In today’s solid-fueled power plant, managing emissions and moving materials more defines the task than the traditional work of making megawatts. That’s the message that emerged from the coal and solid fuels track at this year’s ELECTRIC POWER.
Whether it’s the traditional fuel, coal, or the hot new trend of cofiring coal with biomass, moving stuff around presents major challenges.
The Hidden Costs of Biomass
Ezra Bar-Ziv of Ben-Gurion University described the experience of mixing raw wood-derived biomass with coal at two large power plants in Israel. The aim was to test the environmental benefits of combining coal with a cleaner fuel. Those benefits, Bar-Ziv said, were significant: limiting the growth of CO2 emissions; substantially lowering SOx, NOx, and rocks (particulates); and the absence of mercury in the biomass fuel.
Biomass, Bar-Ziv noted, “can be used in coal-fired boilers without objections from environmental protection authorities.” Biomass also qualifies as “sustainable,” Bar-Ziv said, as long as “the biomass growth rate equals the rate of biomass burning,” meaning no net loss of the fuel.
But the technology has little market penetration so far. According to Bar-Ziv’s figures, there are only 15 commercial projects in the world that feature cofiring coal and biomass. Of those, eight are in Europe and only one is in North America. (There are many more cofiring pilot and feasibility projects. See “Biomass Cofiring: A Promising New Generation Option” in the April 2011 issue of POWER.)
So, asks Bar-Ziv, “if cofiring biomass is good, why are there so few commercial plants?” His answer is that moving and mixing the fuel with coal is tricky and generates several kinds of costs:
- Biomass is bulky, which entails expensive logistics.
- Its high moisture means it is expensive to transport.
- Its low heat value means it requires a lot of fuel to produce power results.
- Biomass is hygroscopic, meaning that it takes up and retains water, burns irregularly, and produces undesirable tars, which must be dealt with.
- It is difficult to pulverize.
What to do? Make biomass more like coal by torrefaction, says Bar-Ziv. Torrefaction involves heating the biomass to 200C to 300C in a reduced-oxygen environment. In short, it is a mild form of pyrolysis. The result of torrefaction is a fuel that has greater heat content and a decreased ratio of oxygen to carbon; it’s also a more easily handled product. “Grindability is improved dramatically, and the power required for pulverization to the suitable size range reduces by a factor of 10, reaching the power required to pulverize coal,” says Bar-Ziv. He called the product “biocoal”; others call it biochar (see “Utilities Increase Renewable Energy Capacity” in this issue).
Lessons from Biomass Transferred to Coal
Even some ranks of coal can benefit from the same basic idea—heating to reduce moisture and make a fuel that has higher heat value and is handled more efficiently. That’s what Great River Energy, Minnesota’s second-largest utility, has done at its two-unit, 1,200-MW Coal Creek Station, the largest power plant in North Dakota. James Kennedy of the Worley Parsons consulting firm, described the Coal Creek project for the coal track.
The mine-mouth plant burns a fuel rated at 6,200 Btu/lb and 38% moisture. Unfortunately, noted Kennedy, the plant was designed to burn 6,800-Btu fuel, meaning that the two units were burning 9% more fuel than they should, which lowered efficiency and increased costs across the board. Using Department of Energy funding, Great River Energy mounted a project to use waste heat from the plant to dry the lignite and improve plant performance. After running a pilot project in 2005, the engineers installed full-scale equipment in 2009. Resulting “performance [was] right on the predicted curve,” with a 9% reduction in fuel moisture.
After a year of operation, the new equipment has seen over 90% availability with more than 90% of the plant’s coal processed through the new dryers. The drying process has also produced significant air pollution reductions: 54% less SO2, around 40% less mercury, 32% less NOx, and a CO2 reduction of 4% due to more efficient operation.
