DOE PUSHES CLEAN COAL TECHNOLOGY PROGRAM

The U.S. Department of Energy continues to press its clean coal technology (CCT) campaign in several parts of the country.

Among recent developments:

Encoal Corp., Houston, a subsidiary of Shell Oil Co.'s Shell Mining Co., broke ground late in October for a plant near Gillette, Wyo., to demonstrate a mild gasification process called Liquids from Coal (LFC).
Western Energy Co., Butte, Mont., plans a $69 million test to improve the quality of western coal. The project at its Rosebud coal mine near Colstrip, Mont., will remove moisture and ash forming impurities from the coal and reduce its sulfur content.
Southern Co. Services Inc., Birmingham, plans an $8.56 million test in Florida of ways to reduce nitrogen oxides at coal burning power plants.
A DOE contractor is working on a concept that would allow gas turbines to operate on coal. DOE said combining a conventional coal combustor, with its hot, impurity laden exhaust, with a relatively fragile gas turbine has been a challenge for the industry. Hague International, South Portland, Me., proposes to keep the coal gases out of the turbine.
Tallahassee, Fla., plans to repower an aging oil and gas fired boiler with the nation's largest circulating fluidized bed combustion boiler. DOE will contribute $74.7 million, or 27% of the cost.

Those and other projects are part of DOE's $5 billion campaign to help foster a new generation of coal technology (OGJ, Nov. 5, p. 55). the result could be increased use of coal and decreased use of oil and gas as boiler fuel, especially in power plants.

ENCOAL PLANT

The Encoal plant at Shell's Triton Coal Co. Buckskin mine is designed to process 1,000 tons/day of subbituminous coal from the Powder River basin of northern Wyoming into two clean burning products: 500 b/d of a low sulfur oil, similar to No. 6 fuel oil, that can displace oil in industrial and utility boilers and 500 tons/day of a solid fuel similar to bituminous coal but without sulfur pollutants.

Both fuels can meet or exceed the nation's strictest environmental requirements.

M.W. Kellogg Co., Houston, has the contract for engineering, procurement, and construction. Start-up is planned for early 1992.

Developed by SGI International of La Jolla, Calif. and Shell Mining, the LFC process is suited for subbituminous coals found in Wyoming, North Dakota, Montana, Alaska, and on the Gulf Coast.

The LFC process uses controlled temperatures and operating pressures to chemically convert coal. Coal is first heated to remove moisture and then cooked to about 1,000 F. in a pyrolyzer.

That releases a gaseous product, and the coal then is cleaned in a cyclone and cooled to condense a liquid product. The remaining solid fuel is nearly moisture free and has a high heating value.

DOE said the key to the LFC process is a computerized control system that sets operating conditions to optimize the quality of product fuels depending on specific market needs and the composition of the feed coal.

The products then will be tested in utility and industrial boilers.

Tek-Kol, a Shell Mining and SGI partnership, owns the LFC technology.

DOE is providing half the cost, or $36.8 million, of the 2 year demonstration. If the project leads to commercial sales or licensing of the technology, Encoal will repay the government's financial contribution.

DOE said if the project is successful, a commercial plant 10 times the size of the demonstration plant could be built and operating by the mid-1990s.

Encoal's plant is among 13 projects selected in the third round of the CCT program. It is the first participant in the third round to negotiate a contract with DOE and pass congressional review.

WESTERN ENERGY VENTURE

Western Energy plans to upgrade low rank subbituminous coals typically found in the western U.S. and favored for their low sulfur content. The coals have high moisture levels-often nearly half of their total weight-and are costly to transport and often difficult to burn.

An advanced coal conversion process will reduce the moisture to about 1%, boosting the heating value from as low as 5,500 BTU/lb to 12,000 BTU/lb.

The process also changes the surface characteristics of the coal to prevent reabsorption of moisture. The sulfur content of the upgraded coal will be only 0.3%, enabling it to be burned without pollution controls.

In the first stage of the process, hot gases from a separate heating plant pass through tumbling coal to remove loosely held water. The second stage further heats the coal to remove chemically bound water and sulfur compounds. Then the coal is cooled and moved through vibrating screens and separators that remove additional sulfur and other minerals.

DOE said an advantage is that the process operates at low pressures, while other experimental coal upgrading techniques require higher pressures and relatively expensive pressure vessels for the drying step.

The plant will produce about 300,000 tons/year of upgraded coal. It will be integrated with existing coal crushing facilities at the Rosebud mine.

DOE plans to provide $34.5 million, or half the 3 year demonstration cost.

Design and construction will require 2 1/2 years. If the plant is successful, Western Energy plans to build a privately financed, commercial scale plant processing 1-3 million tons/year by 1997.

SOUTHERN CO. PROJECT

A Southern engineering subsidiary will test three ways to reduce nitrogen oxides from "tangentially fired" or "corner fired" boilers at its Lansing-Smith plant near Panama City, Fla. About 179 of those boilers are operating in the U.S.

Nitrogen oxide forms in a typical coal boiler when combustion temperatures approach 3,000 F. but can be reduced by "staging" the combustion so the air-to-fuel ratio is controlled at critical places inside the boiler.

