Pulverized coal combustion (PCC) dominates power generation and will continue to do so for the foreseeable future. Due to aging of the existing fleet of PCC plants and global increase in electricity demand, especially in emerging economies, a fleet of new highly efficient PCC plants is likely to be deployed.
Thermal efficiency of a power plant is one of the key parameters affecting the fuel cost, emissions (both non-greenhouse (GHG) and GHG), and capital cost. An increase in plant efficiency reduces coal consumption and fuel costs and lowers the amount of flue gas treated by the flue gas cleaning system, thus resulting in lower emission compliance cost. The published data concerning performance of advanced PCC generation is almost exclusively for the bituminous (hard) coals with no or very little information available for lower rank coals. Addressing this information gap and quantifying the effects of fuel quality on efficiency of USC and A-USC plants are the main goals of the study discussed in this article.
Increasing steam parameters with the resulting increase in turbine cycle efficiency is one of the most effective ways of improving plant efficiency. The state-of-the-art ultra-supercritical (USC) technology can reach a steam temperature of 600°C at the superheater outlet and net efficiency of 47% (LHV) for bituminous (hard) coals. The new target for advanced ultra- supercritical (A-USC) technology is a main steam temperature in excess of 700°C and net unit efficiency estimated at 50% (LHV) for hard coals.1
In addition to increasing steam parameters, improvement in coal quality is an effective method to increase the efficiency of PCC plants. This is particularly important for the advanced PCC technologies (USC and A-USC) operating at high steam parameters. The negative effect of high coal-moisture content on efficiency rises as steam parameters increase, reducing the operating benefits of the USC and A-USC.
Low-rank, high-moisture coals constitute about 50% of the world coal reserves.2 Given such coals’ abundance and low cost, a significant portion of advanced PCC generation built in the future will be fueled by low-rank coals. To achieve the highest operating efficiency, capacity, and availability, smallest equipment size, and lowest CAPEX and OPEX will require reducing coal moisture.
The effects of coal quality improvement achieved by thermal dewatering of high−moisture coals on net efficiency and capital cost of the USC and A−USC plants are discussed below. The results were obtained by predicting performance of a reference 860−MWgross plant for four high−moisture coals.
Thermal efficiency of a power plant is a key parameter. (Courtesy of Great River Energy)
The effects of coal quality improvement achieved by thermal dewatering of high−moisture coals on net efficiency and capital cost of the USC and A−USC plants are discussed below. The results were obtained by predicting performance of a reference 860−MWgross plant for four high−moisture coals.
EFFECT OF COAL QUALITY ON EFFICIENCY
The use of high−moisture coals with a low HHV (higher heat− ing value) results in higher coal and stack flue gas flow rates, higher station service power, and lower net plant efficiency compared to that of hard coals (see Figure 1). In addition, the mill, coal pipe, burner, and coal−handling equipment mainte− nance requirements are higher. The properties of coals used in our analysis are summarized in Table 1.
FIGURE 1. Effect of coal rank and steam parameters on net unit efficiency
As shown in Figure 1, coal quality significantly impacts plant efficiency—for low−quality coals, plant efficiency is signifi− cantly lower compared to the bituminous (hard) coals. This negative effect is higher for plants with high steam param− eters, thus reducing the benefits of advanced PCC operation. For example, an USC PCC plant firing high−moisture German lignite will have 7.3%−points lower net efficiency compared to the same plant using hard coal. Considering the high capital cost of the advanced PCC technology, such a reduction in per− formance is highly undesirable.
Pre-drying of High-Moisture Coals
Given that HHV increases as the total coal moisture (TM) is reduced (the average improvement in HHV is in the range of 100−120 Btuƒlb per 1%−point reduction in TM), countries with large resources of high−moisture, low−quality coals are devel− oping coal dewatering and pre−drying processes to improve unit efficiency, plant operation, and economics, as well as to reduce emissions from existing and future−built power plants firing low−rank coals. However, many thermal drying processes are either highly complex or require high−grade heat to remove moisture from the coal. This significantly increases the process cost, which represents a major barrier to industry acceptance of this technology.
Two previous IEA Clean Coal Centre (CCC) studies identi− fied two low−energy−based coal pre−drying technologies: the U.S.−developed DryFiningTM and German−developed WTA (Wirbelschicht−Trocknung mit interner Abwämenutzung, fluidized bed drying with internal waste heat utilization) as commercially available and suitable for implementation at power plants burning high−moisture coals.3,4
TABLE 1. Properties of the coals used in our analysis
DryFiningTM: A Low-Temperature Coal Drying and Refining Process
DryFiningTM is a novel low-temperature coal drying and cleaning process that employs a multistage moving bed fluidized bed dryer (FBD). The process uses low-grade heat rejected in the main steam condenser and sensible heat from the flue gas leaving the boiler to decrease moisture content of the high-moisture coals, such as North Dakota (ND) lignite. The technology was developed by a team led by Great River Energy (GRE). The DryFining system has been in continuous commercial operation at Coal Creek Station in North Dakota since December 2009 and has processed in excess of 40 million tons of raw ND lignite.
