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Title: USE OF COAL DRYING TO REDUCE WATER CONSUMED IN PULVERIZED COAL POWER PLANTS

Abstract

U.S. low rank coals contain relatively large amounts of moisture, with the moisture content of subbituminous coals typically ranging from 15 to 30 percent and that for lignites from 25 and 40 percent. High fuel moisture has several adverse impacts on the operation of a pulverized coal generating unit, for it can result in fuel handling problems and it affects heat rate, stack emissions and maintenance costs. Theoretical analyses and coal test burns performed at a lignite fired power plant show that by reducing the fuel moisture, it is possible to improve boiler performance and unit heat rate, reduce emissions and reduce water consumption by the evaporative cooling tower. The economic viability of the approach and the actual impact of the drying system on water consumption, unit heat rate and stack emissions will depend critically on the design and operating conditions of the drying system. The present project evaluated the low temperature drying of high moisture coals using power plant waste heat to provide the energy required for drying. Coal drying studies were performed in a laboratory scale fluidized bed dryer to gather data and develop models on drying kinetics. In addition, analyses were carried out to determine the relativemore » costs and performance impacts (in terms of heat rate, cooling tower water consumption and emissions) of drying along with the development of optimized drying system designs and recommended operating conditions.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Lehigh University
Sponsoring Org.:
USDOE
OSTI Identifier:
882470
DOE Contract Number:
FC26-03NT41729
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; BOILERS; COAL; COOLING TOWERS; DRYING; ECONOMICS; EVAPORATIVE COOLING; FLUIDIZED BEDS; HEAT RATE; KINETICS; LIGNITE; MAINTENANCE; MOISTURE; POWER PLANTS; SUBBITUMINOUS COAL; VIABILITY; WASTE HEAT; WATER

Citation Formats

Edward K. Levy, Nenad Sarunac, Harun Bilirgen, and Hugo Caram. USE OF COAL DRYING TO REDUCE WATER CONSUMED IN PULVERIZED COAL POWER PLANTS. United States: N. p., 2006. Web. doi:10.2172/882470.
Edward K. Levy, Nenad Sarunac, Harun Bilirgen, & Hugo Caram. USE OF COAL DRYING TO REDUCE WATER CONSUMED IN PULVERIZED COAL POWER PLANTS. United States. doi:10.2172/882470.
Edward K. Levy, Nenad Sarunac, Harun Bilirgen, and Hugo Caram. Wed . "USE OF COAL DRYING TO REDUCE WATER CONSUMED IN PULVERIZED COAL POWER PLANTS". United States. doi:10.2172/882470. https://www.osti.gov/servlets/purl/882470.
@article{osti_882470,
title = {USE OF COAL DRYING TO REDUCE WATER CONSUMED IN PULVERIZED COAL POWER PLANTS},
author = {Edward K. Levy and Nenad Sarunac and Harun Bilirgen and Hugo Caram},
abstractNote = {U.S. low rank coals contain relatively large amounts of moisture, with the moisture content of subbituminous coals typically ranging from 15 to 30 percent and that for lignites from 25 and 40 percent. High fuel moisture has several adverse impacts on the operation of a pulverized coal generating unit, for it can result in fuel handling problems and it affects heat rate, stack emissions and maintenance costs. Theoretical analyses and coal test burns performed at a lignite fired power plant show that by reducing the fuel moisture, it is possible to improve boiler performance and unit heat rate, reduce emissions and reduce water consumption by the evaporative cooling tower. The economic viability of the approach and the actual impact of the drying system on water consumption, unit heat rate and stack emissions will depend critically on the design and operating conditions of the drying system. The present project evaluated the low temperature drying of high moisture coals using power plant waste heat to provide the energy required for drying. Coal drying studies were performed in a laboratory scale fluidized bed dryer to gather data and develop models on drying kinetics. In addition, analyses were carried out to determine the relative costs and performance impacts (in terms of heat rate, cooling tower water consumption and emissions) of drying along with the development of optimized drying system designs and recommended operating conditions.},
doi = {10.2172/882470},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Mar 01 00:00:00 EST 2006},
month = {Wed Mar 01 00:00:00 EST 2006}
}

Technical Report:

