Improving Precipitator Performance at Pulp and Paper Mills [save as PDF]
Opportunities for reducing opacity from power and recovery boilers
Challenges Unique to Paper Mills
Paper mills face some unique challenges when it comes to optimizing the performance of electrostatic precipitators (ESPs). Resistivity, for example, is a key issue for power boilers fired by wood waste (bark burners) because often the ash is high in calcium. Carbon carryover (loss on ignition, LOI) and heavy inlet ash loading can also limit ESP performance.
For recovery boilers, dust resistivity is typically not a problem, but collection is made difficult because the recovered chemical compounds tend to consist of very fine particles and the dust loading can also be relatively high, leading to significant suppression of inlet field current levels.
Proper operating temperature, good gas-flow distribution, good mechanical alignment and optimized rapping are needed to comply with emission limits on a consistent basis.
Recovery boiler precipitators tend to suffer from corrosion of both the casing/shell and internal components, because the flue gas is high in acid and moisture content. Wet bottom precipitators are especially susceptible to this problem. For example, because of baffles between fields, there is typically little or no flow just above the liquor level. In these locations, shell wall temperatures fall below the acid dew point, and sometimes even below the moisture dew point. Corrosion occurs in these areas and other spots where flow and temperature are both low.
Ensuring that flue gas and shell temperatures are well above the acid dew point is the key to minimizing corrosion. To do that, it’s necessary to find and fix sources of air in-leakage (cold outside air getting in), and maintain good insulation. In some cases, it may be necessary to add heat tracing for parts of the shell.
Opacity and Emission Limits
Because of the challenges already discussed, it's not uncommon for a paper mill to meet emission requirements with relative ease, yet still have trouble complying with opacity limits. This is especially prevalent with recovery boilers. Routinely meeting emission limits requires optimizing boiler operation (not exceeding boiler firing by more than 5-10% of design) and gas-flow distribution into and out of the precipitator. It’s also necessary to perform regular ESP maintenance, including minimizing air in-leakage.