SO2 levels in today's coal-fired power plants can be reduced by wFGD (wet flue gas desulphurisation) or spray dryer absorption (SDA) - for example, NOx by selective catalytic reduction (SCR) and mercury by coal washing. Individually, though, these are not enough and flue gases need further processing for proper reductions. Perhaps the most promising technique is J-Power Entech's ReACT (regenerative activated coke technology). Combined with other methods, it retrofits to coal-fired plants to cut SO2 and NOx to only single-digit ppm levels - comparable with natural gas fired plants.
ReACT filters out pollutants using activated coke as a dry regenerable adsorbent, and is inserted downstream of a primary particulate control device, typically an ESP or fabric filter (FF). It can capture more than 98% of any remaining SO2 and SO3, 30% to 60% NOx, 90% mercury and 50% particulates. The technique is interesting to utilities burning PRB (Powder River Basin) or other low-sulphur coals. And, because the process uses only 1% of the water of conventional wFGD, it suits sites with difficult water supply, treatment or discharge.
It even produces salable sulphuric acid byproducts. The process has been used on several sites in Japan, and demonstrated in the US at the North Valmy power station by Hamon Research-Cottrell as part of an EPRI (Electric Power Research Institute) project.
Inserted downstream of electrostatic precipitator
Ammonia is injected into the upstream flue gas, which then contacts a slowly moving bed of activated coke in pellet form as the process sorbent. Activated coke is easier to produce and as a result less expensive than activated carbon. It is produced by steam activation at around 900°C. It has a surface area of 150m²/g-300m²/g, less than activated carbon but much higher than the metallurgical coal.
The activated coke removes SO2, SO3, NOx, Hg, and other pollutants through adsorption, chemisorption and catalytic reactions that are enhanced in the presence of ammonia. The process does not consume water or lose water to waste disposal streams, and performance improves with lower back-end flue gas temperatures. The total carbon surface area in contact with flue gas and the contact times are much higher than those for activated carbon injection of an ESP or FF. Impact with the coke pellets across the moving bed also captures particulates. The pollutant-laden activated coke sorbent is processed in a thermal regenerator vessel which completes the reduction of NOx to N2 and driving off the SOx.
Activated coke at flue gas temperatures is fed through a lock hopper to the top of the regenerator. The coke falls downwards through three indirect heat exchanger sections for preheating, heating and cooling the regenerated coke, which is then discharged through a lock hopper.
The sulphur-rich gas (SO2, N2, CO2 and H2O) flows to an acid plant for conventional production of marketable sulphuric acid, and there are no steam or SO3 plume releases. The activated coke can adsorb fairly large amounts of mercury, which is retained in the coke and screened to remove fines before being returned to the absorber.
According to H James Peters, executive vice-president of products and technology for Hamon Research-Cottrell, fresh make-up activated coke replaces losses due to fines separation after regeneration and carbon consumed in regenerator reactions. "Make-up is around 4,000kg/y/MW to 6,000kg/y/MW. This mercury-free material is either disposed of, used as fuel or re-used as a carbon sorbent at other sites."
Using and disposing of the carbon
The granular activated coke is repeatedly recycled between adsorption and regeneration towers, with mechanical wear gradually reducing the size of the granules. Reactions in the regenerator also use some carbon, and a make-up stream of activated coke replaces these losses. The activated coke supply rate is however less than 1.5% of the circulating rate.
Unlike other catalysts, which tend to deteriorate over time with activity loss due to aging, fouling, and contamination, the performance of the ReACT's activated coke actually increases over time. The regeneration improves the activity by exposing fresh surface and micropores through carbon reactions, and increasing the number of functional groups on the catalyst surface from residual sulphur, oxygen, and nitrogen. Regenerating the sorbent greatly reduces site logistics for reagent make-up processing and waste handling compared with other FGD processes.
Clean coal generation?
ReACT has the type of performance that will be needed to meet stricter controls. Most notably, a retrofit has made J-Power's 2×600MW Isogo plant near Yokohoma in Japan the world's cleanest (lowest emissions intensity) coal-fired plant. It burns low-sulphur coal in high-efficiency ultrasupercritical boilers with low-NOx burners and controls, along with primary SCR, electrostatic precipitator (ESP) and ReACT.
When Isogo was repowered in 2002, the plant was more than 30 years old. J-Power replaced the two vintage coal-fired units with two 600MW units, more than doubling generating capacity while actually cutting emissions. Isogo now typically operates in the single-digit ppm concentration range for SO2 and NOx, less than 5mg/Nm³ for particulates, and better than 90% control of both elemental and oxidised mercury. Which of course just leaves the CO2.
Improved power-generating techniques will help reduce carbon dioxide emissions, with the best coal-fired plants (and Isogo is one of the most efficient) still only reaching just above 40%. Proper zero-emissions, though, will need CO2 capture and storage (CCS). It is the great hope for making the coal generating industry truly clean, but whether it will be practical and cost effective is a different matter. With another weak agreement at the recent Cancun climate summit, we are running out of time.