- Dec 16, 2010Clean Coal Cometh by H. James Peters November 1, 2010
Above is the Isogo No. 2 system that has been running successfully since 2009.
Going for multi-pollutant control with mercury waste minimization, a system in Japan is able to control SO2, SO3, Mercury, N2 and NOX with a completely dry system, achieving pollution control rates about on par with gas-fired power production
In an August poll in Pollution Engineering's e-newsletter, over 31 percent of respondents said they believe "clean coal" to be a myth. Yet that has not dissuaded industry from trying. Perhaps this old myth is closer to becoming reality than many think. For the United States, which just so happens to be sitting on planet Earth's largest coal deposits, the possibility of clean coal energy is too good to pass up. And though it cannot eliminate CO2 emissions, a technology for cleaning mercury and other contaminants to impressive levels is currently operating just an ocean away.
In 2002, the Isogo Power Station in Yokohama, Japan, decided to replace two coal refinery units. When the second unit went online in 2009, the company was recognized for operating the world's lowest emissions intensity levels for SOX and NOX coal-fired power. The facility typically operates in the single-digit ppm concentration range for SO2 and NOX (against permit levels of 10 ppm and 13 ppm at Isogo No. 2). Particulate is less than 5 mg/Nm3 (0.005 lb/mmBTU), with over 90-percent control of elemental and oxidized forms of mercury.
The Isogo plant uses a completely dry system, , a multi-pollutant control technology developed by Hamon Research-Cottrell Inc., Somerville, N.J., for control of SO2, SO3 and mercury. The system is based on adsorption onto a moving activated coke bed, and further reduction of NOX to N2, with production of the saleable byproduct sulfuric acid, from the regeneration of the coke for return to the adsorber.
The basic process concept originated with Bergbau Forschung in Germany during the 1960s, with additional development through the 1980s by Mitsui Mining, and subsequent full commercialization by EPDC (now J-Power) as an advanced-generation multi-pollutant control. That company acquired the technology from Mitsui in 2005, and now operates three full-scale units at Takehara (1995), Isogo No. 1(2002) and Isogo No. 2 (2009), where the systems are used for utility, refinery, incineration and sinter applications.
General process description
The patented technology involves a three-stage process. Flue gas is contacted with a slowly moving bed of activated coke pellets. The activated coke removes SO2, SO3, NOX, mercury and other species through adsorption, chemisorption and catalytic reactions, which are enhanced in the presence of ammonia. Particulate is also reduced across the moving bed.
The adsorber design is determined based on SO2 control requirements and gas space velocity of 400 hr -1. Activated coke residence times of 80 hours are typical, with SO2 control levels greater than 95 percent. Activated coke surface area and long contact time in the adsorber bed is extremely favorable for mercury adsorption. NOX control also occurs as a co-benefit and can be further enhanced by incremental ammonia injection.
Pollutant-laden activated coke from the adsorber is transferred to a thermal regeneration stage, where pollutants are desorbed as a sulfur-rich gas stream. The regenerated activated coke stream is cooled and fine particulates are separated before returning to the adsorber. Make-up coke in pellet form is added to replace the separated fines. The third stage receives the sulfur-rich gas stream as a saleable byproduct.
Activated coke regeneration and mercury retention
In the regenerator, three heat-transfer zones accomplish preheating, final heating and cooling of the activated coke, with a sufficient time and temperature between the zones to approach complete desorportion and release sulfur rich gas (SO2, N2, CO2 and water) for production of marketable sulfuric acid.
Mercury is thermally desorbed at temperatures that are greater than 400°C to the vapor phase in the regenerator. Mercury desorption is complete at the bottom of the heating zone at 450°C, and activated coke and the fine particulates separated at the regenerator discharge are mercury-free.
The desorbed gases flow upward and counter current to the activated coke. At 200°C at the top of the heating zone, the activated coke effectively re-adsorbs mercury vapors such that all of the mercury that had been removed from the flue gas is confined in a small section of the regenerator.
The volume of activated coke in this zone allows mercury accumulation for two to three years before reaching saturation. At each of the 600 MW plants operated by Isogo, approximately 100 tons of mercury-laden activated coke is disposed during scheduled regenerator downtime every three years or approximately 0.06 tons/MW-year.
A comparison of the make-up carbon and disposal requirements for a plant using once-through activated carbon with a fabric filter or electrostatic precipitator compared to the regenerative activated coke process illustrates mercury-laden material disposal can be a huge issue. This is because fresh make-up activated coke replaces losses due to particulate separation after regeneration. Make-up for a 200-MW plant is approximately 1,000 tons per year (about 5 tons per year per MW). For a regenerator serving a typical 200-MW coal fired boiler, the disposal quantities for mercury-laden solids is approximately 12 tons per year or (0.06 tons per MW-year).
For typical activated carbon systems, 90-percent mercury control requires injection of powdered activated carbon (PAC) into the flue gas at rates between 2 lb per MMacfm and 10 lb per MMacfm, with higher rates generally associated with injection upstream of an electrostatic precipitator, and lower rates with injection upstream of a fabric filter.
When powdered activated carbon is injected upstream of the primary particulate control, whether it be an electrostatic precipitator or a fabric filter, the fly ash becomes contaminated with mercury-laden carbon, which may render otherwise saleable fly ash for cement unusable for that purpose, so that it must be landfilled.
The comparison table shows a dramatic reduction in the volume of mercury-contaminated waste for the regenerated activated coke process.
Above is a temperature profile of a typical ReACT system.
The process offers advantages for utilities, as requirements for mercury control are being added to increasingly stringent SO2 and NOX regulation:
Adsorption does not require water use.
Adsorption of SO3 resolves downstream plume visibility issues.
Regeneration of sorbent greatly reduces site logistics for reagent make up, processing and waste handling compared with other FGD processes.
Marketable byproduct sulfuric acid is produced.
Primary particulate control is upstream (existing), preserving beneficial use of ash.
Mercury control is obtained at greater than 90 percent levels for elemental and oxidized forms.
Mercury laden disposal streams are minimized.
SO2 control greater than 95 percent.
NOX control can be designed in the 20-80 percent range.
The repowering project replaced two 265-MW conventional PC boilers and their pollution control systems (electrostatic precipitator, wet flue gas desulphurization) with two 600-MW ultra-supercritical boilers and advanced pollution control. The emissions performance cited is on a par with natural-gas fired facilities. Such technology should be of interest for United States energy planning, as industry strives to make "clean coal" a reality. PE
H. James Peters
For more information, contact H. James Peters, V.P. Technology for Hamon Research-Cottrell Inc., in Somerville, N.J., at james.peters@.... ReACT is a trademark product licensed by J-Power Entech to Hamon Research-Cottrell for the U.S. market.
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