CNG-CBG White Paper PDF
CNG-CBG White Paper PDF
CNG-CBG White Paper PDF
Compressed Biogas
Technology
CNG and CBG are both compressed, methane-based gases. They can be used interchangeably as fuel in
CNG vehicles. CNG is made from pipeline natural gas. CBG is made from the methane-rich biogas
produced from the anaerobic decomposition of organic wastes at landfills, wastewater treatment plants, or
other facilities with digesters (e.g. food processing plants, dairy farms). While pipeline natural gas is
CNG-ready and requires no conditioning (cleaning) before compression, CBG requires conditioning for
the raw biogas before compression. After conditioning, CBG is functionally equivalent to CNG. The
technology involved in each step of the fuel production process is described below:
1. Conditioning (clean-up) System – A biogas conditioning system (only required for CBG) removes
moisture (H2O), increases the methane (CH4) content by removing carbon dioxide (CO2), and
cleans the gas by removing hydrogen sulfide (H2S), siloxanes, and other trace constituents.
2. Fueling Station – A fueling station typically includes a compressor and fuel pump. The compressor
compresses the cleaned biogas and/or natural gas from ~10 psi to ~4,000 psi. The compressed fuel
is usually expressed in terms of DGE (diesel gallons equivalent) or GGE (gasoline gallons
equivalent). Many fueling stations will have more than one compressor for system back-up or so
both CNG and CBG can be used at the same time. The fuel pump dispenses the compressed fuel to
vehicles at a rate (DGE or GGE per minute) dependent on compressor size and number, the amount
of compressed fuel storage, and other project specifics.
3. CNG Vehicles – Petroleum-fueled engines will not run on CNG or CBG fuel, so a petroleum
vehicle fleet must be transitioned to CNG. CNG engine technology has improved significantly
over the past several decades, and the performance and efficiency of current CNG engines is
comparable to petroleum engines. CNG engine types include dedicated (CNG only), bi-fuel (runs
on either CNG or gasoline), or dual-fuel (runs on both CNG and gasoline). Fleets can be
transitioned either by retrofitting existing petroleum engines, or purchasing new vehicles designed
to run on CNG.
CNG/CBG Production Process
As of December 2012, total estimated fuel prices for CNG and CBG are both roughly $2.00 cheaper than
diesel ($4.00/DGE) and gasoline ($3.30/gal or $3.75/DGE) at current Wisconsin prices. Given current price
differences, fueling a vehicle fleet with CNG instead of diesel would result in significant annual cost savings.
The table below shows the annual fuel costs savings for CNG and CBG at two system sizes and three potential
diesel prices (capital expenditures not included). CNG fuel tax credits (pending at the federal level) also have
the potential to increase cost savings.
Potential Diesel Prices
Fuel System Size*
$3.50/gal $4.00/gal $4.50/gal
50 cfm (200 DGE/day) $139,000 $175,000 $212,000
CBG
100 cfm (400 DGE/day) $277,000 $350,000 $423,000
50 cfm (200 DGE/day) $124,000 $161,000 $197,000
CNG
100 cfm (400 DGE/day) $248,000 $321,000 $394,000
*Size description: cubic feet per minute (cfm) of raw biogas into system; DGE/day of
CNG fuel produced by system
The figure below shows the historical price comparison for diesel, CNG, and CBG. Conventional natural gas
and petroleum prices are projected to remain steady or gradually rise over the next 10-20 years, so the future
price difference between CNG, CBG, and petroleum fuels is expected to remain relatively constant. While
CBG is currently cheaper to produce than CNG, the higher capital costs associated with CBG (due to gas
conditioning) have the potential to offset this difference.
$5.00
$4.50
$4.00
$/DGE (gallons of diesel equivalent)
$3.50 Diesel (purchase
price including
$3.00 taxes)
$2.50 CNG (production
price + taxes)
$2.00
$1.50 CBG (2012
production price +
$1.00 taxes)
$0.50
$0.00
Vehicle Fleet – Identifying a vehicle fleet to use the CNG is one of the most important project
considerations. Ideal fleets for CNG projects are
o driven many miles
o return to the same location daily
o have low fuel economies
Commonly considered fleets include refuse trucks, municipal or county vehicles, law enforcement
vehicles, and school or transit buses. While CNG vehicles operate similarly to comparable petroleum
vehicles, maintenance facilities that service CNG vehicles must meet certain regulatory requirements that
could require facility upgrades.
Location and Sizing of CNG System – The location of a fuel source will largely determine which
vehicle fleets could best utilize the fuel. The fueling system should be sized based on both on the
amount of CNG and/or CBG that is economically available and the fuel needs of the vehicle fleet.
Grant/Funding Sources – Depending on how a project is structured, there are many funding options
to support CNG systems and vehicles in Wisconsin including:
Payback Period – Estimated payback periods for CNG projects can vary significantly depending on
a number of factors including:
Price of diesel/gasoline – The larger the price difference between CNG and the alternative, the
larger the return.
Price of natural gas
Vehicle Fleet attributes – Ideal fleets for CNG projects are driven many miles, return to the
same location daily, and have low fuel economies.
Biogas source – What, if any, investment is required to produce or capture the biogas?
System operations and maintenance costs – The cleaner the raw biogas, the lower the O&M
costs to produce CBG.
Renewable Fuel Credits (RINs) – Credits can provide additional revenue for each unit of CBG
fuel produced.
Conclusion
CNG and CBG represent a significant opportunity for vehicle fleet owners. The main driver for undertaking a
CNG project is fuel cost savings, but energy independence and environmental benefits are also important
drivers. CNG projects are customizable and have many potential applications.
As society’s need for secure, reliable, efficient energy sources and systems continues to grow, SEH is actively
involved in developing new technologies, and finding creative ways to leverage existing technologies to help
you keep pace with this rapidly changing market. Contact SEH for additional information on CNG-CBG
opportunities.