CNCG and Liquid Methanol Incineration in a Recovery Boiler, 2003 Fall Technical Conference

In kraft pulp production a large volume of foul smelling sulfur containing gases is generated. The majority of the odor causing sulfur is in the Concentrated Non-Condensable Gases (CNCG) and Stripper Off Gases (SOG). These gases are typically treated by oxidation. The United States Environmental Protection Agency issued a National Emission Standard for Hazardous Air Pollutants (NESHAP) that requires all chemical pulping facilities to collect and treat these gases and certain process condensates. CNCG and SOG gas treatment in a recovery boiler has some specific advantages, and in recent years has become an acceptable option to other treatment methods.

The main advantages for CNCG and SOG incineration in a recovery boiler are that the sulfur contained in the gases is captured in active form in the green liquor. The heating value of the gases is also recovered in high pressure steam generated in the recovery boiler. The main disadvantage is the potential for operational difficulties and instabilities that can result in increased emissions and lost capacity. Special safety issues that apply to recovery boiler operations are also very important [1].

In many cases permanent use of auxiliary fuel is required for a recovery boiler CNCG burner. The most common auxiliary fuel is natural gas. This can be quite costly to operate, especially if the use of auxiliary fuels is not needed to meet the mill steam demand. A solution to correct this is to rectify the SOG into liquid methanol, and then use this internally produced liquid methanol as auxiliary fuel. There are additional advantages of producing liquid methanol from an environmental protection standpoint. First, it provides a straightforward way to determine and confirm compliance with the NESHAP requirement to collect and treat a prescribed amount of methanol. Second, replacing auxiliary fuel (which for most cases is a fossil fuel) in an NCG burner with the produced methanol creates an overall reduction in emissions of combustion byproducts.

This paper discusses the design, start-up and operational experience of a liquid methanol rectification system and recovery boiler CNCG burner. In this case liquid methanol is stored and used as auxiliary fuel for the recovery boiler CNCG burner. Mill operational experience with the system is discussed and information of actual system performance is given. The impact this practice has had on operating parameters and emissions of the boiler is presented.