Supercritical Pulverized Fuel Power Plants in China

Takeo Yamada
Jun Wada
 
 

1.  Introduction

Along with rapid economic growth, China will keep their significant energy consumption growth rate in early years next century. China is the second largest energy consuming country in the world following the USA and the world's largest coal producer and consumer. Coal consumption in China was 1,373 million tons in 1997 and coal accounts for 73.5 % of their total primary energy consumption. Although China is trying to diversify their energy supply source structure, they will likely need to still depend on coal next decades because of its competitive cost and inadequate other energy source availability.

Power generation industry in China also heavily relies on coal. Installed capacity of total electricity generation in 1998 was 270 GW, of which 70 % was coal-fired. Many of their plants are old and small in size. Capacity additions of electricity generation in the foreseeable future will be enormous to meet high growth rate of electricity demand and coal-fired power generation will remain a major source of the electricity supplies in the future. The State Power Corporation of China estimates that their installed capacity will be 536 GW and coal will hold 69 % of total capacity in 2010.

China is now facing significant environmental issues, which affect all parts of society and should be solved immediately. Many of environmental issues facing China are caused by large-scale and poor coal use and Clean Coal Technologies (CCTs) have a great potential to solve the issues.
 
 

2. Present Status of Supercritical PF (SCPF)

Most of coal-fired plants built today throughout the world use sub-critical pulverized fuel (PF) technology. The conventional sub-critical steam pressure cycle at 16 MPa and 540 deg-C has been dominant design and electricity generation efficiencies of the plants are in the range of 37% to 39% based on LHV, depending on local conditions, cooling seawater temperature. Efforts to improve the efficiencies with higher steam cycle operating pressures and temperatures have been kept for decades. The first attempt to introduce supercritical steam cycle was made at Philadelphia Electric Company's 350 MW Eddystone #1 plant, which was built and commissioned in 1958 aiming to improve net efficiency of the power plant. It was designed to use 34.4 MPa, 649 deg-C main steam and 566 deg-C reheat steam. The steam used in this plant was supercritical condition, far beyond critical condition of water (22.1 MPa and 374.2 deg-C). The need for high creep strength under such high pressure and temperature led to the use of austenitic stainless steels for hot and pressurized parts such as the superheater tubes. However, austenitic stainless steels inherently are sensitive to thermal fatigue and the plant suffered steam line failures due to long term embrittlement and fatigue combined with low thermal conductivity. As a result, slightly lowered steam condition has been applied for new plants, typically at 24.1 MPa/538 deg-C for main steam and 566 deg-C for reheat steam, which is so called supercritical cycle (SC) and had been common with newly built coal-fired power plants for a decade in Japan. Early problems in the first generation SCPF have been overcome and the reliability and availability of SCPF plants are now comparable to conventional (sub-critical) PF plants. The nominal design efficiencies of SCPF based on LHV are around 41-42%, much higher than the conventional PF power plants. This SCPF technology is now popular design for power plants and most economical in Japan. Since lately invented ferritic steels, typically containing 9-12% chromium, enable to keep the main steam temperature higher, it is possible to design the power plants at more severe steam conditions than SC, which is called advanced supercritical or ultra-supercritical (USC). In 1990s, a number of USC were constructed and commissioned in Japan, main steam temperature of which has been raised up to 610 deg-C (Table-1). However, main steam pressure still remains in that of SC level at present. Europe is trying to develop new technology, which uses much higher steam conditions, such as 32.5 MPa/610 deg-C, using superior high creep strength metals.

Comparison of SCPF and conventional PF performance and costs has been made by Coal Industry Advisory Board (CIAB, Regional Trends in Energy-Efficient, Coal-Fired,

Power Generation Technologies). An analysis of comparative performance and cost was carried out for a plant with two (2) 600 MW pulverized coal-fired plants in a European and Asian location.

