By the National Energy Technology Laboratory
The use of low-rank coals (lignite and subbituminous) has seen a significant increase in recent years. Because of the low sulfur content of such coals, many units have adopted fuel switching to meet sulfur emissions specifications, and other units have been built specifically to burn low-rank coals. However, a major disadvantage of low-rank coals is their high moisture content, typically 25 to 40 percent. When such coal is burned, considerable energy is required to vaporize the moisture it contains, thus raising the heat rate of the power plant and lowering its efficiency.
Fuel moisture has many effects on unit operation, performance, and emissions. As fuel moisture decreases, the fuel’s heating value increases so that less coal needs to be fired to produce the same electric power, thus reducing the burden on the coalhandling system. Drier coal is easier to convey as well, which reduces maintenance costs and increases availability of the coal to the handling system. When the crushed coal is gravity-fed into bunkers, the drier coal flows more readily than the wet coal, causing fewer feed hopper bridging and plugging problems. Drier coal is easier to pulverize as well so that less mill power is needed to achieve the same coal fineness. Finally, with less moisture in the fuel more complete drying of coal can be achieved in the mill, which results in an increased mill exit temperature, better conveying of coal in the coal pipes, and fewer coal pipe plugging problems.
The mixture of pulverized coal and air from the pulverizers is combusted in the burners. With drier coal, the flame temperature is higher since there is less moisture to evaporate. At the same time, heat transfer processes in the furnace are modified. The higher flame temperature results in a larger radiation heat flux to the furnace walls. Also, drier coal results in less moisture in the flue gas, which changes the radiation properties of the flame. The change in the flame emissivity also affects the radiation flux to the wall. With a higher flame temperature, the temperature of coal ash particles is correspondingly higher, which could increase furnace fouling and slagging, reducing heat transfer and resulting in a higher flue gas temperature at the furnace exit. However, the reduction in coal flow rate as fuel moisture is reduced also reduces the amount of ash entering the boiler, which leads to less solid-particle erosion in the boiler and decreased boiler maintenance cost.
The flue-gas flow rate from a furnace firing dry coal is lower than one firing wet fuel, and the specific heat of the flue gas is lower due to its lower moisture content. A lower flue gas flow rate also results in a lower rate of convective heat transfer. Therefore, despite an increase in initial flue gas temperature with drier fuel, less heat will be transferred to the water or steam in the boiler convective pass.
Drier coal is expected to lower the temperature of flue gas leaving the economizer and air reheater (APH). APH performance will also be affected by changes in the ratio of air and flue gas flows through the APH and changes in specific heat. Improved overall process efficiency will result from drier coal as the auxiliary power decreases due to decrease in forced draft, induced draft, and primary air fan power as well as decrease in mill power.
Previously, a number of proposals have been advanced to dry low-rank coals prior to combustion, but none of these efforts has resulted in a successful commercial operation. The two major problems with drying schemes before this have been the cost of the energy required and the fact that low-rank coals become pyrophoric when dried beyond a certain point. The Great River Energy Lignite Fuel Enhancement Project overcomes these problems by using waste heat to dry the coal and removing only about 25 percent of the moisture, enough to appreciably improve plant performance but not enough to cause handling problems.
The objective of this project is to demonstrate an economic process of moisture reduction of lignite, thereby increasing its value as a fuel in power plants. The project is being conducted at the Great River Energy’s Coal Creek Station in Underwood, North Dakota. The demonstration activities focus on using low grade condenser waste heat and flue gas in the plant to lower the moisture content of the coal by about 10 percentage points (e.g., reduce the lignite moisture from 40 to 30 percent). A phased implementation is planned: In the first phase, a full-scale prototype dryer module was designed for operation of one of the pulverizers on one of the two 546 MW units at the Coal Creek Station.
The objectives of prototype testing were to gain operating experience, confirm pilot results, and determine the effect of air flow rate, bed coils, bed depth, and coal feed rate on dryer operation in order to optimize performance. The lessons learned from the prototype were incorporated into the design of the dryers being installed in the second phase. A total of four dryers will be built for Unit 2. Although operating with wet lignite requires seven pulverizers, six will provide all the dried lignite required by the boiler.
Following successful demonstration in the first phase, Great River Energy is designing and constructing a full-scale, long-term operational test on a complete set of dryer modules needed for full power operation of one 546 MW unit (four dryers). The coal will be dried to a number of different moisture levels. The effect of coal drying on plant performance will be measured with respect to increase in plant efficiency and availability, reduction in emissions, and improvements in plant economics. The dryer design and operating conditions will be determined for optimal plant performance.
