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Hybrid SCR technology

Hybrid SCR technology cuts costs for deNOx systems

Entropy Technology & Environmental Consultants, LP (ETEC; Houston, Tex.; etecinc.net) has received a patent for its Slip-Stream fluegas recirculation (FGR) technology, a cost-effective method to achieve nitrogen oxides (NOx) control levels from combustion processes using selective-catalytic-reduction (SCR) . The process combines FGR technology with SCR as a hybrid approach to cut the costs of SCR.

Most NOx formed during combustion of natural gas and light oils is from the high-temperature oxidation of atmospheric nitrogen. Thermal NOx formation is an exponential function of temperature, and a square-root function of the oxygen concentration. ETEC’s hybrid concept controls these parameters to reduce NOx output. In the process (figure), a slip-stream of fluegas from a hot-gas fan downstream of the combustion unit (such as an SCR fan or an induced draft) is recirculated back into the flame zone. This reduces the flame temperature and thus can reduce NOx formation by up to 80%, says Ravi Agrawal, principal officer at ETEC. Unlike alternative FGR methods, which require a separate fan to recurculate the fluegases, ETEC’s process uses the existing hot-gas fan, thereby reducing capital costs, he says. Although other combustion-control technologies, such as low-NOx burners, have been used with SCR technology, FGR has never been used with SCR, he adds.

Compared to standard SCR, the hybrid technology can reduce catalyst requirements by as much as 80%, resulting in “substantial” savings in equipment, space and structural steel, says Agrawal. In addition, ammonia requirements are reduced by 50–80%. Savings in NH3 usage alone would pay back Slip-Stream FGR costs in less than six months, he says. ETEC is in preliminary discussions to demonstrate the technology.

By  Gerald Ondrey
Chemical Engineering