Furnace Selective Catalytic Reduction (SCR) Air Preheater – NOx compliance is becoming more difficult as different counties increasingly fall within the Environmental Protection Agency definition of NOx polluters. For furnaces, the least-capital-expenditure-intensive way to comply has been to install low- NOx or ultra low- NOx burners. However, every time I visit a facility with a real bad tube impingement problem, I ask them whether the problem occurred after they switched burners. The answer is always “yes.” Low- NOx burners, by design, produce a larger flame than the same BTU burners they replace.
Unless you also replace the box, you need a computer flow model of the furnace when replacing burners to make sure the new burners aren’t disrupting design flow patterns for the furnace box. This mistake has caused many plants to suffer from poor operating furnaces with unusual burn patterns including flame impingement on tubes, extreme hot spots and uneven firing. The other problem with low- NOx burners is that they require tight fuel-gas composition control. The expected NOx level can become higher or the fuel-air mixture can become dangerous if fuel composition changes.
SCR units remove the NOx after leaving the furnace. Furnace box operations don’t change and there’s less worry about how much NOx is produced when making furnace adjustments. The big disadvantage is costs. It’s a separate unit attached to your flue gas and requires controls, plot space, maintenance and operator time.
But, to make it worthwhile, tack-on an air preheater to your SCR at minimum capital cost. The SCR already needs the fans and ducting system that an air preheater needs, so the energy the exchanger unit saves turns the SCR into a moneymaker. Depending on current temperature, SCR with preheaters can save 10%-30% of fuel cost.
Carbon Trading Projects – Carbon trading or mandatory greenhouse gas reduction is on the horizon. Environmental engineers are probably going crazy trying to figure out where to buy those carbon credits. Unknown aspects and costs of carbon emissions make planning for market credit very difficult. So, this is an opportune time to put together your list of carbon-reducing energy-efficiency projects. Items that you have looked at in the past will likely have carbon credits. Dust off your project list, calculate carbon reduction for the project, and find the carbon reduction per project dollar amount. This list will be invaluable for your environmental department as they explore trading credits. Your company then can either buy credits or do projects.
Water Conservation Controls – Some counties have restricted fresh water usage to the point that plants have reduced production. To decrease fresh water usage use an automated blowdown system for your cooling water system. Some plants take cooling water samples and then adjust chemicals and blowdown rate. Others blowdown periodically (instead of continuously) and add new water and chemicals as needed. For these scenarios, water usage is either too high, equipment protection is too low, or both. To use minimum amounts of fresh water, maximize equipment, and use minimum amounts of chemicals, the best solution is an automated blowdown that constantly adjusts water rate and chemical injection.
The same is true for boiler blowdown. Boiler feed water works more efficiently if automated so that dissolved solids are minimized without wasting hot water. The steam system’s condensate return also can be improved. Some plants won’t return some condensate because of possible contamination. Contamination may be intermittent or something that happened in the early 1980s, but it may happen so it deters collection. If the stream is large enough, add an analyzer and diverter to monitor for the contaminant and then divert water away from condensate return. The added advantage is that it also alerts your operating staff that something in the system is contaminating the condensate.
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Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him at GFaagau@putman.net.