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Canada, New England, and California all have Carbon Credit
programs to achieve GHG reduction goals. Several forms of biomass
diversion from landfills, farms, and other biomass – dependent GHG
sources are already in operation to support significant GHG
reductions. Examples of GHG reductions are given, and the carbon
impact of the different commercially available biomass to GHG
reduction processes are described.
The three groups of commercially guaranteed biomass conversion
1. Power Generation, Steam Generation, and CHP:
from the combustion of biomass wastes. This industry, with about
100 independent units in operation in the US, is based upon pulp
mill technology and includes fluid beds, pulverized fuels, and
suspension grate technologies matched to well-proven emissions
controls. High pressure steam is generated and drives turbines to
While fluid beds can use fuels up to 65 % water, e.g. sludges,
most units use waste woody biomass from a variety of sources, and
provide an important regional waste disposal service in the
Circular Economy. The GHG reduction is site specific – landfill
diversion of biomass is strongly GHG Negative, while 100 %
forestry waste fuel is approximately GHG neutral.
2. Anaerobic Digestion: AD is firmly
established as the leading method of converting high moisture
content organic wastes first to methane-rich gas, thence to power,
pipeline gas, CNG / LNG. AD’s lower conversion efficiency and
higher specific capital cost is offset by a consistent GHG
reduction due to the sources of its biomass feedstocks, and in
many cases by wider socioeconomic benefits in disposing of wastes.
3. Thermal Conversion to Hydrocarbons:
This category includes simple pyrolysis, classical and advanced
gasification processes, and direct catalytic conversion, with end
products ranging from crude distillate oils, synthesis gas for all
applications, and catalyst-tailored products. These processes
generally have a high conversion efficiency, and their GHG
reduction impact is largely a function of their biomass sources.
Power Generation, Steam Generation, and CHP
Over 100 US biomass Independent Power Producers, and our many
biomass industrial and municipal Combined Heat and Power
generators, do not grow and burn trees – they cannot afford to do
so at current power prices. They all use some form of waste
biomass fuel, often diverting the wastes from landfills. They
cannot afford to plant and harvest either trees or other ‘energy
crops’, even though such plantations have been tried.
The range of waste biomass fuels is wide – forestry wastes,
including sawmill wastes, wastes from wood products manufacturing,
non-recyclable waste paper, recycled paper mill wastes and
sludges. In addition, biomass plants use wood waste diverted from
municipal landfills to avoid methane generation, clean wood from
construction and demolition sites, utility transmission line
right-of-way clearance and urban tree removals – a huge volume.
Finally, there is process waste from biodiesel and cellulosic
fuels and chemicals production, agricultural wastes, straw and
husks from grain crops and grain processing.
Some biomass plants, such as municipal Waste-to-Energy plants,
combine recycling with power generation. The US has about 80 WtE
biomass power plants, operating in compliance with emissions
regulations. These third generation WtE plants, like those in
Europe and the newer WtE plants being built in China, exhibit high
reliability and extremely low emissions as the result of several
decades of continuous process improvement.
The carbon footprint of biomass power plants is generally neutral
as determined by UE EPA and US Department of Energy. Each location
should calculate its own particular carbon footprint. Some biomass
power is strongly carbon-negative, owing to the reduction in
landfill methane emissions when biomass is diverted, even with
landfill gas recovery, as shown below. Additional reductions in
carbon footprint can be achieved by the use of biodiesel and
renewable LNG or CNG in trucks and equipment – a growing trend.
Carbon Negative – a Simplified Calculation.
Dry wood consists of a mixture of cellulose, hemi-cellulose, and
lignin. The ash-free chemical composition of wood can be
represented either as C6H(H2O)6,
or more simply as CH2O. CH2O is used below
for the approximate calculation of the amount of methane, CH4,
and carbon dioxide, CO2, that is released from a
landfill when woody material undergoes anaerobic decomposition.
Anaerobic decomposition is the result of the exclusion of air, the
presence of water, and the presence of anaerobic and
methane-forming bacteria, similar to the conditions in a swamp
where methane, or marsh gas, is generated.
2 CH2O (water, bacteria), 2 x 30 = CH4 ,16,
+ CO2 , 44
One ton of dry, ash-free, wood in a
0.27 ton CH4
Plus 0.73 ton CO2
The typical 25 MW biomass power plants use from 1.05 to 1.1 dry
ton of wood per net MW-hour; using 1.05 tons/ MWhr:
One MWhr of biomass power from
diverted biomass avoids the formation of 0.28 tons of methane in a
As a GHG gas, methane is approximately 21 times as powerful as
CO2. So one MWhr of diverted biomass power avoids the release of
approximately 6 tons of CO2 equivalent, plus the CO2 also
generated for a total of 6.7 tons of CO2 E per biomass MWhr.
