Outline of technology
1.Background
Soot and dust are regulated, like NOx, under the provisions for by-fuel kind/-boiler size concentrations. At present, however, there are some regions subject to the total amount control which limits the overall emissions from each region as in the case of sulfur and nitrogen oxides. In coping with such regulations, an electrostatic precipitator was employed in Yokosuka Thermal Power Plant for the first time in the world in 1966 for mainly other thermal power plants to follow suit. Development of pressurized-
bed combustion and coal gasification combined-cycle technologies is also under way for high-efficiency power generation, using cyclones and ceramics/metal precision-removal filters.
On the other hand, trace materials-related control is now being intensified such as through the addition of boron (B) and selenium (Se) to waste water materials to be regulated and U.S. regulations on mercury (Hg).
2.Technology
(1)Electrostatic precipitation method
Outline
The electrostatic precipitation method is a method to remove soot/dust in exhaust gas in accordance with the theory that any dust negatively charged by corona discharge at the discharge electrode adheres to the positive dust-collecting electrode. The soot/dust that adhered to the electrode is allowed to come off and drop by giving a shock to the electrode with a hammering device. The dust collection performance depends upon the electrical resistance of soot/dust, preferring particles of 104-1011 -cm in the apparent electrical resistivity range. In the case of pulverized coal thermal, electrical resistance of many particles is high, urging various measures to be taken against high-electrical resistance particles.
As one of such measures, temperature conditions for dust collection are adjusted. Electrical resistance changes as shown in Fig.2. Those successfully developed/commercialized from the use of such characteristics are a high-temperature electrostatic precipitator run at 350O
C, a higher temperature than that of conventional low-temperature electrostatic precipitators (130- 150O
C) to lower electrical resistance and an advanced low- temperature electrostatic precipitator run, with its electrical resistance lowered by operating it at a dew point-or-lower temperature (90-100O
C). Other than these, successful commercialization is found in the moving electrode method which brushes off soot/dust by moving the electrode to prevent back corona due to dust accumulation at the electrode and a semi-wet electrostatic precipitator where a film of liquid is shed to wash away dust. Another measure has also been taken with electric discharge methods commercialized, including an intermittent charge method to supply a pulsed voltage on the order of several milliseconds and a pulse charge method for tens of microseconds-order energization.
At present, leading pulverized coal-firing plants built in and after 1990 are provided with very low-temperature electrostatic dust-
DC high voltage Flue gas (from boiler) Power supply Discharge electrode Collected soot/dust Dust collecting electrode
Fig. 1 Theory of Electrostatic Precipitator
Electrical resistivity ( -cm)
Low-alkali /high-sulfur coal
Low-alkali /low-sulfur coal
Advanced Low-temperature ESP Conventional low-temperature ESP
High-alkali /high-sulfur coal
High-alkali /low-sulfur coal
High-temperature ESP
Members in charge of research and development:
Mitsubishi Heavy Industries, Ltd.; Hitachi, Ltd.; Sumitomo Heavy Industries, Ltd.; others
1) "Thermal/Nuclear Power Generation Companion, 6th edition", Thermal/Nuclear Power Generation Association, 2002 2) Sato, "Design and Actual Operations of High-Temperature Electrostatic Precipitators", Thermal/Nuclear, Vol. 34, No. 4, 1983
3) Tsuchiya et al, "Technology Development of Low Low-Temperature EP in Coal Thermal-Purpose High-Performance Flue Gas Treatment System", MHI Technical Journal Vol. 34, No. 3, 1997
4) "Toward the Realization of Integrated Gasification Combined-Cycle Power Generation", CRIEPI Review, No. 44, 2001 5) Coal-Based Next-Generation Technology Development Survey
/Environment-Friendly Coal Combustion Technology Field (Trace Element Measurement/Removal Technology), NEDO results report, 2003
References
(2)High-temperature dust collection method
Outline
This is under development as a technology to remove soot/dust from hot gas for pressurized-bed combustion and integrated gasification combined-cycle (IGCC) power generation processes. This technology utilizes a multi-cyclone, ceramics- or metal- based filter and the granular-bed method using silica or mullite material where soot/dust is removed. Multi-cyclones are mainly used for rough removal. Those used for precision removal include ceramic or metal filters of a cylindrical porous body into which SiC and other inorganic materials and iron-aluminum alloys are formed (Photo 1). Such filters used here were developed by Nihon Garasu Kogyo or overseas by Paul Schumacher. Such technologies, now under durability evaluation/demonstration, are used for pressurized-bed combustion power generation at Hokkaido Electric Power’s Tomato-Atsuma Thermal Power Plant, with their verification tests conducted not only for IGCC at 200-ton/day Nakoso IGCC Pilot
Plant but also under the EAGLE Project. It is also expected to be used at an IGCC demonstration unit (250KkW) slated for initial operation in 2007.
(3)Mercury removal method
Outline
Among trace contents of coal, mercury is cited as a material released into the atmosphere at the highest rate, with about 30 percent of the failure to be removed at precipitators/desulfurizers proved to be released. However, bivalent mercury (Hg2+) is nearly all removed, leaving nonvalent one (Hg) as discharge matter. A method to remove this nonvalent mercury is under active review. The Center for Coal Utilization, Japan, though still in the process of basic research, selected active carbon, natural
inorganic minerals, and lime stone as materials which absorb mercury and is now evaluating their absorption characteristics. The research results prove that, if the method to inject active carbon or an FCC ash catalyst into the flue for the removal of metal mercury is combined with removal at the flue gas desulfurizer, 90% or more mercury can be removed easily.
Photo 1 SiC Ceramic Filter
Members in charge of research and development: Mitsubishi Heavy Industries, Ltd.;
Hitachi, Ltd.; Kawasaki Heavy Industries Co., Ltd.; others
Members in charge of research and development: Center for Coal Utilization, Japan
1) Nakayama et al, "Development State of 3 Dry Gas Cleanup Methods in Dry Gas Cleanup Technology Development for Integrated Gasification Combined-Cycle Power Generation", the Japan Institute of Energy Journal, Vol. 75, No. 5, 1996
2) Shirai et al, "Coal Gasification Gas-Based Fixed-Bed Dry Desulfurization Technology Development", the Society of Powder Technology, Japan’s journal, Vol. 40, No. 8, 2003
3) M. Nunokawa et al, "Developments of Hot Coal Gas Desulfurization and Dehalide Technologies for IG-MCFC Power Generation System", CEPSI proceedings 2002 Fukuoka
References