with the fuel supply limitation, translates into electricity costing about 20% to 30% more than that from conventional fossil-fueled power plants.
Here, too, the CFB offers a solution. Due to its fuel flexibility, a large-scale (300 MWe or larger) CFB power plant can be built to burn a combination of coal and several types of biomass. This arrangement provides both the environmental benefit of reducing CO2 emissions and the economic benefit of affordable electricity. It is also flexible enough to utilize more biomass when it is available or to fall back on coal when biomass is unavailable to meet consumer demand.
This concept can produce a substantial reduction in CO2 emission over a conventional coal-fired plant. A supercritical 600-MWe CFB plant burning 20% biomass is estimated to produce 32% less CO2 emission than a conventional coal plant (Figure 2). The good news is that this technology is available today and can produce affordable electricity.
2. CO2 air emissions from a 600-MWe Plant. Emissions are in units of million tons per year. Source: Foster Wheeler Research and Development Team
Flexi-Burn Technology for Even More Flexible Solutions
Although a 30% reduction in CO2 emissions is a big step in the right direction, projections show that we need to do much more to significantly reduce the risk of global warming. What makes this task so challenging is that our CO2 emissions are tied to our energy use, and to realize any reduction in CO2, we must first offset all new emissions connected to the growth in our energy use.
To take carbon reduction to the next level, Foster Wheeler is developing Flexi-Burn CFB technology. Flexi-Burn will allow the CFB to produce a CO2-rich flue gas and be part of a practical carbon capture and storage (CCS) solution . Flexi-Burn technology has the potential to reduce coal plant CO2 emissions to the atmosphere by over 90% while minimizing the cost impact and technology risk to consumers.
Flexi-Burn technology uses a mixture of oxygen and recycled CFB flue gas (Figure 3). The flue gas thereby becomes rich in CO2 (containing over 90% CO2 on a dry basis) rather than rich in nitrogen, as when air is used. Because the flue gas is nearly all CO2, Flexi-Burn CFB technology doesn’t need expensive and energy-intensive equipment to remove the CO2 from its flue gas and has the potential to produce carbon-free electricity at a very low cost as compared to other technologies.
3. Flexi-Burn CFB technology diagram. Source: Foster Wheeler
But the challenge goes well beyond capturing CO2 from coal power plants. Transporting and storing CO2 in underground locations, like saline aquifers, is unproven and requires years of study to understand the cost and risks. And no one really knows how much consumers are willing to pay for truly carbon-free electricity. Due to the uncertainties of carbon regulation, the cost and risk of CCS technology, and consumer behavior, it is nearly impossible to predict when wide-scale market acceptance of CCS power plants will occur.
To cope with the market uncertainty from a technology standpoint, Flexi-Burn has been designed for market flexibility. The plant will be able to operate in either economical air mode (as all coal plants operate today), providing affordable and competitive electricity to today’s market, or in CCS mode later—without requiring any significant modification to the plant. This technology gives power producers the flexibility to adjust plant operation in response to a changing and uncertain market for carbon-free electricity and will likely allow faster adoption of CCS technology.
Flexi-Burn also provides investment flexibility by allowing a plant to be built in functional stages. The plant can be built first for full functionality in the economical air mode, allowing power generators to defer the CCS portion of the plant’s capital investment until it is justified by market and policy conditions. This flexibility will allow investment in the power capacity we need to meet our growing demand while giving investors the flexibility they need when facing uncertain carbon markets.
As we are seeing today, the uncertainty surrounding the carbon issue is preventing needed investment in our power generation infrastructure. This hurts not only the consumer but also the environment. Without new, modern power plants coming onto the grid, power generators have no choice but to keep relying on old, inefficient, and polluting power plants to meet consumer demand.
Circulating fluidized bed technology is an important part of the solution to meet the world’s energy needs while conserving natural resources and preserving our environment.
—Robert Giglio and Justin Wehrenberg (firstname.lastname@example.org) are with Foster Wheeler Power Group.