Landfill gas is becoming increasing important as a source of fuel for process heating. This is primarily due to its carbon footprint, which can be harnessed to do work instead of being vented or flared into the atmosphere. One manufacturer conducted a study to determine if its existing infrared burners could be used effectively with this supply gas or if modifications to the burners needed to be made.
Initial research has been completed with the process infrared burners using carbon-dioxide-modified natural gas. Radiant emissivity was achieved in the 550 to 610 BTU content range while maintaining a constant air/gas mixer setting. During combustion, the emitter temperature was 1,700°F (927°C). Radiant fire also was achieved and maintained as low as 475 BTU by adjusting the air-gas mixer to reduce the total percent of natural gas.
- Determine the lower limit of infrared radiance through
the downward adjustment of the BTU content in an air/gas/carbon dioxide
- Define process performance viability with low BTU content gas typically available from a landfill pipeline without expensive upstream control equipment.
A cylinder of carbon dioxide was piped in to dilute down the air/gas stream. A two-stage pressure regulator, flow meter and low pressure regulator were connected in series to provide a flow rate of 35 to 45 ft3/hr and a pressure of 0.25 to 0.5 psig.
The company next evaluated its operating and control module an alternative to a standard air/gas train. The self-contained unit includes an air blower, mixer, control valves, pressure switches, ignition control module, flame safeties and PC boards. It measures approximately 15 by 18 by 12". The air input orifice diameter to the mixer valve was increased to the maximum possible; unfortunately, only a minimum of 730 BTU content gas could be achieved. So, a standard air/gas train was used instead with zero pressure regulator, valves and air blower.
The percent oxygen in the total mix was measured by a handheld oxygen analyzer. The burner surface temperature was measured by a pyrometer.
FindingsFirst, testing showed it was necessary to add carbon dioxide to effectively reduce the caloric content of the gas. Only adjusting the air/gas mixer to provide a lower energy content mixture resulted in an increase in the flow rate, producing a high, yellow luminous flame in the burner.
Second, the average BTU content of landfill gas - provided by the city of LaGrange, Ga., to several companies that use gas from this pipeline - was 585 BTU/ft3. The variance around this norm is 550 to 610 BTU/ft3. Bright red infrared radiance in an all-metal burner was maintained throughout this range with an emitter temperature of 1,700°F without requiring mixer adjustment. This indicates viability from a process control standpoint.
Third, with a fixed air/gas mixer, the percent oxygen varies inversely with the BTU content. The higher the percent oxygen in the total mix, the lower the BTU content of the gas. Thus, measuring the percent oxygen in the mix can define the BTU content of the gas.
The effects of landfill gas contaminants such as hydrogen sulfide and other non-methane organic compounds were not evaluated. Actual landfill gas needs to be tested in another study to determine long-term effects on performance of the all-metal infrared burner.