TIPS uses nucleate condensation to create droplets from exhaust streams that capture particulates and other pollutants such as acid compounds and mercury. These droplets are mostly liquid CO2 created by a high-pressure phase transition from gaseous to liquid states during combustion. These high pressures are achieved by feeding pressurized oxygen into the coal combustion chamber instead of ordinary air at atmospheric pressures. Developed by the ThermoEnergy Corp., of Little Rock, AR, TIPS can be built into new power plants or retrofitted into existing plants. TIPS can play a key role in reducing greenhouse gas emissions while improving the energy efficiency of coal combustion power plants.
Standard Clean Coal Combustion Technology
Contrary to its public image, coal is a much cleaner source of energy than it was in the past. New and improved technologies have greatly reduced the emissions produced per ton of burning coal. The term “clean coal” applies generically to a range of technologies designed to greatly reduce the emissions from coal-burning power plants. It can include coal that has been prewashed chemically of minerals, heavy metals, and other impurities prior to burning (especially pyretic sulfur which is removed by float/sink separation). It also includes the treatment of exhaust gases with steam to make the CO2 in the flue gas easier to recover and sequester.
Born of the Clean Coal Initiative of the 1980s, today’s clean coal technology includes the now standard fluidized bed combustion (FBC) technique in both its bubbling, circulating, and pressurized styles. In general, FBC involves the suspension of coal particles with upward blowing jets of air during incineration. The resulting mix of fuel and air results in greater burn efficiencies and heat generation. But for all its advantages, FBC still represents only 2% of the world’s total coal combustion. All FBC technologies utilize the injection of powdered limestone into pulverized coal particles that removes up to 90% of sulfur emissions and moderately reduces nitrous oxide emissions.
Bubbling FBC operates at atmospheric pressures and operates a steam turbine only. Sorbent chemicals (limestone is typically used) are injected into the turbulent mix to facilitate removal of sulfur dioxide. The bubbling process is useful for combustion of high-ash coals burning at temperatures between 800°C to 900°C, operating boilers with capacities as large as 25 MW. Using relatively slow blower velocities and a heavy sand to provide a definable surface, the improved bed stability allows for superior sorbent interaction with SO2. Heat exchange for producing the steam that turns the turbines occurs in tubes buried in the bedding.
Circulating FBC uses high velocity air streams. This results in the absence of a defined bed as the particles are permanently suspended in the flue gases. Flue gases and unburned particles pass through a cyclone where the larger particles drop out for return to the bed. This can happen dozens of times until complete incineration is accomplished. This method gets the most energy out of low-grade, high-ash coal. Direct injection of limestone into the bed dispenses with the need for a secondary desulphurization chamber.
Circulating FBC is used to operate boilers with capacities in the 230 to 300 MW range.
Similar to the atmospheric FBC technologies described above, pressurized FBC confines the bed in a chamber where pressures can exceed 16 times atmospheric pressure. This allows for a 95% reduction in sulfur dioxide emissions as well as further reductions in nitrous oxide. The coal is fired with a dolomite limestone solution to remove impurities from the flue gases. Operating temperatures run as high as 900°C. Heat transfer to transform water into steam occurs in deep bed heat exchangers. Unlike other FBC technologies, a pressurize combustion chamber allows for the efficient use of all kinds of fuel, including biomass, at much higher efficiencies. However, some pressurized FBC facilities generate large amounts of NO2.
Coal Combustion Emissions
Even the cleanest coal burning technology of today produces some emissions. The first class of emission is particulates. Primarily, particulates are the ash and soot from coal combustion. Excessive articulates can result in odor, visibility, dust and respiratory problems. Also found in particulates are incombustible mineral matter. Fortunately, mature and widely used technologies are available to remove particulates from coal emissions. These include electrostatic precipitators and filter fabrics. Precipitators use an electric fields generated across collection plates to create a charge on the particulates. The particles are than attracted to the plates and removed from the emissions stream. Fabric filters, installed in bag houses constructed in the smoke stacks, use textiles with small opening sizes to physically catch particulates. Either technology is capable of removal rates as high as 99.5%.
