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In
Landfills and Biomass Reactors
Introduction
STI has a steam injection process for landfills and Biomass Reactors. The steam is very effective in
enhancing the digestion of organic materials by methanogens. This is very helpful in recovering airspace in
landfills however in closed landfills this means the organic material will eventually be exhausted and the
generation of methane will stop. The steam also accelerates the digestion in Biomass Reactors and
produces the maximum amount of biogas in the shortest amount of time. With a reactor so hungry for
organic material it can be challenging to maintain a steady flow of feedstock especially on weekends when
there may not be any trucking available or due to bad weather.
With all this biogas being produced and 40% of it being CO2 it is obvious that algae production would be a
good use for the CO2. By injecting algae into the steam stream and then into a landfill or Biomass Reactor a
gas to energy project can be more sustainable.
Algae
There are over 65,000 known species of algae in the world. Alga is basically seaweed that is found in
saltwater as well as freshwater, the type we are interested in, is fresh water. Also the two main freshwater
types we are interested in are the “heavier than water” type and the microalgae, which contain oil, therefore
they float in the water column. This type of algae can contain between 20 to 60% oil, which is used to make
biodiesel. This same oil will be converted into biogas very rapidly by methanogens. The methanogens will
also convert the cellulose of the cell walls or lipids although not as fast.
The heavier than water type is of interest only for the Biomass Reactor as feedstock because it would be
easier to harvest and de-water than microalgae, although it has very little oil. It grows as a carpet on the
bottom of the tank and can be raked up and shook to remove water, it would then be ground and then loaded
into the reactor the same as green waste.
It has been reported that algae will grow 20 to 30 times faster than food crops. The optimal temperature for
alga growth ranges from 70o to 80o F. This should be able to be maintained by injecting the exhaust from
generators.
Most microalga will double its population every 48 to 72 hours while some can split 16 times within 24 to 48
hours. Therefore, if 100 tons per day is required for feedstock then at least 400 tons should be maintained
to allow for a 100-ton off-take per day.
Another challenge is to inject the exhaust from generators or biogas scrubbers (10,000+ scfm) into the algae
beds and not evaporate the water or increase the temperature of the water to the point where it will cook the
alga. On average it requires 2.5 pounds of CO and CO2 to make 1 pound of alga. Other nutrients and
control of the pH are also required.
Many algae reactors use clear acrylic tubes filled with water and CO2 is injected into the tubes. This is not
practical with the high volumes of hot exhaust from power plants.
It has been reported that the photosynthesis occurs within ½” of the surface and as the density of the algae
increases, the light is prevented from penetrating down into the water. STI is proposing to use large tanks
with light emitting tubes to provide more surface area. A mixing chamber will be used to mix the exhaust with
the water from the reactor tank, which will carbonate the water and help cool the exhaust prior to entering the
reactor tank. This should provide a constant volume of water to absorb the exhaust.
Algae Injection - Landfill
Currently, if STI steam treats 5 acres of a landfill to a depth of 75 feet with 5,000 gallons of water/steam per
acres it will produce about 6,000,000 cubic feet of LFG per day. This process will last about 1.2 years before
the process is moved to another 5 acres.
In an active landfill this recovered airspace can be refilled with refuse and can be steamed again when the
area is full. This allows much more refuse to be placed in the landfill. The gas from this five acres can
produce 12 –15 Mw/hr of power. The exhaust from the power plant can be sent to algae beds to mitigate
exhaust emissions and to grow alga. For every 100 tons of algae that is injected into the steam stream and
then into the landfill, another 3 Mw/hr can be generated.
For closed landfills the same process is applied but since the landfill is closed there is a limited amount of
organic waste available. To extend the life of the gas field hence the life of the gas-to-energy project, algae
can be harvested, mixed into the steam stream and injected into the landfill extending the life of the gas
field. Since we are only filling the void space and not airspace it does not violate the closure plan.
If 100 tons of algae are injected into a closed landfill per day then an additional 3 Mw/hr of power can be
generated or without adding more power, the life of the gas field will be extended for another 5 years. Only
microalgae would be used in this process.
Biomass Reactor Injection
The Biomass Reactor can be just as productive as a landfill if the feedstock is available. As stated above
there are two types of alga, microalgae and seaweed type. The biomass reactor can use any type of dry
organic material as feedstock. The microalga must be killed and the cells fractured by injecting it into the
steam stream.
The “heavier than water” algae, can be harvested like seaweed, dried, shredded and loaded into the reactor
as feedstock like green waste.
Like the landfill approach, if 100 tons of algae are injected (either microalga and/or seaweed) into a biomass
reactor per day then 3 Mw/hr worth of feedstock can be used in place of importing feedstock. With the right
conditions and alga type, it may be possible to provide enough alga to make the project self-sufficient.
Table 1
Algae Production
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