As population grows and solid loads increase, a biosolids plant in Texas was determined to find the best way to handle its biogas production increase. The plant currently utilizes less than half of their biogas for beneficial use on-site. One portion of biogas fuels a combined heat and power (CHP) system for power and heat generation. Another portion fuels hot water boilers for digester heating. However, the biogas production rate exceeds the demands of the CHP and hot water boilers. The remaining biogas is routed to a flare.
To mitigate flaring and better understand the value of the produced biogas, CDM Smith conducted an evaluation was conducted for different alternatives over the next 20-year period for utilizing the biogas produced at the site. This assessment examined using biogas in the following ways:
CHP: As currently in place at the facility, biogas can fuel the CHP engine(s) to produce electricity and heat. The CHP-generated electricity is put onto the grid. The facility receives credits to offset electricity costs.
Digester Heating: In addition to the CHP engine, boilers fueled from biogas can provide digester heating. Renewable Natural Gas (RNG): Biogas can be upgraded to biomethane quality, then used as RNG in a natural gas pipeline. Upgrading technologies include liquid scrubbing, pressure swing adsorption, and membrane scrubbing. Compressed Natural Gas (CNG): Biogas can be upgraded to RNG and further compressed to CNG, then used as vehicle fuel.
This paper focuses on the greenhouse gas lifecycle and air permitting evaluations for the different alternatives. These evaluations were two of the criteria considered to determine the preferred alternative for handling the current and upcoming increase in biogas production at the plant.
.jpg "Christopher L. Campbell, MS, ENV SP photo")