Because the Coal Creek project was a first-of-a-kind venture, what lessons can be learned for those who wish to replicate the technology? Kennedy suggested two that deal with fuel handling. The first is crusher location. “Crushers,” he said, “need to be as close to the dryer inlets as possible to minimize localized agglomeration at hoppers along the material transport system.” The second is fuel size, he said: “Wet fines need to be minimized to prevent accumulation and bed elutriation (the separation of lighter and heavier particles),” and “Oversize chunks need to be minimized to prevent bed stagnation zones.”
Kennedy added that Great River Energy expects even greater gains in the future from its lignite-drying operations: lower NOx emissions as the furnace is retuned based on the new performance experience and “substantially reduced routine pulverizer, boiler, and air quality control system maintenance costs.”
In summary, said Kennedy, the Coal Creek project produces “coal drying as needed”; it avoids the need for protracted fuel storage and the concomitant risks of spontaneous combustion. The moisture reduction means lower fuel throughput, net heat rate improvement, reduced flue gas volume, and reduced service requirements. On top of those benefits, the project lowers air emissions for criteria pollutants and CO2 as a result of increased efficiency.
One of the major drivers of improved air quality from coal-fired plants in the past 20 years has been the installation of wet flue gas desulfurization equipment. But wet scrubbers have created unique and daunting materials-handling issues, as described by Richard McCartney of engineering and construction firm Roberts & Schaefer Co.
“All of the wet scrubber addition projects require limestone to be delivered to the power plants and the gypsum by-products disposed of,” McCartney said (Figures 1 and 2). “This requires that two separate material-handling facilities be added to the power plant. The limestone facility includes delivery, unloading, stockout, storage, reclaim, and silo fill systems. The gypsum facility includes stockout, storage, reclaim, loading, and disposal systems.”
|1. Input. Progress Energy Florida’s Crystal River Station (Units 4 and 5) is located in Crystal River, Fla. The limestone-handling facility is designed to receive limestone from back-dump trucks at two above-grade receiving hoppers with drag chain reclaim conveyors Courtesy: Roberts & Schaefer Co.|
|2. Output. At the end of the flue gas desulfurization process, plants have to dispose of gypsum. At American Electric Power’s Mitchell Plant in Cresap, W.Va., barge loadout is through a transfer house and onto a barge loadout shuttle conveyor equipped with a telescopic chute. Courtesy: Roberts & Schaefer Co.|
All this, of course, comes on top of the coal-handling requirements. So the modern solid-fueled power plant is a complex network of materials-handling gear: belts and screws and trucks and barges and chains and cranes. Here’s how McCartney describes just the options for handling the stockout and reclaiming of the gypsum output from the wet scrubbers: “Stockout methods can form a single conical pile, a circular pile, or a long triangular pile. Storage includes both open and enclosed. The minimum storage capacity is based on the production cycle and the shipping/disposal schedule.
“As with the stockout systems, there are many reclaim systems that accommodate reclaim of gypsum. Reclaim methods can vary from fully automated to manual with mobile equipment assistance. The arrangement of the stockout pile and the type of reclaim equipment used dictates the percentage of the stockpile that will be automatically reclaimed without the use of mobile equipment.”
How does a plant operator dispose of all the gypsum pouring from the scrubber? McCartney explains, “The major methods for disposing of gypsum at power plants are barges, trucks, and conveyors. The disposal method is based on the location of the power plant and the opportunities to sell the gypsum within that region. Power plants along major rivers like the Mississippi and Ohio ship their gypsum by barge. Coastal power plants ship the gypsum by ocean barges, while power plants in the East or West usually ship their gypsum by truck. Occasionally, conveyor transport is used. The typical disposal systems are:
- Conveyor(s) directly to a wallboard plant or to a landfill area on the plant property.
- Trucks loaded by conveyor chute or mobile equipment (front-end loaders).
- River or ocean barges loaded by a loadout conveyor with a loading chute.
- River or ocean barges loaded by a loadout shuttle conveyor with a telescopic chute.”
— Kennedy Maize is a POWER contributing editor and executive editor of MANAGING POWER.