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A range of other low-rank coal-water fuels that are being developed includes a variety from Alaska that can sustain combustion without need for additional oil. Most coal-water fuels will burn cleanly in steady state, external-combustion applications such as boiler fuel. Some coal-water fuels will burn in internal-combustion engines except that the use of such fuel in such engines became problematic. Gas turbine engines that were run on coal-water fuel had carbon deposits form on turbine blades. Similar carbon deposits appeared on the piston rings and valves of diesel engines that were also run on coal-water fuel.
Coal Combustor Technology
Research has been undertaken at the University of Pennsylvania that involves a specialized combustor for coal. The combustor is still under development and promises to solve the carbon deposit problem that results from burning powdered coal in internal-combustion (gas) turbine engines. The combustor can apparently be modified to burn coal-water fuel in gas turbine engines. Gas turbine engines need to rotate at maximum RPM with turbine inlet temperature at maximum in order to operate at peak efficiency.
The coal combustor is being developed for use in high-powered, internal-combustion turbine engines such as those found in thermal power stations. Bladed turbine engines are generally unsuitable for low-powered (under 500-Hp) applications, however, an alternate engine is under development. The "Star Rotor" turbine is positive-displacement Brayton-cycle engine that is based on the layout of a gerotor gear pump and is presently under development at A&M University of Texas. The coal combustor engine could be adapted for use on the Star Rotor engine. The projected performance of this engine would be better suited for commercial road transportation applications.
The positive-displacement turbine of the star rotor enables it to deliver power efficiently over a wide range of engine speed. The positive-displacement compressor can deliver enough air to the combustion chamber over a wide range of engine speed to maintain an optimal turbine inlet temperature. Power output of the star rotor will vary with engine speed. This engine will need to drive through either a multi-speed gearbox (manual, automatic or automated) or electrical transmission in order to deliver optimal performance in commercial transportation. A liquid coal fueled version of the star rotor engine could be used in the commercial transportation industry depending on how (market-driven) fuel prices evolve over the long-term future.
Coal Fuel Economics
If world oil prices remain high (over $50-per barrel) over the long-term future, there may be some economic gain in the development of alternate fuels such as coal-water fuel and engines such as the star rotor. The mass produced cost of coal-water fuel is estimated at near $20 per barrel, however, it has lower heating value (7000 to 9000-BTU/lb) than diesel fuel or gasoline. A long-term, market-driven price differential between these fuels would be the only way by which to develop coal-water fuel for the commercial transportation industry. Such a fuel may be introduced to transportation companies in selected regions where vehicles are used in local services and may be re-fueled at a limited number of stations. Alternate fuel such as coal-water fuel may be used in a range of external-combustion engines that are presently being developed.
Thermo-acoustic Engines
One type of such engine is the "thermo-acoustic" engine that converts heat energy to low-frequency sound waves that in turn drives a linear alternator. These engines could be built in modules of 50-Kw or 100-Kw each and could operate at comparable efficiency to the best diesel engines in present use. That thermo-acoustic engines convert heat to electricity at high efficiency will enable them to recharge onboard batteries that are used for energy storage in a range of hybrid vehicles. These vehicles will include municipal transport hybrid buses, hybrid taxis, hybrid trucks that are used in local service as well as in hybrid railway locomotives that are used as shunters or used to haul commuter trains.
The performance and efficiency of such hybrid vehicles can be improved by the use of ultra-capacitors. These solid-state devices can outperform batteries insofar as rapidly absorbing and discharging large amounts of electrical energy. Their use in battery-powered and in hybrid vehicles can greatly improve acceleration, recover energy during deceleration, reduce overall energy consumption and greatly extend the life expectancy of storage batteries and the engines that recharge them. Electrically powered drag racing cars in the USA are using ultra-capacitors for energy storage.
