Tests at a 4,000-tpd plant in Pennsylvania showed that the use of a wet scrubber utilizing limestone raw feed as the scrubbing medium could significantly reduce emissions from the production process.

Within the system, the raw feed limestone reacts with the SO2 contained in the offgas to form an inert buffered calcium sulfite/bisulfite/sulfate slurry, which is then recycled to two locations within the operation process: the spray tower, which cools the kiln gas from 400 degrees to 180 degrees C by injecting slurry into the hot gas stream; and the roller mill, which takes the kiln discharge and crushes the limestone to a fine powder.

The slurry is injected into the mill to control dusting as the limestone is crushed. This results in no net discharge of liquid from the process, thus avoiding trading one environmental problem for another.

The DynaWave Reverse Jet scrubber quenches the hot inlet gas and removes SO2 and particulate. The scrubber consists of an open duct, into which the limestone slurryscrubbing liquid is injected counter-current to the gas to form a region of extreme turbulence, effectively cleaning the gas. The design of the equipment permits the recirculation of slurry containing 15% to 20% suspended solids without pluggage or solids settling, erosion, or equipment downtime. This high solids handling ability allows the recycling of the slurry to the plants spray tower and roller mill to meet existing gas cooling, gas conditioning, and dust control requirements.

Raw limestone at the plant is relatively large in size (13%>325 mesh, versus no more than 5%>325 mesh in other systems) and contains a high percentage of species such as silica and quartz, which are highly erosive to piping and scrubber equipment. Monsanto EnviroChem systems, the supplier of the DynaWave scrubber, claims that proper materials selection and careful design to avoid slurry settling and erosion allows the equipment to cope adequately with this particular set up.

The wet scrubber also is said to have the ability to handle sudden unpredictable upset conditions in the upstream wet electrostatic precipitator, which can increase the system's particulate loading from 37 mg/AM[superscript]3 (15 kg/h) to 625,000 mg/AM[superscript]3 (280,000 kg/h) almost instantaneously. This upset condition occurs for three to five minutes, one to five times per month.

The scrubber features belt-driven slurry pumps with casings and impellers constructed of CD4MU nickel-hardened, erosion-resistant alloy and pinch-style elastomer diaphragm valves.

Slurry settling is discouraged through the use of continuous agitation and movement of the liquid in the tanks and through recycle loops in the piping, and a series of automatic water flush valves throughout the piping system, which are interlocked to slurry flow.

Performance The SO2 and particulates removal results have met all expectations since the scrubber was installed. The only problems to date have been a limited amount of initial erosion on some of the slurry recirculating piping and valves due to impurities such as silica and quartz in the limestone feed, together with those in the iron admixture. This erosion was eliminated by redesigning of those portions of piping to eliminate excessive liquid turbulence and the elimination of the iron from the scrubber feed.

An initial second problem was buildup of slurry carryover on the chevron demister in the top of the scrubber vessel.This was resolved by automating the chevron washing spray system to increase the consistency and frequency of the washing. Overall, the system has operated reliably with little downtime and no problems of slurry pluggage.

Other installations The first installation of Reverse Jet Scrubber technology in the UK cement industry will be at the Castle Cement plant in Ribblesdale. A mid-1998 startup is expected.