A growing trend for kraft paper mills is to use an existing power or recovery boiler for destroying noncondensable gas (NCG) and stripper off-gas (SOG.) This is a viable alternative to burning them in the lime kiln (where it can cause production problems) or in a dedicated incinerator.
The effectiveness of NCG incineration requires satisfying both temperature and residence time criterion. The United States Environmental Protection Agency (EPA) handbook “Control Technologies for Hazardous Air Pollutants” prescribes that achieving complete destruction requires NCG entering a furnace to have a residence time of at least 0.75 seconds at a temperature of at least 1,600°F.
Before delivering NCG to any boiler, an engineering study of the furnace and backend equipment must be completed. Design factors include optimal delivery location, species destruction criteria, corrosion concerns, and the impact on emissions. Additionally, the minimum steam generation rate at which this time and temperature residence time criteria are satisfied is determined.
In the majority of cases, the chemical recovery boiler has been selected as the primary location for incineration. A benefit of opting for the recovery boiler is the opportunity to capture and return the sulfur contained in the NCG and SOG streams back to the pulping process. Power boilers can also serve as effective locations to incinerate NCG and SOG after fully understanding the potential for acid dew corrosion in the backend and developing a mitigation strategy.
Jansen recently evaluated the feasibility of introducing NCG and SOG for incineration in a power boiler located in the US Midwest. Operating data along with boiler details were used to construct a heat and mass balance model of the boiler. These cases considered a range of steaming rates and fuel blends available to the client. Jansen’s model was used to evaluate various scenarios to predict the impact of burning these gases to identify the required upgrades. The calculations predicted a residence time of 0.8 seconds for the gases above a temperature of 1,800°F. These satisfactory conditions confirmed the adequacy of the boiler to accommodate NCG and SOG incineration.
Besides identifying the optimal location to deliver the gases for satisfying destruction criteria, Jansen’s model also identified potential corrosion conditions in the tubular air heater (TAH). The increased sulfur dioxide (SO2) produced from burning NCG and SOG revealed concerns regarding acid dew point temperatures in the TAH. Jansen identified modifications to the air ducting arrangement within the TAH and recommended a steam coil air heater to help mitigate corrosion.
Experience has shown that delivering NCG and SOG to power and recovery boilers is an economical solution for destroying these gases. With a thorough engineering evaluation and modeling, decisions can be made to optimally deliver the NCG for destruction without harming other boiler equipment.
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Author: Joseph M. Klover, P.E. – Process Engineer