External-combustion Piston Engine
The Proeschel group of Ohio has developed and patented an annular design of heat exchanger that offers effectiveness levels in the range of 90-percent. This group has incorporated their unique heat exchanger design in experiments that involve a modified Ericsson-cycle, external-combustion piston engine. The coal combustor technology from University of Pennsylvania could be used in such an application to assure clean exhaust emissions.
The Proeschel-modified Ericsson-cycle engine could drive electrical generation equipment in a hybrid vehicle or drive through a multi-speed transmission. It could theoretically approach the thermal efficiency of a diesel engine while burning a coal-water fuel. The power output of the Proeschel engine would be considerably less than that of a gas turbine engine. It seems aimed at a market niche where low-powered gas turbine engines would operate at very low thermal efficiency. The market niche for the Proeschel-modified engine in the commercial transportation industry would be hybrid municipal buses, hybrid taxis and possibly hybrid shunting locomotives.
Limited Intercity Transportation
Relative future fuel prices will determine the acceptance of coal-water fuel as a commercial transportation fuel. If the fuel gains acceptance in several nearby cities as a fuel for local transportation, the infrastructure that would evolve could support a limited amount of intercity transportation. Intercity commercial transportation would require the use of engines that will have higher power output than engines used in local and municipal operation. Road vehicles would use multi-speed automatic (automated) or manual transmissions whereas railway transportation would use electrical transmission systems. The external-combustion Ericsson-cycle engine from the Proeschel group and the internal-combustion version of the Star Rotor engine would be prime candidates for road-based, intercity commercial transportation. The Star Rotor could also be adapted to operate as an external-combustion engine.
External-combustion Turbines
The success of a coal-fueled Star Rotor engine would depend on the long-term performance of the specialized combustor from University of Pennsylvania. As an option, it could be adapted for operation in external combustion engines. A single-shaft Star Rotor could include a part-load combustion fan (with variable-pitch blading) that could supply additional air into the combustion chamber to maintain optimal turbine inlet temperature during part-load operation. Most of the air that would otherwise enter the combustion chamber in an external-combustion version of the Star Rotor engine would be fresh, hot uncombusted exhaust air from the turbine. An external-combustion Star Rotor operating on coal-water fuel could see service in railway traction service as well as in the commercial intercity road transportation industry.
The performance and efficiency of external-combustion engines will depend on the performance of the heat exchangers. The Proeschel annular heat exchanger design can be made from any a variety of several highly specialized, high-temperature alloy steels. The use of high-temperature heat exchangers and recouperators would assure that external-combustion engines would operate at high thermal efficiency. A competing heat exchanger to the Proeschel design would be the rotating Ljungstrom unit. It offers comparable effectiveness, rotates at low speed (3-RPM) and has successfully been used as a recouperator to improve thermal efficiency in gas turbine engines. Kyocera America offers several types of silicon-nitride that can be used for heating elements in a Ljungstrom heat exchanger. One variety of silicon-nitride has a thermal shock resistance of 750-degrees F and a peak operating temperature of 2500-degrees F.
Heating elements made from this version of silicon-nitride could be arranged in series in a rotating Lungstrom heat exchanger so as to spread the thermal shock load over several elements in a Lungstrom heat exchanger and extend the service life of the unit. The combination of high heat capability and high thermal shock tolerance in the heat exchanger could raise thermal efficiency in external-combustion turbine engines. The efficiency of so-equipped external-combustion turbine engines could approach the efficiency levels of internal-combustion turbine engines. The part-load efficiency of the external-combustion engines could further be assured by using a fan to pump additional air into the combustion chamber during part-load operation.
External-combustion turbine engines that are of bladed design or positive-displacement design could operate on coal-water fuel and be used in railway motive service. Smaller versions of the positive-displacement Star Rotor engine and the competing Proeschel-modified Ericsson-cycle engine would be more appropriate for use in road commercial transportation services. The hot exhaust from external-combustion coal-fueled engines could be used to drive bottom-cycle engines to improve efficiency.
