Waste Gas as an Alternative Fuel
Be it syn-gas, methane, digester or flare gas, there are several control schemes and burner designs that can be used to burn the waste gas thus lowering your overall fuel bill. The first assumption here is that you have a need for steam or hot water at your facility. The other issues that need to be considered are the total supply of the gas, BTU content, can you store the waste gas for a period of time, and how stable you need the output to maintain your end process. The BTU content normally needs to be in the 250-500BTU/SCF range to burn on its own. If lower than that, mixing the waste gas with natural gas will raise the combined BTU content to allow for combustion.
Assuming the BTU content is over 500/cu ft, the simplest and least costly method is to use a dual head / dual gas train burner that will burn either the waste gas or the natural gas. You would monitor the waste gas pressure and if sufficient, burn only waste gas through the waste gas section of the burner head. As you use the waste gas, the pressure will drop. At some pressure point, the burner would shut down and restart on natural gas using the natural gas section of the burner head. This allows you to keep producing steam and also allows for the collection of more waste gas. A built in time delay (hours) will keep the burner on natural gas until a supply of waste gas has been collected. At the end of the time delay, the burner will again shut down and restart on waste gas. This control scheme will allow you to use all the waste gas available and still maintain your boiler load. Most burners we provide use this very simple control strategy.
If waste gas storage is an issue or the supply of waste gas is erratic, you can move up to a staged waste gas / natural gas control scheme. Using a linkageless control system, you can program in several fuel curves that step up the mixture of waste gas / natural gas. The burner for this application has dual gas trains. For example, you can start with a 25% waste gas / 75% natural gas mixture and if the waste gas supply increased move up to a 50% / 50% mixture. Then move up to a 75% waste gas / 25% natural gas mixture. The burner control system senses the supply of waste gas and adjusts the mixture to compensate. For this control scheme, the burner needs to come down to pilot or low fire to allow it to switch fuel curves.
If the supply of waste gas is large and you need tight control of the end process, you can use a fully metered system. Instruments are installed to measure the flow of the standard gas and the waste gas. This system has two control loops, one will vary the waste gas to use all that is available and the other will vary the standard fuel to meet the steam or hot water load. While more expensive and more complicated to set up and maintain then the two above mentioned control schemes it does give you tighter process control and requires a minimum amount of waste gas storage.
The key to proper waste gas burning is knowing what you have available (waste gas data) and how your end process needs to operate. Will full information, an appropriate control scheme / burner can be designed to meet your needs. For more information, please go www.webster-engineering.com and click on the "Alternative Fuels" page. Call Webster Engineering at 620-221-7464 or your local Webster Representative.