Next in importance and more difficult to remove are the trace elements found in coal. These include toxic elements such as mercury, selenium, and arsenic. Also found in coal are radioactive thorium and uranium. Fluidized bed combustion in itself goes a long way to remove trace elements from coal emissions. Activated carbon silos have been successfully used to remove 90% of mercury from coal emissions. Sorbent injection of activated carbon into the flue gas has also shown success. The addition of hydrated lime can reduce selenium emissions.
Oxides of nitrogen (especially NO2) are not found in coal but are formed during the combustion process. These emissions are carried up the flue and into the atmosphere, contributing to such problems as acid rain, smog, and elevated ozone levels. NOx is also a significant greenhouse gas. Over 90% of NOx emissions can be removed using specialized burners and catalysts. Ammonia vapor is used as a reducing agent in the catalytic reduction process, producing harmless nitrogen and water vapor. While these technologies have been fully developed, they have not found extensive use outside of the developed countries.
Sulfur is present in many coals and can combine with oxygen during combustion to create sulfur dioxides. Their presence in the atmosphere can lead to the formation of acid rains and acidic clouds. As with NOx, SOx emission reductions as high as 90% can be achieved with existing technology. Again, fluidized bed combustion greatly eliminated sulfur emissions. Wet lime/stone gypsum method utilizes water mixed lime injected into an absorption tower to react with SO2 and produce gypsum. The gypsum is then removed for disposal or reuse as construction materials such as drywall.
Most notorious and hardest to eliminate of the coal combustion emissions is CO2. Critics point to coal combustion as potentially the greatest source of climate change. Improved combustion efficiencies have significantly reduced CO2 emissions. New zero emissions technologies being tested and developed allow for sequestering of CO2 for storage or industrial reuse.
While 90% to 95% removal rates are impressive, greatly expanding our reliance on coal as an energy source increases the need for technologies that effectively remove 100% of emission from coal combustion. This is where ThermoEnergy technology holds such promise. ThermoEnergy is not limited to the burning of coal. It can utilize coal, natural gas, oil, and various types of biomass as fuel stock. Though it operates on the slightly modified Rankine Cycle used by all steam engines, by altering the combustion process it can convert CO2 emissions into liquids for easy capture and removal from the exhaust.
The Rankine cycle is a four stage process. First, water is pumped into a boiler is pumped into a boiler under increased pressure at a constant volume. Second, an external heat source raises the temperature of the water in the boiler and flashes it into steam under constant pressure and expanding volume. Third, the steam continues to expand (at decreasing pressure and increasing volume) as it turns a turbine which generates electricity. Fourth, the steam passes through a condenser where heat is removed, temperature drops and the steam reverts back to water vapor at constant pressure and decreasing volume. These four parts (pump, boiler, turbine, and condenser) make up a standard steam engine. Heat is added at the boiler and removed at the condenser. Work is performed by the turbine as is required by the pump. The addition of external heat to the system ensures that more work is done than is required for the engine to function.
The TIPS process modifies the standard Rankine cycle somewhat. It utilizes a unique pressurized oxygen-fuel technique. In the TIPS process, combustion takes place at high pressures (700 to 1,300 psia). Increasing the pressure of combustion shifts the temperature and which water, CO2 emissions and toxic trace elements such as mercury and selenium condense from gas to liquid. Potentially acidic NOx emissions are also removed during the process. The key to the TIPS method of CO2 removal is the air separation unit that removes nitrogen from the air stream prior to combustion of the coal fuel. Combustion occurs in the presence of highly pressurized O2, not ordinary air (which is mostly nitrogen). Pre removal of nitrogen from the air stream prior to combustion prevents the formation of NOx pollutants. Nitrogen removed from the air by the separators is emitted as harmless N2. The result is the removal of NOx emissions in a manner that is simpler and more economical than post combustion removal of NOx pollutants by flue gas scrubbing and amine absorption.
