Provide engineering support to develop a cleanup process for a biogas derived from cow manure and upgrade the biogas to both pipeline quality natural gas and engine fuel for electrical power generation.
ANAEROBIC DIGESTOR BIOGAS PURIFICATION SYSTEM PROCESS DESIGN
The client, an engineering firm, was contracted by the ultimate client, a renewable energy development company, to evaluate systems for the purification of anaerobic digester derived biogas. This methane rich stream contained various contaminants including hydrogen sulfide, water, oxygen, and carbon dioxide which required a varied degree of removal depending on the gas end use. For this project, upgrading the biogas to both pipeline quality natural gas and engine fuel for electrical production were considered. Process Engineering Associates, LLC (PROCESS) was contracted to provide engineering support for the development of gas cleanup options for each case. Project tasks included: development of computer process simulations, mass and energy balances (HMBs), process flow diagrams (PFDs), and preliminary capital and operating cost estimates.
Designs for biogas cleanup were completed based on compositions of gas derived from a specific anaerobic digester system (ADS) currently processing agricultural biomass (dairy cattle manure). The focus of the design work was the removal of hydrogen sulfide (H2S), carbon dioxide (CO2), and oxygen, sufficient to meet natural gas pipeline and engine fuel specifications.
Upgrading the biogas to meet engine fuel specifications required the removal of H2S and water. For this case, fixed bed H2S scavenging systems were evaluated based on removal efficiencies, bed life, and ultimately the overall economics associated with bed regeneration or replacement. Water removal was accomplished via common drying technologies. Using PROCESS‘ licensed commercial computer process simulation software, CHEMCAD, mass and energy balances for the system were completed and PFDs were generated.
Production of pipeline quality natural gas was accomplished via a two-step membrane system. In the two-step process, CO2 and oxygen permeate across the membrane, substantially purifying the biogas. Fractions of H2S and water in the membrane feed also permeate, simplifying upstream equipment requirements. To maintain high methane recoveries, a portion of the permeate is compressed and recycled to the feed. Methane (CH4) rich raffinate produced by the system is exported as pipeline quality natural gas. Using multicomponent membrane permeation data supplied by the client, PROCESS developed a worksheet calculation to determine the membrane’s raffinate and permeate component recoveries and incorporated the results into a simulation of the system.
Results from the simulations were compiled into PFDs, and equipment sizing data were determined for the purpose of generating system preliminary capital and operating cost estimates. Process level (duty) equipment specifications were then completed to support future detail engineering efforts.
- Alternative Fuels
- Gas cleanup process design