By Teresa Hansen, Associate Editor
Rising natural gas and fuel oil prices have users of all kinds looking for cost-effective fuel alternatives. Landfill gas (LFG) is one viable and popular substitute for these fuels. Many companies are successfully implementing LFG product conversions to existing oil- and/or natural gas-fired boilers. For example, a major U.S. automaker successfully converted three natural gas boilers to boilers capable of burning natural gas and LFG. The first of the three projects, which has been operating since early 2002, has an approximate LFG flow of 100,000 standard cubic feet per hour or approximately 50 to 60 MMBtu per hour.
Because of its extensive track record in the application and combustion of non-standard gases, the automaker selected Hamworthy Peabody Combustion to conduct the LFG conversion projects. The first two projects were completed during a plant outage and LFG availability schedule. The boilers were successfully commissioned on both LFG and natural gas, and both projects have been in service for some time with no operational or availability issues. Because the first two projects were so successful, the automaker contracted with Hamworthy Peabody to construct the third turnkey project.
Landfill gas flame
One of the three projects required only a modification to an existing burner, while the other two required Hamworthy Peabody to supply and install new vertically up-fired burners to burn LFG. For all three projects, Hamworthy Peabody supplied new PLC-based burner management systems and new LFG piping from the LFG supplier’s flange and combustion control modifications. In addition, Hamworthy Peabody’s scope included the mechanical and electrical installation, as well as unit commissioning.
Each of the three projects involved retrofitting the existing watertube boilers to burn the LFG. The vertically up-fired boilers were originally field-erected stoker fired. The conversions included removal of the stoker, modification of the existing stoker support steel and installation of a “floor.” No pressure modifications were required.
To maximize LFG usage, Hamworthy Peabody designed the boiler combustion control system so that all available LFG is burned and natural gas co-firing provides any additional heat input required. Because at times the available LFG flow rate can be less than that required for a full load, the balance of heat input must be provided by co-firing with another fuel source, such as natural gas or fuel oil. This is accomplished through the combustion control system, which uses a pressure control loop for the LFG. The combustion control system is fully metered and cross limited to measure both air and fuel flow accordingly to assure proper and safe fuel/air ratios and maintain safe, efficient firing of the multiple fuels.
In addition, during the initial design and operational review, the customer noted that boiler availability was critical, adding another reason for Hamworthy Peabody to design a co-fired system. The specific systems provide a constant natural gas pilot to assure stability and a constant flame source, should there be an upset at the landfill. This constant natural gas pilot negates the requirement for a boiler repurge should the landfill gas supply become interrupted. If the LFG supply is interrupted, the combustion control system automatically starts or increases the main natural gas burner to maintain steam production.
Another special issue that is common to almost every LFG firing application, and had to be considered in these installations was the presence of H2S. Because landfill gas typically contains a small amount of H2S that can react with moisture in the fuel and form small amounts of sulfuric acid, all the surfaces that come in contact with LFG are stainless steel to avoid contamination.
Although there are issues to consider when firing LFG, as this automaker has discovered, the fuel is a viable option to combat the rising costs of natural gas and fuel oil. The installations are field-tested and proven, and are saving the company money.