To construct a pilot plant to demonstrate the feasibility of removing sulphur and nitrogen oxides from flue gases when burning coal e. g. converting an oil-fired boiler into a coal-fired one, using low temperature catalysts. See projects CS/15/85/DE and CS/1 /88/DE.
This contract is concerned with the construction and testing of a pilot plant which was completed in April 1985 and tested up to March 1986. Further contracts, CS/15/85 and CS/00001/88 cover the construction, commissioning and testing of a demonstration plant.
After initial difficulties the FDG-plant was operated at a removal rate
of 90 %.
In both reactors of the DENOX-plant catalysts of japanese and german production were tested. The catalysts differ mainly in the geometric structure (quadratic or triangel) and the specific geometric surface area (430 m2/m3, 750 m2/m3 or 860 m2/m3).
After 8 500 respectively 5 700 hours of operation at temperatures between 290 deg. C and 350 deg. C the japanese catalysts showed neither decrease of the NO-removal rate nor of their activity.
The same results have been attained with the catalysts of german production after an operation time of about 2 000 hours.
Further tests at 270 deg. C reaction temperature for 2 100 hours resulted neither in ammonium salt deposits on the catalyst surface nor on the surface of the heat exchanger. At the given construction of the catalysts there was also no loss of NO-removal rate in comparison to the operation at higher reaction temperatures
During normal operation the ammonia slip was distinctively below 1 vpm.
Measurement of SO3 behind the DENOX-reactor yielded concentrations of about 3-5 vpm.
For cleaning the flue gas preheater 10 l rinsing water per m2 heating surface are necessary to yield a 95 % cleaning capability.
After two and a half year operation time the experience shows that a DENOX plant downstream the FDG allows the use of catalysts with a distinct higher specific geometric surface area respectively a smaller pitch compared to catalysts for high dust SCR DENOX plants (SCR-plant between steam generator and air preheater). Compared with the high dust configuration the necessary catalyst volume for a DENOX plant downstream the FDG is about 50 % lower. Due to the comparatively low catalyst activity loss during the operation time a catalyst lifetime of 5 years is expected.
The pilot plant is comprised of a flue gas desulphurisation system (FGD) wet scrubber with limestone as absorbed and gypsum as final product. The flue gas flow rate is 10,000 m3/h NTP (wet). There is also a downstream DENOX plant in two x 5,000 m3/h lines (wet):
Flue gas to be cleaned is tapped as a partial flow of the flue gases from Units 2 and 4 of the Heilbronn thermal power station (hard coal burning power units with slag tap firing, in each case) downstream of the electrostatic precipitator: then it is fed to the absorber by a booster fan.
The flue gas flows through the absorber from bottom to top. Within the spraying zone, the flue gases are brought into contact with the scrubbing suspension flowing in the oppposite direction. In this way, the oxides of sulphur, hydrogen chloride and hydrogen fluoride are, to a large extent, removed.
Following the FGD plant, the flue gas flow is split into two partial flows and supplied to both DENOX lines.
The flue gases which emerge at +/- 50 deg. C from the absorber are heated to the required 250-350 deg. C reaction temperature prior to entry into the DENOX reactors: these are heated by propane surface burners.
As the process causes no heat loss within the DENOX reactor it is possible to retrieve the heat content of the flue gases prior to discharging to the chimney with a heat exchanger system. During continuous operation all that is then necessary is to heat the flue gases by that temperature difference corresponding to the terminal temperature difference of the heat exchanger system.
Prior to entry into the DENOX reactor, ammonia in the form of ammonia/air mixture is injected into the flue gas in an amount corresponding to the NOX mass flow to be reduced.
Within the two DENOX lines of the pilot plant, two catalysts of different compositions are on trial: these exhibit optimum NOX removal rates over differing temperature ranges.
Within each reactor, four catalyst layers are installed 3. 5 meters apart, in reactor sections which may, for ease of access, be withdrawn.