Low-rank coals (i.e., lignite and brown coals) have been estimated to account for approximately 50% of global coal reserves, with as much as half of those reserves considered to be economically recoverable.1 While the principal deposits are concentrated in the U.S. and the Russian Federation, significant reserves also exist across Asia. Over 50 billion tonnes of proven recoverable low-rank coal resources have been identified in China alone and significant reserves of lignite exist in India, Pakistan, and Thailand. These low-rank coals are considered to be low grade because of their high moisture content and low heating value. Such coals also may have higher sulfur content, requiring the application of additional technologies for its capture and removal.
In common with other relatively low-value fuels, no free- market mechanism exists for low-rank coals used in power generation because their low energy content usually makes transport over longer distances uneconomic. For this reason, lignite-fired power plants are commonly built adjacent to lignite mines. A power plant and surface mine then form a single economic entity in which lignite is transported by dedicated infrastructure, typically a conveyor belt, and delivered directly to the nearby power plant.2
“CFBC … is thought likely to feature strongly in Asia’s future fossil fuel power generation fleet.”
The role of some coal-fired power plants has changed significantly in recent years as co-combustion of biomass and waste become more commonplace. The utilization of low-rank coals and other fuels in an efficient and environmentally benign way poses many technical challenges. Circulating fluidized bed combustion (CFBC) is particularly well suited to utilize low-rank coals and other challenging fuels, which is why there are about 130 such plants in operation in Asia and more under construction.
BENEFITS OF CFBC
CFBC was first demonstrated in the late 1970s, but it was not used for power generation until 1985 (see Figure 1 for a generic CFBC process). The basic concepts behind CFBC rely on gas velocities high enough so that particles are entrained and carried out of the boiler. Flue gas passes through solid separators (typically cyclones) that return ash and other solids to the lowest part of the combustor and thus prevent unburnt fuel from leaving the furnace. This creates a recycle loop through which fuel particles can pass 10 to 50 times until complete combustion is achieved. The prolonged combustion time results in much lower temperatures (800–900°C) than those found in pulverized coal combustion (1400–1600°C), which has traditionally led to lower operating efficiencies in CFBC.
FIGURE 1. A generic power plant based on CFBC technology
The flexibility of CFBC means that low-rank coal, waste, and biomass can be used in the same boiler.
Although CFBC has been used for decades, higher-efficiency plants are now successfully operating on increasingly large scales. A 460-MW supercritical CFBC unit at the Lagisza power plant in Poland has been operating since 2009 and a 600-MW supercritical unit recently began operation at Baima in China,3 while the four 550-MW supercritical units of the Samcheok power plant in South Korea, scheduled to begin operation soon, constitute the world’s single largest CFBC power plant. Unit sizes have been steadily increasing, with 600- to 800-MW supercritical CFBC systems now commercially available and larger units under development.
CFBC power plants are particularly well suited to burn low- grade fuels and mixtures of such fuels. A large amount of inert bed material makes it possible to have considerable variation in fuel properties, or to change fuels during operation without significant disruption to the combustion process. Circulating solids also improves heat transfer and makes it possible to burn high energy content fuels while maintaining the combustion temperature in the region of 850–900°C. This relatively low combustion temperature minimizes fouling and slagging of heat surfaces since ash melting and softening points are generally much higher than CFBC temperatures. The low temperatures also make emissions control more straightforward as the amount of NOx created is relatively lower and solids circulation provides a long residence time for fuel and limestone particles, resulting in high SO2 capture efficiency and lower limestone consumption. In fact, the supercritical unit at Lagisza has demonstrated up to 95% SO2 removal with a calcium to sulfur ratio of two or 99.8% removal at a calcium to sulfur ratio of three. Additionally, CFBC plants have exceptional flexibility and can operate effectively while running at as little as 30% of full load.
Increasingly liberalized and volatile fuel markets, coal sources of highly fluctuating quality, and the trend toward biomass and/or waste co-firing in some regions are all factors that can make CFBC a more attractive technology for power generation. For example, if the price of higher quality imported coal becomes too great, a switch to lower grade, locally sourced coal can be made without significantly altering the performance of a CFB boiler. In this way, the flexibility of CFBC can act as a contingency against variation in fuel supply.
CASE STUDY: THE SURAT HIGH-SULFUR INDIAN LIGNITE CFBC PLANTS
India’s demand for coal-based electricity is forecasted to increase dramatically over the next few decades, and utilization of relatively low-quality coals, including high-sulfur lignite, will be necessary to meet this demand. Thus, much like the rest of developing Asia, there is a strong potential for increased deployment of CFBC power plants in India.