The energy cost of separating the nitrogen from the air and pressuring the remaining oxygen prior to combustion is about 20% of the energy contained in the coal. The capital costs of an air separator unit are less than that of a standard desulfurizing unit. Continued technological improvements in the operation, design and manufacture of air separator units will continue to reduce both operating and capital costs. Furthermore, preremoval of nitrogen from the air eliminates the heat of combustion that is lost in the nitrogen exhaust and the chemical bonding of nitrogen and oxygen. All in all, the economics of a fully developed TIPS process compare favorably with standard flue-gas cleanup systems operating at atmospheric pressures.
Pressurized combustion increases the pressure of the exhaust which changes the phasing of the exhaust constituents. Each of these exhaust elements have a solid, gaseous, and liquid phase which depends on the combination of pressure and temperature acting on the exhaust. These phases are shown on a phase diagram where the boundaries of the three phases meet at a triple point (the triple point of mercury, for example, occurs at negative 38.8344°C, at a pressure of 0.2 mPa). For most substances, the solid phase occurs mostly to the side of the triple point with lower temperatures. Liquids usually occur in regions with both temperatures and pressures higher than the triple point. Gases typically occur at temperatures higher than the triple point and pressures lower than the triple point. This brief description is an oversimplification since phase boundaries tend to be curved and phase regions can extend into adjacent quadrants. Since operating temperatures of coal combustion preclude the existence of the solid forms of these exhaust elements, increasing the pressure will cause the exhaust to phase transition to a liquid state instead of remaining gaseous.
At the high operating pressures used by TIPS, these liquid exhausts can be easily collected by a simple condensing heat exchanger. Potentially acidic gases, mercury, selenium, and (most important) CO2, can be removed at condensed volumes 2,500 to 3,500 smaller than conventional gaseous exhausts. A condensing heat exchanger is an example of a phase change heat exchanger. Boilers are an example of phase change heat exchangers that cause liquid water to transition into gaseous steam by the addition of heat. The condensing heat exchanger, in effect, runs this process in reverse by cooling the exhaust by the removal of heat and transition of gaseous flows to liquids. Some designs of condensing heat exchangers can remove up to 98% of the heat present in flue exhausts. ThermoEnergy engineers have designed several turbine reheat steps utilizing the “waste” heat from the liquid CO2 and the condensing heat exchanger resulting in cost savings from $0.01 per kWh to $0.04 per kWh depending on the size of the unit and the local market for peak power.
Nucleate condensation is an added feature of the TIPS process which efficiently and effectively removes particulates. The cooling and condensing of CO2 and other exhaust gases results in liquid droplets that prefer to adhere to the surfaces of small particulates. This leads to the formation of larger droplets that continue the particulate accumulation process. The biggest droplets, carrying their mass of particulates, merge into streams that are easily removed. Also known as steam hydroscrubbing, the nucleate condensation performs particulate capture and removal at very high efficiencies. So the use of high pressures to generate CO2 exhausts in a liquid form does double duty by removing the mass of particulates.
So what happens to the CO2 liquid that is removed from the exhaust? CO2 sequestering can result in a stockpile of useful industrial materials. When pumped into oil fields, the pressure generated by the CO2 can push out crude oil and enhance oil and gas recovery operation from marginal fields. Similarly, CO2 can be pumped into coal seams to increase the production of coal bed methane. In this way depleted oil and gas reservoirs can get a new life. Aside from other manufacturing uses, unwanted and unneeded CO2 can be pumped into deep, stable saline formations for permanent deposal. New uses for unwanted CO2 are being discovered; including its use as a nutrient for green algae whose oil content is high enough to allow harvesting as a source of clean biodiesel fuel.