In tangentially fired boilers, coal and air are alternately blown inward from corner mounted burners and angled toward the center of the furnace, creating a circular swirl of combustion gases.

Southern will test a concentric firing system that creates an air-rich boundary around the inside walls of the boiler and a fuel-rich fireball in the center. Because there is little air in the center, little nitrogen oxide formation is expected. Combustion will be completed when "overfire" air is added above the primary combustion zone.

The second test series will reduce the air flow in the main combustion zone to create a fuel-rich area where little nitrogen oxide is formed. More overfire air will be added through ports above the primary combustion zone to complete combustion in a cooler region of the boiler, where temperatures are below the nitrogen oxide formation threshold.

The third process is a concentric clustered, tangentially fired system that combines and enhances features of the first two. Coal nozzles will be grouped in clusters, creating fuel-rich combustion zones. Two overfire systems will be used, allowing for more gradual staging than a single system.

The three processes will be tested during 3 years. DOE is contributing $4.15 million of the project's cost, the Electric Power Research Institute $1 million, and Southern the rest.

HAGUE HEAT EXCHANGER

Hague has designed a ceramic heat exchanger which, positioned between the coal combustor and gas turbine, serves as a barrier to impurities of coal gas while allowing its heat to flow through a stream of clean, high pressure air. The hot air, in turn, spins a gas turbine.

DOE said conventional metal heat exchangers begin to deteriorate when the temperature exceeds 1,700 F., but Hague's ceramic version is expected to withstand the 3,500 F. blast of a coal combustor.

It also will be able to transfer enough heat to raise the air stream temperature to as high as 2,300 F., the inlet temperature of a modern day, high efficiency gas turbine.

DOE said the indirect fired gas turbine, if successful, could be 50% more efficient than today's conventional steam turbine plants because the gases leaving the turbine remain hot enough to boil water into steam, which is then used in a steam turbine generator.

It said the combined cycle approach could raise thermal efficiencies to 45-50%, compared with the 33% efficiency of today's coal burning power plant,

DOE said the high efficiencies mean less pollutants are produced per unit of energy, including carbon dioxide. It said a power plant running at 48% thermal efficiency will release nearly a third less carbon dioxide than one operating at 33% efficiency.

Unlike other advanced approaches to coal fired combined cycles that either gasify the coal or burn the coal directly in specially hardened turbines, the Hague approach would use standard coal combustion and gas turbine equipment. Sulfur pollutants would be removed with conventional flue gas cleanup equipment, while nitrogen oxides can be limited by controlling coal burning.

Hague plans to prevent fly ash particles in the coal gas stream from sticking to the heat exchanger tubes and robbing efficiency by electrically charging the ash particles before the gases enter the heat exchanger. An electrical field on the heat exchanger tubes would deflect the particles toward collection devices.

Hague will test the ceramic heat exchanger and its components, using 300 hr of coal fired at a scale equivalent to 3,000 kw.

DOE is providing $2.2 million toward the 30 month, $4.2 million effort. The balance is being provided by Hague, Foster Wheeler Development Corp., E.I. Dupont Corp., Stone & Webster Engineering Corp., Pyropower, Consolidated Edison, Florida Power Corp., Empire State Electric Energy Research Corp.

TALLAHASSEE PROJECT

Tallahassee's repowering project will be integrated with existing turbine/generator equipment at the city's Hopkins station, about 3 miles west of Tallahassee.

The 250,000 kw unit will allow the city to modernize its 23 year old plant, exceeding environmental standards for emissions while cutting reliance on oil and gas fuels.

The Tallahassee circulating fluidized bed unit will be one of the largest demonstrations of an advanced technology under the CCT program. The combustion process will capture potential air pollutants, avoiding the need for costly cleanup devices.

In a circulating fluidized bed process, crushed coal and limestone injected into the bottom of a furnace are suspended by upward jets of air. As coal particles burn in a turbulent motion, they are reduced in size and carried up and out of the furnace. Solid particles are separated from hot gases in a cyclone device and circulated back through the furnace, resulting in more efficient fuel use.

As coal and limestone particles mix in the turbulent bed, more than 90% of sulfur pollutants are captured by the lime. These solid, sulfur laden limestone particles are removed from the bottom of the furnace and cyclone, along with coal ash. Formation of nitrogen oxides is inhibited by relatively low temperatures in the furnace and staged introduction of combustion air.

Smaller circulating fluidized bed boilers are operating with a variety Of fuels, but the Tallahassee project will be the largest in the nation and the first to burn eastern high sulfur coals.

DOE said data from the project, expected to be available in the mid-1990s, will be given to utilities facing the need to replace or repower aging plants. Circulating fluidized bed units can be built in less than 4 years and added to a plant in modules.

DOE expects to issue a final environmental report for the project within a year, enabling construction to begin in mid-1992. Bechtel Power Corp. will design and build the advanced combustion system, and Foster Wheeler U.S.A. will provide the boiler. A 2 year test will begin late in 1995.