NEWS RELEASE MAY 2015
Hundreds of Options Facing Power Plants Purchasing New or Upgrading Wet Calcium FGD Systems
The wet calcium (lime and limestone) process is only one option for removing SO2. Dry systems, ammonium sulfate, seawater, amine and other processes are also alternatives. Assuming that the purchaser has decided to purchase a new or upgrade an existing wet calcium FGD system, he must then make hundreds of decisions about processes and components. These systems represent up to 15 percent of the entire power plant investment. So it is important that all options be considered and the best selected. Power Plant Air Quality Decisions is a program providing utilities with a continuing and thorough analysis of issues and options.
The first set of options involves system design. If you select the less expensive limestone process, you have low operating but high capital cost. If you opt for lime, the reverse is true. If you need to elevate the discharge gas temperature to meet local regulations, then you need to consider gas-to-gas heat exchangers before and after the scrubber. But you should avoid this expensive and maintenance prone process if you can.
There are a variety of scrubber designs falling into four main categories: spray tower, tray tower, sump and hybrid. Spray towers require large quantities of slurry. Tray towers use much smaller pumps but the fan horsepower is greater. Sump scrubbers are compact but also require more fan horsepower. A hybrid version is the rod or pipe scrubber which is being touted by at least one major supplier. A double contact scrubber is offered by another. The conventional wisdom has been that laminar flow and optimum droplet dispersion causes the best results. The purchaser should be cautioned to consider that turbulence rather than laminar flow may provide the best results.
When spray towers are used, the performance of the nozzles is critical. Hollow cone or full cone, downflow vs. upflow and other arrangements need to be analyzed. Mist eliminators are also critical. Escaping mist adds to emissions and can cause maintenance problems. Design considerations include:
- Chevron or non-chevron
- Shape and number of turns
- Spacing between each chevron
- Number of mist eliminator stages
- Pressure drop vs. efficiency
The selection of materials is critical due to the abrasive and corrosive environments. The inlet to the scrubber and the scrubber walls has proved most challenging to materials suppliers. Stainless, alloys, titanium, FRP, plastic and rubber lining, and non-metallic mineral linings are all being utilized. One problem is that the corrosive impact is controlled by the operator. If he recirculates more slurry and bleeds less, the chloride level can rise to levels which eliminate most material choices. The temperature is also controlled by the operator. If the pumps fail, the scrubber can quickly exceed temperatures beyond the FRP limit.
Big recirculating slurry pumps may be required to move hundreds of thousands of gallons per minute. Many improvements have recently been made to reduce maintenance and improve efficiency. The purchaser needs to determine which vendors have made these improvements.
Slurry valves are equally important. Two options are knife gate and butterfly valves. Both are being used with butterfly valves showing more use in Europe.
The power plant has to decide whether to make wallboard quality gypsum or just a material for disposal. The gypsum quality is influenced by the oxidation blower. Both single-stage and multi-stage blowers are available. Cost, energy consumption and other factors differ.
Recent component improvements are important enough that the purchaser of a new system should make sure that the system he purchases incorporates them. Operators who are upgrading systems should also make sure they are aware of these advances.
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