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  • This is the fourth Quarterly Report for this project. The background and technical justification for the project are described, including potential benefits of reducing fuel moisture, prior to firing in a pulverized coal boiler. A description is given of the equipment, instrumentation and procedures being used for the fluidized bed drying experiments. Experimental data were obtained during this last quarter on the effects of particle size on drying rate for a North Dakota lignite. Other experiments looked at drying a PRB coal. The tests comparing drying rates with lignite particles of different diameters were carried out with particle top sizesmore » from 2 to 9.5 mm and covered a range of air velocities. The results show that drying rate increased with air velocity, but that, within the accuracy of the data, the data for all four particle size distributions follow the same curve. This suggests the higher drying rates associated with the larger particles are due to higher air velocities and not to any inherently different drying rates due to particle size. The drying data with the PRB coal show qualitatively similar behavior to that observed with lignite. However, quantitative comparisons of the drying rate data obtained so far for the two coals show the PRB dried at rates which were 14 to 20 percent lower than the lignite, for comparable process conditions. The equilibrium relationship between relative humidity and coal moisture was refined using a correction for temperature. This reduced the scatter in the coal moisture versus relative humidity data and improved the predictions made with the first principle drying model.« less
  • This is the fifth Quarterly Report for this project. The background and technical justification for the project are described, including potential benefits of reducing fuel moisture, prior to firing in a pulverized coal boiler. A theoretical model, for computing the effects of dryer design and operating conditions on performance of a continuous flow fluidized bed dryer, operating at steady state conditions, is described. Numerical results from the model, compared to data from a pilot scale lignite dryer located at Great River Energy's Coal Creek Station, show good agreement. The dryer model was used to perform parametric calculations on the effectsmore » of dryer design and operating conditions on dryer performance and required in-bed heat transfer. Other analyses show the first order effects of firing lignite and PRB coals, dried to various moisture levels, on flow rates of coal, combustion air and flue gas, fan and mill power and unit heat rate.« less
  • This is the sixth Quarterly Report for this project. The background and technical justification for the project are described, including potential benefits of reducing fuel moisture, prior to firing in a pulverized coal boiler. Coal drying experiments were performed with a Powder River Basin coal to measure the effects of fluidization velocity and drying temperature on rate of drying in a batch drying process. Comparisons to computational results using the batch bed drying model show good agreement. Comparisons to drying results with North Dakota lignite at the same process conditions confirm the lignite dries slightly more rapidly than the PRB.more » Experiments were also carried out to determine the effects of inlet air humidity on drying rate. The specific humidity ranged from a value typical for air at temperatures near freezing to a value for 30 C air at 90 percent relative humidity. The experimental results show drying rate is strongly affected by inlet air humidity, with the rate decreasing with more humid inlet air. The temperature of the drying process also plays a strong role, with the negative impacts of high inlet moisture being less of a factor in a higher temperature drying process. Concepts for coal drying systems integrated into a power plant were developed. These make use of hot circulating cooling water from the condenser, steam extraction from the turbine cycle and thermal energy extracted from hot flue gas, in various combinations. Analyses are under way to calculate the effects of drying system design and process conditions on unit performance, emissions, and cooling tower makeup water.« less
  • This is the seventh Quarterly Report for this project. The background and technical justification for the project are described, including potential benefits of reducing fuel moisture, prior to firing in a pulverized coal boiler. Coal drying experiments were performed with lignite and Powder River Basin coals to determine the effects of inlet air moisture level on the equilibrium relationship between coal moisture and exit air relative humidity and temperature. The results show that, for lignite, there is a slight dependence of equilibrium moisture on inlet humidity level. However, the equilibrium relationship for PRB coal appears to be independent of inletmore » air humidity level. The specific equilibrium model used for computing lignite coal dryer performance has a significant effect on the prediction accuracy for exit air relative humidity; but its effects on predicted coal product moisture, exit air temperature and specific humidity are minimal. Analyses were performed to determine the effect of lignite product moisture on unit performance for a high temperature drying system. With this process design, energy for drying is obtained from the hot flue gas entering the air preheater and the hot circulating cooling water leaving the steam condenser. Comparisons were made to the same boiler operating with lignite which had been dried off-site.« less
  • Low rank fuels such as subbituminous coals and lignites contain significant amounts of moisture compared to higher rank coals. Typically, the moisture content of subbituminous coals ranges from 15 to 30 percent, while that for lignites is between 25 and 40 percent, where both are expressed on a wet coal basis. High fuel moisture has several adverse impacts on the operation of a pulverized coal generating unit. High fuel moisture results in fuel handling problems, and it affects heat rate, mass rate (tonnage) of emissions, and the consumption of water needed for evaporative cooling. This project deals with lignite andmore » subbituminous coal-fired pulverized coal power plants, which are cooled by evaporative cooling towers. In particular, the project involves use of power plant waste heat to partially dry the coal before it is fed to the pulverizers. Done in a proper way, coal drying will reduce cooling tower makeup water requirements and also provide heat rate and emissions benefits. The technology addressed in this project makes use of the hot circulating cooling water leaving the condenser to heat the air used for drying the coal (Figure 1). The temperature of the circulating water leaving the condenser is usually about 49 C (120 F), and this can be used to produce an air stream at approximately 43 C (110 F). Figure 2 shows a variation of this approach, in which coal drying would be accomplished by both warm air, passing through the dryer, and a flow of hot circulating cooling water, passing through a heat exchanger located in the dryer. Higher temperature drying can be accomplished if hot flue gas from the boiler or extracted steam from the turbine cycle is used to supplement the thermal energy obtained from the circulating cooling water. Various options such as these are being examined in this investigation. This is the eleventh Quarterly Report for this project. The background and technical justification for the project are described, including potential benefits of reducing fuel moisture using power plant waste heat, prior to firing the coal in a pulverized coal boiler. During this last Quarter, the development of analyses to determine the costs and financial benefits of coal drying was continued. The details of the model and key assumptions being used in the economic evaluation are described in this report.« less