1. Background
The technology behind the wet desulfurization process has also matured but requires a significant amount of service water as well as advanced wastewater treatment measures. Meanwhile, an already commercialized ammonia-based selective catalytic reduction (SCR) process requires not only long-term control of expensive DeNOx catalysts but also measures to prevent ammonia leaks. Development efforts are, therefore, underway for a dry combined desulfurization DeNOx method to remove NOx and SOx simultaneously without requiring any service water, wastewater treatment, or a DeNOx catalyst.
2. Technology
(1) Active carbon adsorption method
Research and development: J-POWER; Sumitomo Heavy Industries, Ltd.; Mitsui Mining Co., Ltd. Project type: Voluntary support project for coal utilization promotion
Overview
The active carbon adsorption method causes a reaction between SO2 in exhaust gas and injected NH3 on active carbon at 120-150°C, thereby converting SO2 into ammonium hydrogen sulfate (NH4HSO4) and ammonium sulfate ((NH4)2SO4) for adsorption/removal while decomposing NOx into nitrogen and water as does the SCR process. Figure 1 shows the process. The moving-bed adsorption tower (desulfurization tower) removes SO2 in the first-stage and, in the second stage (denitration tower), NOx is decomposed. This method first removes SOx and then NOx since, as shown in the figure, the presence of high- concentration SO2 tends to decrease the effectiveness of NOx removal.
Active carbon that has absorbed NH4HSO4 is heated to 350°C or higher in the desorption tower to desorb NH4HSO4 after decomposing it into NH3 and SO2, while the active carbon is regenerated. SO2 can be adsorbed and removed in the form of sulfuric acid (H2SO4) even if NH3 is not injected in the desulfurization tower. However, since the following reaction with carbon occurs during desorption, consuming active carbon, NH3 is added at the time of desulfurization to prevent such active carbon consumption.
H2SO4 → SO3+H2O
SO3+0.5C → SO2+CO2
Coal is used to reduce desorbed SO2 into elemental sulfur at 900°C for recovery. There is another method that oxidizes SO2 into SO3 to recover it as sulfuric acid.
During the development of this technology, carried out at J- POWER’s Matsushima thermal power plant, first, an active- carbon desulfurization method (with an adsorption tower) that can treat 300 Km3N/h (90MW-equivalent) of gas was subjected to verification tests (1983-1986), obtaining removal efficiency of 98% for SOx and 30% for NOx. To improve the DeNOx removal efficiency, a combined desulfurization DeNOx pilot plant that can treat 3,000m3N/h of gas with two towers was tested (1984-1986). SOx was almost completely removed by the desulfurization tower in the first stage, while 80% of NOx was removed. This technology, verified after being up-scaled to a capacity of 150Km3N/h, was introduced in 1995 to the DeNOx unit of the No. 2 unit of the Takehara coal thermal power plant’s normal- pressure fluidized-bed boiler (350MW), and is currently in operation. This technology was also installed as a desulfurizer in 2002 at J-POWER’s Isogo thermal power plant’s No. 1 new unit (600MW) (Photo 1). Although this technology is currently in operation, there are no cases of it being used as a combined desulfurization-denitration system.
Fig. 1 Active carbon method desulfurization process
Photo 1 Active carbon method desulfurizer
(2) Electron beam process
Research and development: Ebara Corporation; Chubu Electric Power Co., Inc.; Japan Atomic Energy Research Institute; others Project type: Voluntary
Overview
The electron beam process, as shown in Figure 2, involves using an electron beam to irradiate SOx/NOx in exhaust gas and injected NH3 to cause a reaction for their recovery as ammonium sulfate ((NH4)2SO4) or ammonium nitrate (NH4NO3) in the downstream precipitator. Byproducts: ammonium sulfate and ammonium nitrate, which are used as fertilizers. Removal efficiencies of 98% or more for SOx and 80% for NOx, at an NH3/NO molar ratio of 1, is obtained at 70-120°C. The NOx removal efficiency also characteristically increases with higher SO2 concentrations, though SOx removal efficiency does not affect the concentration of SO2 at the inlet.
For this process, technology development was undertaken by Ebara Corporation and U.S. partners, including DOE, which made joint contributions from 1981-1987. In Japan, based on the development results, a pilot plant that can treat 12,000m3N/h of gas was built at Chubu Electric Power’s Shin-Nagoya power plant, where the technology was verified from 1991-1994.
Regarding this technology, a plant that can treat 300Km3N/h (90MW) of gas (Photo 2) was built at Chengdu Heat-Electricity Factory, a co-generation power plant in Sichuan Province, China. It is currently being operated for demonstration and is obtaining a NOx removal efficiency of 80%.
Fig. 2 Process flow for electron beam desulfurization process
Photo 2 Electron beam desulfurizer
References
1) Hanada et al., "Dry Desulfurization-Denitration-Technology Dry Active Carbon-Method Sulfur Recovery Formula at Coal Thermal Power Plant" Thermal/Electronic Power Generation, Vol. 40, No. 3, 1989.
2) "Renewing Isogo Thermal Power Plant" pamphlet, J-POWER.
3) Aoki, "Electronic Beam Flue Gas Treatment Technology," Fuel Association Journal, Vol. 69, No. 3, 1990.
4) S. Hirono et al., "Ebara Electro-Beam Simultaneous SOx/NOx Removal," proc 25th Int Tech Conf Util Sys, 2000.
Source: New Energy and Industrial Technology Development Organization (NEDO), Japan Coal Energy Center (JCOAL)
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