The following economic parameters were used:

Four scenarios including conventional plants, SCPF plants, USC plants and USC plants with post-combustion sulfur and nitrogen oxide controls have been evaluated in the report. Comparison between 2400 psig/1000 deg-F (166 bar/538 deg-C) conventional plants, 3500 psig/1000 deg-F (240 bar/538 deg-C) SCPF plants and 4500 psig/1100 deg-F (311 bar/593 deg-C) USC plants will be shown in this paper. The analysis was carried out for two variants of capital cost and coal price. The higher capital cost (800 $/kW) corresponds to that for a plant built in a developed country, and the lower capital cost (620 $/kW) corresponds to that for a similar plant constructed in a developing country. The lower coal price (16.50 $/t) might be that for a mine mouth coal plant in a developing country, and the higher coal price (44.00 $/t) might be the landed price of internationally traded coal at a coastal power plant. The performance of both technologies is shown in Table-2.

Table-2

Comparison of Efficiency and Environmental Performance

Between Conventional PF, SCPF and USC Technologies

Fuel Cost Reduction Emission Reduction (40 years)

(million $/year) ( thousand tons)

Technology Efficiency 16.50$/t 44.00$/t CO2 SO2 NOx Particulates

Conventional 38% base base base base base base

3500 psig SCPF 41% 3.0 5.2 14,500 61 22 222

(8%) (8%) (8%) (8%)

4500 psig USC 45 6.0 10.9 27,200 120 43 432

(15%) (15%)(15%) (17%)
 

SCPF plant has 3% higher efficiency than conventional plant and emission over 40 years of SCPF will be 8% less than that of the conventional plant. USC plant has 7% higher efficiency than conventional plant and emission over 40 years will be 15 % less than that of the conventional plant.

Fuel cost reduction of SCPF over the conventional plant is estimated to be $3.0 million and $5.2 million every year for lower coal price and higher coal price, respectively. Fuel cost reduction of USC is twice as much as that of SCPF, $6.0 million and $10.9 million for lower coal price and higher coal price, respectively.

The capital cost comparisons between the different technologies for the higher capital cost case are shown in the Table-3. The plant would have two units with low NOx burners, high efficiency dust collection equipment, once-through seawater cooling, including the switch yard and all the facilities for a new site location and a 60 month construction schedule. The capital cost estimates include the plant equipment, structures, switchyard and coal unloading facilities. Land, development, financing and owners costs are not included.

Table-3

Capital Cost Comparison

conventional SCPF USC

Boiler 142.94 $/kW 153.09 $/kW 163.52 $/kW

Boiler plant piping 27.81 $/kW 31.03 $/kW 31.81 $/kW

Feedwater systems 28.06 $/kW 28.62 $/kW 29.18 $/kW

Turbine-Generator 79.20 $/kW 82.37 $/kW 83.95 $/kW

Turbine plant piping 16.25 $/kW 15.44 $/kW 15.43 $/kW

Remainder of plant 509.17 $/kW 500.69 $/kW 487.17 $/kW

Total plant cost 803.43 $/kW 811.07 $/kW 811.08 $/kW

(base) (101.0%) (101.0%)

The incremental capital cost associated with a SCPF or USC plant compared to a conventional sub-critical plant is not significant (1%). This is because the capital cost increase specific to the SCPF plant associated with superior materials and other design features are counter-balanced by capital cost savings due to the fact that the boiler and ancillary equipment can be smaller as a result of the increased efficiency.

When the higher capital cost is used in the analysis, going from conventional PF to SCPF in the lower coal price case reduces the electricity cost by 0.08 cents/kWh, and in the higher coal price case by 0.23 cents/kWh. The corresponding reductions in going from conventional to USC are 0.14 cents/kWh in the lower coal price case and 0.48 cents/kWh in the higher coal price case. When the lower capital cost is used in the analysis, the corresponding reductions in going from conventional to USC are 0.13 cents/kWh in the lower price case and 0.47 cents/kWh. To summarize the studies, SCPF and USC have increased generating efficiency and lower generating costs than those of conventional over the broad range of international experience with capital and fuel costs and the economic competitiveness of SCPF and USC is generally applicable.
 