Differences and Similarities With Other Technologies
There are two primary competitors with ThermoEnrgy in the clean coal combustion market, Futuregen and IGCC. Both are designed to be integrated CO2 sequestration and hydrogen production facilities. They are also designed to be a zero emissions power plants. The facilities will utilize coal gasification technology with combined cycle electricity generation to produce electricity and hydrogen from the coal. So they are not traditional coal combustion power plants, but coal gasification facilities designed to create a synthetic gas consisting of hydrogen and carbon monoxide. The reaction of this gas will produce additional hydrogen and CO2.
However, since this is a hydrogen and synthetic coal gas production facility and comparison with TIPS would be apples to oranges. For example, TIPS has a simpler operation with fewer units and uses existing sensors, instrumentation and process controls. Its technology is tested and reliable. TIPS can be used to retrofit existing coal burning plants and is suitable for a variety of fuels.
TIPS is a straightforward way to utilize our nation’s vast reserves of coal in a clean, environmentally safe manner.
TIPS can also be combined with coal-gasification power plants to increase their energy efficiency, hydrogen production and exhaust gas clean-up. Not showing up on any spreadsheet, but vital nonetheless, is the part TIPS can in reducing greenhouse gas emissions.
Potential and Promise
Thermoenergy claims that, “The company has ... developed the ThermoEnergy Integrated Power System process, an advanced power plant design that converts fossil fuels and biomass into energy without producing air emissions.” Two major issues affect the future use of America’s vast coal resources: the need to develop an economic method of capturing toxic pollutants (such as mercury) and the cost effective capture (and potential reuse) of CO2. Though still under development, TIPS has the proven potential to achieve both aims. Further improvements to oxygen separation technology (resulting on both lower capital and operating costs) will greatly improve the profitability of high-pressure oxygen fuel technologies. Additional positive economics can be achieved by the resale of toxic and green house pollutants for industrial uses.
The latest federal budget has authorized up to $2.3 million for TIPS research and development. ThermoEnergy and its collaborators are using this grant money to speed up the development of this technology. This development program has three goals: 1) partner with a key industry participant, 2) build a prototype facility, and 3) commercialize the TIPS process. Another grant, totaling $1.5 million, will pay for research and development of a TIPS based retrofit package that will serve as an economical upgrade for existing coal-fired power plants with the goal to convert them to zero air emission facilities. As part of this development program, ThermoEnergy Corp. has committed $310,000 of federal funds to develop compact zero air emission power plants based on the TIPS technology for medium to heavy industry. These units fall in the category of combined heat and power units (CHP). The objective is to allow the economical substitution of natural gas with alternate fuels. The ability to switch fuels gives industry much needed operational flexibility and the ability to take advantage of price drops in different fuels. The cost of natural gas has risen 600% in the past five years, so there is considerable opportunity for cost savings. Any industrial facility located in a USEPA air pollution “nonattainment” zone can only change fuels if the new fuel is as clean as or cleaner than its existing fuel. Given its near zero emissions, TIPS based CHP is the only potential power source in the 50-MW to 100-MW range that will allow for a fuel switch. International markets (those countries adhering to the Kyoto Protocol) will benefit greatly from this technology.
Canada and the United Kingdom have also shown a strong interest in TIPS technology. These two nations are represented by CANMET (the Canadian energy laboratory) and Reaction Systems Engineering (located in Kent, UK) “These two entities represent some of the most respected international experts in the field of advanced power generation systems,” says Dennis Cossey, CEO of ThermoEnergy Corp., “We are both fortunate and pleased to have people of this caliber involved with our project.” The Canadians are no less enthusiastic. “We are very excited to be part of this team and to add our expertise to develop this process which has a high potential to play an important role in the next generation of clean power generation systems,” says Bruce Clements of CANMET.
DANIEL P. DUFFY , PE, is an environmental engineer employed by URS Corp., Akron, OH.
DE - November/December 2006
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