Clean Energy Systems, Inc. (CES) is developing a new fossil fuel-based "zero-emission power plant" (ZEPP) that integrates aerospace technology and conventional power systems. The ZEPP technology involves replacing conventional power plant steam boilers and flue gas cleaning equipment with a gas generator, adapted from rocket engines. If successful, the ZEPP will be highly efficient, will emit virtually no air pollutants, and will be capable of effectively capturing and sequestering carbon dioxide (CO2) at a cost comparable to or below that of today's best combined-cycle systems without CO2 capture. Although CES has made progress, the technology still faces considerable challenges.

In a conventional fossil fuel-fired steam-electric generating unit, air and fuel are combusted in a boiler. Water circulating in a network of tubes in the boiler is transformed into steam by the heat generated by the combustion process. The high temperature, high pressure steam powers a turbine that drives a generator to produce electricity. The steam is then cooled and converted to liquid form in a condenser, where it is transported back to the boiler for re-use.

In units that control for air pollution, exhaust gas is treated either during or after the combustion process to limit pollutants such as particulate matter (PM), sulfur dioxide (SO2) and nitrogen oxides (NOx). In conventional pulverized coal-fired power plants (coal has the highest carbon content of the fossil fuels), carbon dioxide (CO2) concentrations in the flue gas range from three to 15 percent by volume. Due to the low concentrations of CO2 and huge volume of flue gas in a conventional plant, it is very difficult and costly to separate and capture CO2.

A conventional steam boiler typically operates at temperatures ranging from 1,000 to 1,100 degrees Fahrenheit (F), which caps efficiencies at about 45 percent. In fact, most existing boilers are significantly less efficient: the average efficiency of the existing coal-fired electric generating fleet in the U.S. is about 35 percent. This means that only 35 percent of the heat of combustion is transformed into electricity, with the remaining 65 percent going up the stack (typically ten percent) or being discharged with the condenser cooling water (typically 55 percent).

In contrast to a boiler in a conventional power plant that uses air in the combustion process, the gas generator developed by CES burns a combination of a gaseous hydrocarbon fuel and oxygen (as opposed to air) to produce a mixed gas of steam and carbon dioxide at high temperature and pressure. The use of oxygen to combust the fuel is referred to as "oxy-fuel" combustion. Any gaseous fuel composed predominantly of carbon, hydrogen and oxygen can be combusted in the gas generator; suitable fuels include natural gas, landfill gas, or syngas from coal, refinery residues or biomass. The combustion products power a steam turbine that drives a generator to produce electricity. From the turbine, the exhaust gases enter a condenser/separator where they are cooled and separated into their components, water and CO2. A simplified schematic of the CES process is shown in Figure 1.

The use of oxygen rather than air as a combustion agent eliminates the presence of nitrogen in the flue gas, avoiding the formation of NOx and the large volume of exhaust gas typical of a conventional power plant boiler. For this reason, an oxy-fuel combustion unit is only a small fraction of the size of a conventional boiler.

Oxy-fuel combustion also results in greater combustion efficiency. The steam generator is capable of producing mixed steam and CO2 at discharge pressures of 1,500 pounds per square inch (psi) and temperatures of up to 3200 degrees F. At these temperatures, efficiencies between 50 and 60 percent are possible with nearly 100 percent CO2 capture. This is comparable to the efficiency of a combined-cycle plant, which does not capture carbon. In addition, much higher CO2 concentrations are produced with oxy-fuel combustion, allowing the gas to be separated and captured more readily than would be possible in a conventional fossil fuel-fired steam boiler.

Although it still faces significant challenges, as we discuss below, every component in the ZEPP process, with the exception of the gas generator and reheater, is commercially proven and standard in power generation or other industries. The gas generator technology, although it has not been used in the electric power sector, has been used successfully in aerospace applications for decades, including in the space shuttle main engines, where hydrogen and oxygen are combusted to produce steam at high temperature (1500 degrees F) and pressure (5000 psi). Likewise, high temperature, high pressure turbines have been used successfully for many years in aerospace applications. And the Department of Energy's National Energy Technology Laboratory (DOE-NETL) has adapted gas turbine combustor design concepts to produce a direct-fired, oxy-fuel reheater that the ZEPP can employ.

In September 2000, DOE-NETL awarded CES funds to design, fabricate and test a ten MW gas generator to operate on oxygen, methane and water. As part of this program, CES tested an industrial-scale gas generator in 2002 and 2003. The goals of the program were to demonstrate a non-polluting gas generator at temperatures up to 3000o F at 1500 psi, with the resulting gas composition, consisting of steam and CO2, substantially free of contaminants. The project successfully met its goals.

In early 2002, the California Energy Commission awarded CES two million dollars, which it subsequently increased to four million dollars, to build and operate a natural gas-fired zero-emission demonstration power plant using CES's gas generator technology. Mirant and Air Liquide are participants in the project. The two-year power plant demonstration at a nominal five MW level began early this year at the Kimberlina power plant site near Bakersfield, California. The CES gas generator powering the Kimberlina ZEPP has logged 350 operating hours as of mid-July 2005 and has been exporting power to the grid since mid-March. Since Kimberlina is less than five miles from existing oil and gas fields, it provides a good opportunity to test and study carbon sequestration, both via injection into deep underground saline formations and through CO2 flooding for enhanced oil or gas recovery.