Bottom-cycle Engines
The energy source for bottom-cycle engines is typically all or part of the heat that is rejected by a high-powered top-cycle engine. Top-cycle engines that operate on coal-water fuel could return thermal efficiency levels of 20% (combustion at 1400-degrees F) to over 30% (combustion over 2000-degrees F). There are two types of engines that can operate as bottom-cycle engines to external-combustion engines that operate on coal-water fuel. One type of engine would be a battery of thermo-acoustic engines while the other type of engine would be a steam engine. BMW is presently testing a steam engine as a bottom-cycle engine in automotive applications.
A thermo-acoustic engine would convert the exhaust heat to sound waves and then to electricity. Optimal bottom-cycle engine performance would be attained when the commercial vehicle is operating is sustained high-power operation, such as intercity operation. An electric motor would need to be added to the vehicle drivetrain if it is not so equipped. External-combustion turbine engines that operate at 20%-efficiency on coal-water fuel could have exhaust temperatures at 600-degrees F.
A bottom-cycle thermo-acoustic engine could operate at 28%-efficiency on this heat and the overall combined efficiency of the compound system could exceed 40%. A high-temperature, external-combustion turbine engine could operate at 32%-efficiency with an exhaust temperature of over 1000-degrees F. The bottom-cycle thermo-acoustic engine could operate at 35%-efficiency and the compound system could operate at an efficiency of over 50%.
Recent developments in small-scale steam power technology have revolved around the use of super-critical steam where pressure exceeds 3210-psia. The Enginion group of Germany and Cyclone Power in the USA are among the leading companies in the development of super-critical steam engines. In 2002, Enginion installed a single-acting, uniflow steam engine (inlet injectors, exhaust valves) into a Skoda (Volkswagen) automobile. The engine used steam at extreme high pressure (4000-psia) and high temperature (1200-deg F). It delivered the thermal efficiency of a diesel engine (40%).
Steam Bottom-cycle Engine
An external-combustion turbine engine that operates at over 30%-efficiency will have enough heat in the exhaust (1000-deg F) to boil water and raise saturated steam. The use of a conventional steam engine as a bottom-cycle engine could raise the combined thermal efficiency to over 44%. The heat in the exhaust would be sufficient to preheat the water for a super-critical steam engine and the combined efficiency could approach a level of 50%. Super-critical steam engines use coil-monotube boilers and may be better suited as bottom-cycle engines in on-road commercial operation. A super-critical steam engine could also operate as the main engine in a commercial road vehicle that runs on coal-water fuel.
Railway companies may be willing to consider a testing a locomotive that uses an external-combustion turbine that operates on coal-water fuel. They may even be willing to consider using a conventional steam engine as a bottom-cycle engine if the problems that pertain to the operation of such an engine can be resolved. The exhaust heat from the turbine engine may be sufficient to generate saturated steam. A small amount of extra fuel would have to be burnt to convert the saturated steam to superheated steam that can be expanded in a (positive-displacement) steam engine. The overall combined efficiency of a turbine (32%) with steam (20%) could exceed 40%.
Conclusion
Coal-water fuel can be processed from an extensive supply of low-rank (low sulphur content) coal that can be found in Alaska and in Alberta. Coal-water fuel can be used a feedstock for plants that produce synthetic fuel from coal and its byproducts via the Fischer-Tropsch process. The process consumes energy and manpower and ultimately raises the price of the final product. There are several types of external-combustion engines that are under development and that can operate efficiently on coal-water fuel.
They may be able to do so at lower cost and at comparable efficiency to internal-combustion engines that will operate on synthetic fuel that was processed from coal-water fuel. There may be cost-savings and efficiency gains to be realized from burning coal-water fuel directly in external-combustion engines. The North American trucking industry has suffered economically due to the escalating cost of (imported) diesel fuel. If world oil prices remain high over the long-term future, a truck equipped with an external-combustion engine operating on coal-water fuel could incur substantially lower fuel costs that a diesel-fuel competitor.