3. Advanced Clean Coal Technology(CCT)

Besides improving steam conditions of SCPFs, another efforts have been made to develop advanced coal-to-power processes. Pressurized Fluidized Bed Combustion (PFBC) and Integrated Coal Gasification Combined Cycle (IGCC) are regarded as principal clean coal technologies which would be effective in reducing CO2 emmission. Table -4 shows their aspects compared with that of SCPF.

PFBC is coal combustion boiler installed in a pressure vessel. Besides generating steam for the steam turbine, pressurized exhaust gas (1-1.5 MPa) from the vessel can also drive gas expansion turbine which produces additional power. Therefore, it has higher thermal efficiency than SCPF if they have the equal steam conditions. In Europe and in the USA the first PFBC plants were introduced in early 1990s with capacity of 70-85MW so as to demonstrate the technology. However, they suffered from a number of troubles which caused shutdowns and sometimes serious damage on equipment. Since these problems have not completely been concurred, PFBC is not yet a well-proven technology as SCPF, though larger PFBC plants up to 350MW are under construction.

IGCC is a combined system of gasifier and combined cycle power plant. Each component, coal gasification alone for chemical process and combined cycle alone for power generaion, had been almost proven. As the combined cycles, firing natural gas, have achieved extremely high efficiency (approx. 48% on HHV basis), IGCC is expected to have the highest efficiency of the CCTs. In late 1990s several commercial-scaled IGCC plants having 250-350MW capacity were introduced using different types of coal gasifiers. They also faced complicated troubles, some of which were the results from system integration of both technologies, gasifier and combined cycle, and the others were due to gasifier scaling up. They are not in the stage where high efficiency of them is demonstrated. Therefore, IGCC is also still an under developing technology.

A certain amount of cost will be necessary before PFBC and IGCC become fully matured. This development cost will directly reflect on newly build plants using these technologies, which may result in relatively higher plant cost than that of conventional PF system.
 

4. SCPF in China

The current coal use technologies in China are characterized as low efficient and high polluting. Most of present coal-fired electricity generating plants in China is small in size, although constructions of large size of plants such as 600 MW and 800 MW are now underway. 75 % of total capacity is shared by small plants less than 300 MW. The small size of plants, combined with inconsistent coal quality and low plant availability, show low electricity generating efficiencies ranging from 27 % to 29%, significantly lower compared to those of developed countries' plants (35-40 %). These plants should be replaced by new high efficient power plants to improve the performance. For next decades, the major capacity addition and replacing of electricity generation in China is still expected to be coal based technologies including conventional sub-critical PF, SCPF, USC, PFBC and IGCC.

Factors to affect the choice of technologies are economical factors, investment cost, operating cost and maintenance cost, reliability and availability and environmental benefits. Achieving greater electricity conversion efficiency produces lower emissions of acid gases and CO2, and it is considered cleaner technology. As mentioned previous chapter, SCPF and USC technologies operate at higher steam temperature and pressure levels than conventional sub-critical PF and they achieve greater efficiencies than conventional PF. PFBC and IGCC technologies use both a steam cycle and gas cycle to achieve further high efficiencies. While the choice of the most efficient technologies such as PFBC and IGCC is beneficial, they have the following crucial disadvantages.

(1) high capital cost

(2) high maintenance cost

(3) unproven and unreliable

(4) operation difficulty

Though SCPF requires special materials and manufacturing capabilities because of the higher temperature and pressure operating conditions. It is generally thought that SCPF is a commercially proven, reliable and cost competitive technology where the specialized materials and manufacturing capabilities are available. And SCPF technology is also perceived upgraded to USC quite easily.

China is just at the beginning of introducing SCPF. At present there are two 600 MW and two 500 MW SCPF plants already in operation. Another four SCPF plants are under construction (Table-5). Though their main steam temperature is no higher than 540 deg-C, it is expected to be raised by steps as new plants are planned. Technology transfer of both manufacturing SCPF plants and specialized materials in Chinese industry is a large potential to diffuse these technologies in China and improve energy supply security and environment. Technology transfer of design and manufacturing for SCPF plants is underway, while specialized materials for the plants are currently imported. Once they obtained the technology for SCPF materials, the introduction of SCPF would be accelerated.