Copyright © 2017 Water Environment Federation. All Rights Reserved.
WSEC-2017-TR-002, RBC Bioenergy Technology Subcommittee, CHP Task Force 9
Characteristics / Applicability
When considering the potential for employing gas-fueled reciprocating engines to operate on
digester gas, there
are a significant number of available manufacturers of reliable equipment.
The process of combusting digester gas for the
purposes of combined heat and power (CHP) has
been executed successfully at a large number of
facilities both globally and within the United
States. Numerous manufacturers of gas reciprocating engines
have demonstrated product
competence in this arena. This section of the paper discusses these proven
manufacturers, engine
performance, and emission performance as they apply to operations on digester gas.
Several years ago, the gas reciprocating engine market experienced a consolidation in
competing brands
offered. General Electric (GE), the owner of Jenbacher gas engines,
purchased Waukesha gas engines,
and Caterpillar, at about the same time, purchased the
MWM gas engine brand. While Cummins also
offered high quality engines. This market
consolidation may have seemed to reduce the competitive
nature and technological
advancement of the engine market, but the acquired brands are still offered in the market
through their new parent companies. All three gas engine
manufacturers, Caterpillar, Cummins,
and GE have continued to invest in their product offerings. Additionally, reputable European gas
reciprocating engine manufacturers such as MTU, Siemens (who acquired Dresser and Guascor)
Liebherr, and MAN have entered the US market for digester gas applications.
When evaluating the potential for generating electrical energy from digester gas, the first
consideration for
the project owner is to match the size of the reciprocating engine to the process
flows and electrical loads. Gas reciprocating engines
range in size from 140 kW
el
to over 9,000
kW
el
. A rough rule of thumb is that a 10-MGD average flow plant would
produce enough
digester gas to power 330 kW
el
, while a 25-30 MGD average flow plant would produce
enough
gas to generate up to about 1,000 kW
el
. It is a rare occasion that a digester gas facility would not
be able to find an ICE generator size that could efficiently accommodate its gas production rate.
Performance of gas engines is based upon the efficiency of the engine in converting chemical
energy from
the digester gas to electricity and heat. Gas reciprocating engines consist of an
alternator (electrical power generator) and an engine
component. The engine component
combusts the digester gas thereby producing mechanical energy. This mechanical energy is then
converted to electricity by the alternator. Heat is recovered from the engine component as a
byproduct of power generation. In the recent past significant improvements in power generation
efficiency have been achieved by most of the ICE manufacturers. Programs, such as the
Advanced Reciprocating Engine System (ARES) funded by the United States Department of
Energy, have
supported these technological advancements. In the case of the ARES program
specifically, it produced
improvements to American manufactured gas engines and responses in
kind from European and other
manufacturers. Overall engine performance is rated on electrical
efficiency, thermal efficiency, and
overall combined efficiency (a summation of thermal and
electrical efficiency). A typical range of fuel-to-electrical power efficiencies is from 35% to 45%
based upon the Lower Heating Value (LHV) of the digester gas. Thermal efficiency is a measure of
the heat recovered from the engine during combustion compared to
the energy input of the
digester gas. Typical thermal efficiencies range from 40% to 50%. As a rule of
thumb, gas
reciprocating engine’s heat is recovered from the engine lube oil, engine jacket water, engine
intercooler, and exhaust. Approximately half of the heat is high temperature heat (700
o
F – 950
o
F)
recovered from the exhaust with the remainder recovered from the lower grade temperature
elements
made up by the combined lube oil, jacket water, and intercooler, (180
o
F – 230
o
F).
Finally, the total
efficiency of a CHP system is the summation of the electrical and thermal