CES hopes that the experience obtained at Kimberlina will pave the way for "first-generation" ZEPPs of modest size (20 to 70 MW), based on currently available steam turbine technology, operating on natural gas, and located where the absence of emissions (CO2 capture and no air pollutants) will have sufficient value to offset the inevitably relatively low efficiencies of the first generation plants. CES has been actively seeking projects that fit these criteria.

CES is pursuing three such projects in Holland, Norway and California. Holland has legislation that provides subsidies up to several eurocents/kWh for electricity from climate-neutral sources, including zero-emission combustion systems. Norway imposes a relatively high tax on CO2 emissions. The U.S may see a demand for ZEPPs as well, particularly in states that regulate CO2 or in other situations in which the captured CO2 has value and/or clean power has added value for any of a number of reasons. Several areas in California fill that bill. The most advanced CES ZEPP project in California involves placement of a facility in an oil field for enhanced oil recovery.

In March 2005, CES entered into an agreement with a Dutch energy development company for engineering services for a 50 MW ZEPP. The agreement calls for CES to prepare a preliminary design and plant proposal for a nominal 50 MW ZEPP to be built in the Netherlands.

CES hopes that these projects will set the stage for "second-generation" ZEPPs within the decade, that will employ oxy-fueled reheaters and intermediate pressure turbines based on gas turbine technology. CES expects that these will exhibit much improved cycle efficiencies and produce electricity at significantly lower cost.

Finally, CES anticipates commercial large scale (400 to 1000 MW) ZEPPs fueled by syngas from the gasification of coal within ten to 20 years. Its vision is that these plants would be emissions-free and would produce power at costs comparable to or below that of today's best combined-cycle systems.

One of the major advantages of the ZEPP technology is that it is very clean from an environmental standpoint. The process avoids creating pollutants through the use of clean gas fuel (e.g., syngas from coal) and a clean oxidizer (oxygen) that are separated from unwanted atmospheric elements such as nitrogen. This avoids the formation of atmospheric air pollutants such as SO2 and NOx. Proper choice of the fuel/oxygen mixture ratio, complete mixing, and careful control of operating conditions (temperature, pressure and residence time) also virtually eliminate carbon-containing pollutants such as CO, VOCs and particulates.

As with Integrated Gasification Combined Cycle (IGCC) technology, CO2 can be separated and collected relatively easily and cost-effectively. CO2 capture systems, such as amine-based scrubbers, are available for conventional fossil fuel-fired power plants, but these systems are extremely expensive to install and operate. In conventional plants, as much as 35 percent of the electrical output is required to separate, collect and sequester CO2, whereas CES anticipates its sequestration process requiring approximately three to five percent of the electricity produced by the plant. Depending on the size of the plant, CES expects that its technology will permit essentially 100 percent CO2 separation and capture at an estimated cost of nine dollars per metric ton, compared to 32 dollars per metric ton for combined-cycle plants. (In neither case do these costs include the transport costs from the generating source to the sequestration site.)

Additionally, the CES power cycle does not consume water and does not produce any effluents. In fact, it is a net producer of water when air-cooled condensers are used. Finally, the technology has a very small footprint compared to conventional boiler technology.

Significant advances in steam turbine technology will need to occur before all of the efficiency-related benefits of the ZEPP technology can be realized. There are no commercially available steam turbines that can withstand the high exhaust temperature and pressure produced by the CES gas generator. (Historically, there has been no demand for advanced steam turbines capable of efficiently using temperatures and pressures of this magnitude, since there were no boilers capable of delivering such energy.) Pairing the CES gas generator with currently available steam turbines would result in sub-optimal performance and efficiency (about 32 percent for coal-fired systems). This translates into relatively high cost. CES estimates that its first generation ZEPP, which will be fueled by natural gas, will be able to produce electricity at a cost of approximately $0.05 to $0.06/kWh. This exceeds the cost of conventional fossil fuel-fired steam electric generating technology, but is comparable to or below the cost of a number of other clean energy sources.

However, supported by funds from DOE, major turbine companies are planning to develop advanced turbines based on proven gas turbine technology. These turbines, expected to be commercialized in the next five to ten years, are intended to operate at steam conditions approaching 1,200 degrees F and 1,200 psi (high pressure turbine) and 2,200 degrees F and 170 psi (intermediate turbine with reheater). Second generation ZEPPs using these advanced turbines are expected to be fueled by natural gas and to operate at efficiencies of approximately 55 percent-similar to today's combined cycle plants without carbon capture and better than combined cycle plants with carbon capture. Predictions are that the cost of operating gas-fired second generation ZEPPs will be on the order of $0.04/kWh, which is comparable to IGCC power plants without CO2 recovery.

Another problem is the fact that oxy-combustion processes require pure oxygen produced, for example, from cryogenic (low temperature) air separation units. There is a four to ten percent energy penalty associated with cryogenic air separation, depending on various factors. To address this problem, DOE-NETL is investigating two advanced oxy-combustion pathways. One is an oxygen transport membrane combustion process, which is expected to reduce the energy required to separate oxygen from air and to reduce energy requirements and capital costs of oxygen production by 30 percent or more. In another effort, DOE-NETL is investigating techniques to improve combustion and gasification processes and thereby increase power plant efficiency and decrease CO2 separation costs.

Of course, none of this addresses the other work in progress: once CO2 is separated and captured, the effort to assure its safe and cost effective long term sequestration.