The combustion of such coal can pose significant challenges, which were overcome by Bharat Heavy Electricals Limited (BHEL), an Indian state-owned power plant manufacturer, as they operated two 125-MWe CFBC units at Surat that were modified specifically to burn high-sulfur lignite (see Table 1 for the units’ design parameters).5
Plant Design
In BHEL’s CFBC units, the pre-crushed lignite is extracted from the storage bunkers by two variable-speed extraction drag-link chain conveyors and fed through rotary valves and slide gates, which can isolate the fuel feed system from the combustor in case of an emergency. The system has two parallel coal feed lines, both of which need to be operated for optimal fuel combustion. Inert material such as bed ash or sized sand, required for initial start-up, is fed to the combustor directly through a rotary valve. Presized limestone stored in silos is gravity fed through variable-speed rotary valves at a rate based on the SO2 content in the flue gas.
Ash handling is hugely important during CFBC operation. At the BHEL CFBC units, ash is removed from four different locations
TABLE 1. Design parameters of BHEL’s Surat plant
in the system: Coarse bed ash exits toward the bottom of the combustor, bed ash from the furnace is removed at bottom heat exchanger, fly ash is removed from the collection hoppers below the convective pass and air heater sections, and fly ash is also captured and removed via the electrostatic precipitator. In order to maintain an appropriate solids inventory in the combustor, bed ash is extracted continuously from the lower combustor and furnace bottom heat exchanger through a cooled ash discharge.
Operating Experience and Lessons Learned
While BHEL’s CFBC units have generally performed well, there have been some challenges around solids handing and deposition in the system. For example, three outages occurred due to ash hold-up in the cyclone at low loads of about 20 MW and another outage was caused by a suspected blockage of the cyclone standpipe when the plant was operating at about 70 MW. An investigation into the incidents concluded that the most probable cause was the recarbonation of calcined lime- stone that had not reacted with SO2.
The limestone also was found to be much finer than recommended. This resulted in high throughput during low loads because, due to an equipment malfunction, the SO2 measurement was not available to control the volumetric feeder of the limestone. In addition, the timing of pulsing air has been subsequently reduced, as it was found that the gas temperature is a key parameter in avoiding the formation of sticky deposits.
The following remedial steps were taken to prevent further outages attributable to cyclone standpipe blockage:
- The limestone feed size was checked continuously with additional sampling.
- The limestone feeder hopper size was reduced.
- The operating procedure was revised to maintain higher combustor temperatures before commencing limestone addition.
- Automatic air pulsing was incorporated at the junction of the cyclone and standpipe to disturb particles and avoid agglomeration.
After incorporation of these changes, the challenges around limestone injection were resolved.
Another operational challenge related to heavy and rapid deposit build-up on the flue gas side of the boiler heat transfer tubes. The deposit build-up was most severe at the low-temperature superheater tube bank. There were also ash deposits in the final-stage reheater tube bank during the initial period of operation. These deposits increased the gas-side pressure drop and in turn forced the operation of the induced draft fans at high loads, causing boiler trips.
The deposits occurred as the boiler was brought online after resolving the cyclone blockage problem when the limestone feed rate was increased to meet SO2 emissions limits. It was suspected that the formation of sticky deposits, as previously observed in the cyclone, initiated the formation of the larger deposits on the tubes. To assess the cause, samples were taken and confirmed that the primary mechanism of fouling was the recarbonation of free lime followed (i.e., CaO to CaCO3) by slow sulfation of the deposit. Improvements in the soot- blowing mechanism along with an increase in its frequency have helped overcome the fouling issue.
After the implementation of high-pressure soot blowers along with a fluidization arrangement for smooth evacuation of the ash falling onto the hoppers, full load operation with limestone addition to ensure sulfur capture of more than 98% (versus 97% design) was achieved.
FIGURE 2. Result of implementation of high-pressure soot blowers on gas path (photo taken inside ductwork)
Although the supercritical CFBC power plant operated by BHEL experienced some challenges, after detailed technical assessments and some operational modifications, both units were able to operate reliably and with low emissions. As an increasing number of CFBC power plants are deployed in India and throughout the rest of Asia, there will be lessons to be learned around optimal operation and additional R&D needed to further increase the efficiency.
CONCLUSIONS
Low-grade coals are a significant energy resource in Asia, but require technological solutions that can cope with the demanding requirements of these fuels. CFBC has demonstrated its effectiveness in handling a wide range of coal types and combinations of coal and other fuels and is thought likely to feature strongly in Asia’s future fossil fuel power generation fleet. Although some operational challenges may be encountered, experience to date has demonstrated that such challenges can largely be overcome and are minimal compared with the advantages offered by the increased deployment of CFBC.
Source: Ian Barnes - Consultant, Hatterrall Associates
- Indo Tambangraya Megah (ITMG)
- Bukit Asam (PTBA)
- Baramulti Sukses Sarana (BSSR)
- Harum Energy (HRUM)
- Mitrabara Adiperdana (MBAP)
- Adaro Energy (ADRO)
- Bumi Resources (BUMI)
- Samindo Resources (MYOH)
- United Tractors (UNTR)
- Berau Coal