Implementation of DryFiningTM at the Coal Creek lignite- fired power plant has improved unit heat rate by more than 5%, simultaneously achieving 30% reduction in SO2 and Hg emissions, 20% reduction in NOx emissions, improving plant availability, and lowering plant water usage.5−10 Pre-combustion Hg removal is currently a topic of considerable interest.
APPLICATION OF DRYFININGTM TO ADVANCED PCC POWER PLANTS
There is significant interest in coal quality improvement through thermal pre-drying in markets experiencing high electrical load growth combined with local resources of low-rank coals. A number of lignites and sub-bituminous coals from
Table 2. Steam conditions
Results
The results for the high-moisture coals presented in Table 1 are depicted in Figures 2 to 4. For clarity, only the results for the USC and A-USC steam conditions are shown here. The improvement in ynet relative to the subcritical steam conditions and raw coal is shown in Figure 2 as a function of the reduction in TM. For all analyzed coals, ynet increases as TM is reduced (coal quality is improved). The efficiency improvement is higher for the higher moisture coals and higher steam conditions; the largest improvement with the lowest TM is achieved with A-USC steam conditions.
For the decrease in coal moisture of 25%-points, high- moisture German lignite, and A-USC steam conditions, the improvement in 𝞰net is 12 %-points; for identical conditions and lower moisture PRB coal, the efficiency improvement is approximately 10.5%-points.
FIGURE 2. Improvement in net efficiency as a function of reduction in TM for USC and A-USC steam conditions
FIGURE 3. Reduction in CO2 emission intensity as a function of reduction in TM for USC and A-USC steam conditions
TM can be reduced by approximately 15%-points using exclusively the power plant waste heat. To achieve deeper coal drying requires process heat. In this study, the low-pressure (LP) steam extracted from the LP turbine was used as a source of the process heat. Because this steam extraction lowers the steam turbine power output, the achieved improvement in unit efficiency is smaller and the eficiency curves change slope at 6TM greater than 15%.
The reduction in CO2 emission intensity EFCO2 relative to the subcritical steam conditions and raw coal is presented in Figure 3 as a function of 6TM for the USC and A-USC steam conditions. The magnitude of the reduction in EFCO2 increases with the reduction in TM and improvement in steam conditions. For the 6TM of 25%, high-moisture German lignite, and
FIGURE 4. CAPEX savings as functions of reduction in TM for USC and A-USC steam conditions
A-USC steam conditions, the reduction in the CO2 emission factor exceeds 28%. For identical conditions and lower moisture PRB coal, the reduction in EFCO2 is lower, approximately 22%. As the figure shows, improvement in coal quality by thermal drying results in significant reduction in CO2 emissions.
The results of the economic analysis performed for the USC and A-USC steam conditions are presented in Figure 4. The capital cost (CAPEX) savings for all analyzed coals are given as functions of the reduction in coal moisture content.
For a new plant, the CAPEX savings increase with the reduction in TM and are higher for the A-USC conditions compared to the USC due to the smaller size and lower cost of the DryFining™ system. The highest CAPEX savings from the analyzed coals were achieved by the Indonesian WARA coal, followed by the
U.S. lignite and PRB coals. For these coals, A-USC steam conditions, and 6TM of 25%, CAPEX savings are in the US$170−220 per kW range, or 6−7.5% of the plant capital cost. For the USC steam conditions, the CAPEX savings are approximately US$75−85 per kW lower.
As the results show, thermal drying of high-moisture coals provides significant capital cost savings for the USC and A-USC steam conditions, and should be considered for all new advanced PCC plants.
CONCLUSIONS
An effective method of increasing net eficiency of PCC plants is to increase steam parameters and improve coal quality. This is particularly important for advanced PCC technology (USC and A-USC), which operates at high steam parameters. The negative effect of high coal-moisture content on eficiency increases as steam parameters increase, reducing the benefits of USC and A-USC operation.
Analyses applying DryFining™ to advanced PCC power plants were carried out to determine the effect of reduced coal moisture content on plant performance, CO2 emissions, and capital cost. Results indicated that plant efficiency increases as TM is reduced and steam conditions are increased; the largest improvement can be achieved with combustion coal having the lowest moisture content at A-USC steam conditions. The efficiency improvement from coal drying increases with higher moisture coals in comparison to lower moisture coals. Plant efficiency is improved due to reduction in fuel moisture from thermal drying, resulting in decreased flow rates of coal, flue gas, and emitted CO2.
In conclusion, improvement in coal quality by thermal drying has a significant positive effect on the power plant efficiency and reducing CO2 emissions. Economic analysis results show that CAPEX savings increase with the reduction in TM. The savings are better for higher steam conditions due to the smaller size and lower cost of the DryFining™ system.
Thermal drying of high-moisture coals provides significant capital cost savings, especially for the USC and A-USC steam conditions, and should be considered for all new PCC plants.
Source: Nenad Sarunac - Associate Professor University of North Carolina at Charlotte, Charles Bullinger, Mark Ness, Sandra Broekema, Ye Yao - Great River Energy
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