Project Synopsis

Provide a feasibility study and subsequent process design for a novel gas cleaning process to upgrade municipal landfill biogas to both pipeline quality natural gas and engine fuel for electrical power generation.

Project Summary

LANDFILL GAS PURIFICATION SYSTEM ALTERNATIVES EVALUATION AND PRELIMINARY DESIGN

The client, an engineering firm, was contracted by the ultimate client, a renewable energy development company, to evaluate systems for the purification of landfill derived biogas.  This methane (CH4) rich stream contained various contaminants including hydrocarbons, chlorinated fluorocarbons (CFCs), siloxanes, hydrogen sulfide (H2S), water, nitrogen, and carbon dioxide which require a varied degree of removal depending on the gas end use.  For this project, the upgrade of biogas to pipeline quality natural gas and as 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 municipal waste landfill.  The initial focus of the design work was on the removal of high vapor pressure hydrocarbons, CFCs, and siloxanes.  Specifically, a method for removing relatively high concentrations of chlorinated compounds and siloxanes was sought due to their potential for producing corrosive and abrasive constituents upon combustion.

At the client’s request, PROCESS' licensed commercial computer process simulation software, CHEMCAD, was used to provide an initial feasibility study for biogas purification using a liquefied carbon dioxide (CO2) scrubber.  In this system, liquid CO2 is introduced to the top of a pressurized column with compressed landfill gas added to the column bottom.  Within the column, a portion of the CO2 vaporizes, resulting in low operating temperatures which cause impurities to condense.  These impurities are carried from the column bottom; vaporized CO2 and clean biogas are carried overhead.  Results of the study showed this operation to be technically feasible and a full plant simulation was then completed.

For the liquid CO2 scrubber plant, purified landfill gas was separated from gaseous CO2 using a two-step membrane system.  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 the simulation.  Using the two-step membrane process, CO2 from the scrubber overhead was recovered, compressed, and recycled back to the scrubber.  The methane rich raffinate produced by the system was exported as pipeline quality natural gas or as fuel for an engine. PROCESS also incorporated vendor supplied efficiency data for hydrogen sulfide scavenging and desiccant dryer systems into the model.

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.  Alternative systems including carbon adsorption, commercial refrigeration purification technologies, and molecular gate processes were then evaluated against this baseline.  For these cases, effluent biogas compositions were derived from vendor supplied data and incorporated into a plant mass balance.  The results for each were then provided to the client to screen the available options for further consideration.

Industry Type

  • Alternative Fuels
  • Landfill Gas Processing

Utilized Skills

  • Feasibility study
  • Design of specialized scrubber systems
  • Gas purification process design
  • Waste recycling process design

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