In response to the first round of the Clean Coal Power Initiative, Great River Energy (GRE) submitted a proposal for a full-scale test of a lignite-drying technology that they had been developing since the 1990s. The previous work included bench-scale research and development, field trials, and preliminary drying studies. These studies convinced GRE of the technical feasibility and economic benefits of lignite drying and prompted the submittal of their proposal. The Department of Energy evaluated and selected their proposal, and a cooperative agreement was awarded on July 9, 2004.
The project team for the Lignite Fuel Enhancement Project consists of GRE, participant and site provider; the Electric Power Research Institute, collaborator; Lehigh University, collaborator; Barr Engineering, lignite handling; and Falkirk Mining and Couteau Properties, lignite supplier. The project is sited at GRE’s Coal Creek Station in Underwood, North Dakota. Coal Creek Station is a mine-mouth plant, burning approximately seven million tons of lignite per year and consisting of two 546 MW, tangentially fired Combustion Engineering boilers. Steam is produced at 2,400 psig and 1,000 degrees F with a 1,000 degree F reheat temperature. The Coal Creek station has eight pulverizers per unit (seven active and one spare). The station has two single reheat General Electric G-2 turbines.
Schematic of Lignite Coal Drying Using Waste Heat From Condenser Water and Flue Gas
The figure above provides a simplified flow diagram of the lignite drying process. Warm cooling water from the turbine exhaust condenser goes to an air heater where ambient air is heated before being sent to the fluidized bed-coal dryer. The cooling water leaving the air heater is returned to the cooling tower. A separate water stream is passed through coils in the fluidized bed-coal dryer (a two-stage dryer is used to enhance heat transfer). The purpose of these coils is to provide additional heat to the fluidized bed to reduce the amount of air required. The dried coal leaving the fluidized bed is sent to a pulverizer and then to the boiler. Air leaving the fluidized bed is filtered before being vented to the atmosphere.
The technical aspects of the project are being implemented in two phases. The first phase involved the construction
and operation of a prototype dryer, a full-sized dryer with a maximum capacity of 112.5 tons/hour (225,000 lb/hour). It was designed to reduce the moisture content of lignite from 38 percent to 29.5 percent and improve the higher heating value from 6,200 Btu/lb to 7,045 Btu/lb. The prototype unit was fully automated and integrated into the plant control system. The first coal was introduced into the prototype dryer on January 30, 2006, and performance testing was carried out in March and April 2006.
Firing drier coal results in improved boiler efficiency and unit heat rate, primarily due to lower stack loss and lower auxiliary power (lower fan, pulverizer, cooling tower, and coal handling power). This performance improvement will allow greater electrical output with existing equipment. Performance of back-end environmental control systems (scrubbers and electrostatic precipitators) will also improve with drier coal due to the lower flue gas flow rate and longer residence time. The reduction in required coal-flow rate and modified temperature profile will directly translate into lower emissions of NOX, carbon dioxide (CO2), SO2, and particulates. For units equipped with wet scrubbers, mercury emissions resulting from firing drier coal would also be reduced. This is due to reduced APH gas outlet temperature, which favors the formation of mercuric oxide and mercuric chloride at the expense of elemental mercury. These oxidized forms of mercury are water-soluble and can therefore be removed in a scrubber.
During testing of the prototype coal dryer in 2006, at a feed rate of 75 tons/hour (14 percent of total fuel rate to the 546 MW unit), there were no major operating problems. The moisture of the total coal was reduced by only about 1.1 percentage points. Yet there were significant benefits in the prototype dryer operation for the 546 MW unit.
Performance measures showed that the lignite flow rate was reduced by 2 percent, pulverizer power was reduced by 3.3 percent, boiler efficiency improved 0.5 percent (absolute), net unit heat rate improved 0.5 percent, NOX emissions decreased 7.5 percent, and SO2 emissions decreased 1.9 percent. These results indicate that there will be significant improvements in operations once the project is fully implemented.
The potential market for GRE’s coal-drying technology is quite sizeable. There are 29 units with a total capacity of 15.3 gigawatts (GW) that are burning lignite directly, and another 250 units with a total capacity of over 100 GW burning Powder River Basin coal. If all these units were to adopt coal drying, the economic and environmental benefits would be quite large.