However, most landfills practice landfill gas recovery. The EPA
model uses a default value of 50 % LFG recovery when calculating
emissions, but it is assumed that in southern California – the
origin of LFG recovery technology – LFG recovery is approximately
65 %, as advised by SCS Engineers and industry sources. Therefore
the net emissions of methane to the atmosphere are approximately:
0.28 tons CH4 / MWhr x 35 % net
emitted = 0.1 ton of methane emission avoided per Biomass MWhr, or
2.1 tons of CO2 equivalent.
Wood waste deposited in a C&D landfill will generate LFG more
slowly than in an MSW landfill, but typically there will be no LFG
recovery at a C&D landfill. Once water accumulates, and oxygen
is depleted, anaerobic decomposition will take place, yielding 6 –
7 tons CO2E per biomass MWhr.
Waste paper is lignin-free wood, and decomposes in the same way,
but more rapidly than woody material.
If a given facility uses 50 % landfill-diverted biomass, and 50 %
carbon-neutral forestry waste, then a pro rata calculation of the
negative carbon impact can be used to calculate the Carbon Credits
A typical 25 MW biomass power plant, using 100 % landfill
diverted biomass, prevents the emission of about 1.2 MM tons per
year of CO2 equivalent.
A typical 60 MW, 2,000 ton MSW/day waste to energy plant, where
100 % of the biomass fraction of the fuel avoids landfill
decomposition, prevents the emission of about 2.9 MM tons/yr of CO2
Most AD feedstocks would be converted to methane and CO2 if
not so processed, therefore the above simplified calculation may
be applied, with adjustment for the individual MAF organic
analysis. Loss of carbon to digestate affects the overall carbon
conversion efficiency of the particular AD process, but does not
affect the negative carbon impact due to conversion. Negative
carbon impact must be calculated on the MAF content of the
feedstock, then converted to wet tons, or to gallons.
The additional socio-economic impact of many AD projects which
eliminate the discharge of livestock manures and their consequent
damage to rivers, lakes, fisheries and tourist business should be
added to their negative carbon impact.
Although the carbon conversion efficiency of most AD processes is
significantly lower than that in a combustion boiler, the heat
rate of the gas engine gensets used for AD is better than that of
a biomass boiler/steam turbine, such that an AD plant has negative
carbon impact of about 5 tons of CO2 E per MWhr,
compared to the 6.7 tons CO2 E for the solid biomass
system. Care must be taken to define the alternative decomposition
pathway of each part of the AD feedstock in calculating this
A typical 5 MW AD facility, using 100 % feedstock that would
otherwise generate methane emissions, prevents the emission of
about 200,000 tons per year of CO2 equivalent.
Thermal Conversion to Hydrocarbons
There are about a dozen commercially guaranteeable biomass
conversion systems available. Some are equipped to handle MSW,
others are limited to less-corrosive forestry wood wastes. In all
cases, the negative carbon impact can be calculated from the
feedstock analysis and the alternative disposal of that feedstock
in the absence of the project.
A good example is the Edmonton, Alberta, Enerkem project which
converts 100,000 tons/yr of RDF from MSW into approx 440 bbl/day
of hydrocarbon, using a classic oxygen blown gasification process
followed by gas clean-up and methanol synthesis.
Gasification processes have excellent carbon conversion
efficiencies; if the resulting syngas is used in combined cycle
power generation, a very efficient overall system results. But the
carbon impact is based upon the feedstock consumption, not the MW
output, so a lower negative carbon impact per MWhr than for a
conventional boiler/turbine is the result.
The Enerkem Alberta project has a negative carbon impact of about
550,000 tons/year of CO2 equivalent, based upon 100 %
of its feedstock being diverted from landfill.
Andrew Grant has a B.A. and M.A. from Cambridge University
and over 35 years’ experience as a manager of biomass conversion
projects. He has been involved in providing guarantees of
performance, environmental impact and cost studies, for coal and
biomass conversion technologies, and has performed due diligence
studies of technologies and of facilities. Andrew is familiar
with a wide range of biomass processing, ranging from wood chips
to rice straw to MSW, and is experienced in greenhouse gas
reduction and carbon footprinting, in the use of waste biomass,
and other emerging technologies.
was originally published
on Biofuels Digest. Biofuels Digest is
the most widely read Biofuels daily read by 14,000+
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