Autumn 2023 EN

BIOGAS JOURNAL | AUTUMN_2023 ENGLISH ISSUE
in the airtight enclosed garage for 18 to 21
days, during which time the volume shrinks
to about one-third. This is the way the process
runs through the five digesters.
The substrate is irrigated with percolate
every two hours on average. The liquid
travels into a drainage channel through the
substrate via a slope in the bottom of the
digester. From there, it is pumped back to
the percolate tank. “In principle, the plant
is simple in design,” says the farmer. It is
equipped with two pumps, an agitator in
the percolate tank, and five irrigation nozzles
in each digester. The heating is done
solely with the percolate, which is directly
heated by the CHP unit through heat exchangers.
No heating pipes are installed.
In addition to horse manure, the residents
of Lower Saxony use approximately 10 percent
of cattle manure and small amounts of
grass clippings. According to Bartels, the
digestate is stored and then applied as a
top fertilizer on the farm’s own fields. As it
was one of the first plants in Germany, Babö
GmbH obtained a favorable price from the
equipment manufacturer. “Wet fermentation
with substrate processing would have
cost roughly the same,” Bartels estimates.
His garage digester plant has an electricity
self-consumption rate of only 3 percent,
which is significantly lower than an upgraded
wet fermentation system. However, it
should be noted that more operating hours
with diesel consumption for the farm loader
need to be considered. Nevertheless, he is
very satisfied with his plant and is convinced
that he made the right decision.
A horizontal fermenter handles
higher TS (Total Solids) content
Frank Bauer from Schrozberg has found a
kind of compromise solution: a plug-flow
fermenter made by the manufacturer Novatech.
The 900 m³ horizontal fermenter
with a slowly rotating longitudinal agitator
can handle TS (Total Solids) levels of up
to 30 percent, as is typical for this type of
digester. Bauer raises turkeys and keeps
35 horses. He wanted a robust technology
to utilize his manure. He currently gets
manure from more than 200 horses, and
combined with turkey and some cattle manure,
the solid manure fraction amounts
to approximately 50 percent. The biogas
plant, built in 2007, still consists of three
cylindrical tanks and two CHP units with a
combined capacity of 500 kW e
. Bauer directly
feeds renewable raw materials into
one of the cylindrical digesters. The material
from the plug-flow fermenter is pumped
into a secondary digester and then into the
digestate storage.
However, Bauer also cannot avoid pre-processing
the horse manure. “I didn’t want
an expensive and potentially unreliable
technology directly in the fermentation
process,” he says. That is why he uses a
mobile biomass shredder. Although it is
primarily designed for woody green waste,
it works perfectly fine with horse manure
as well. “If I use horse manure that is not
shredded, I immediately notice an increase
in power consumption,” explains Bauer,
adding, “30 liters of diesel are enough to
shred around 200 tons of horse manure.
After shredding, the manure pile steams,
and I can see that energy is being lost.” The
manure then has to be brought to the plant
as quickly as possible. Bauer is convinced
that horse manure is “very good for the biology”
because, unlike poultry manure, it
does not involve nitrogen inhibition.
Bunker Silo Digesters
A novel, cost-effective type of dry fermentation
plant is being developed in the current
research project FeBio (Solid-State Biogas
Plant): a bunker silo biogas plant. “The
bunker silo digester is comparable to a long
garage embedded in the ground without a
roof, which is covered with a tarpaulin after
being filled,” explains Eike Ziegler, a
developer at the participating biogas plant
manufacturer Ökobit GmbH. This type is
based on the work of Hans Wolfertstetter
from Upper Bavaria, who has built several
of these plants under the concept of the
“Chiemgau Model”. Following the Chiemgau
model, a plant was also established in
the Eifel region, which now serves as a basis
for standardization and technical improvements
for Ökobit and the FeBio project.
Technical challenges in the bunker silo
design include the practical opening and
closing of the cover tarp. According to Ziegler,
this is resolved by using a rolling and
gas-tight fastening system. In addition, the
pipelines for percolate irrigation should be
easily removable to provide space for filling
the fermenter with a wheel loader. However,
the modular plant should be kept as simple
and compact as possible.
“The aim is to achieve electricity generation
costs of around 18 cents/kWh e
and investment
costs of less than 8,000 euros/kW e
of
installed capacity for the core plant,” states
Christoph Spurk, Managing Director of Ökobit.
The company intends to offer this type
of plant as a cost-effective builder-owner
model. The groundbreaking ceremony for
the FeBio pilot plant recently took place in
Saarland. Biogas will be produced here out
of horse manure in three bunker silo digesters
and converted to electricity using an 80
kW e
CHP (Combined Heat and Power) unit.
With regard to profitability, only one of the
interviewed users of horse manure digestion
is skeptical, while two are satisfied and
one is even enthusiastic. Horse owners can
be new business partners to biogas plant
operators and offer new opportunities, such
as providing the service of spreading horse
manure digestate. Adrian Bartels explains
that some of his equestrian centers get
straw for horse stall bedding from him.
Horse Manure Mobilizes Waste
Collection Trucks
Meanwhile, Königs Pflanzenenergie is
working on its concept of “Mobility with
Horse Manure”. The company has been
operating a biogas purification and injection
system since 2010. A significant
portion of the biomethane now comes
from horse manure, which presents good
opportunities for marketing it as a fuel.
According to Herbert Königs, a Bio-CNG
(bio methane compressed natural gas) refueling
station is currently being built directly
at the biogas plant.
It has already signed a contract for refueling
waste collection trucks. Biomethane
from horse manure allows for additional
income through the sale of greenhouse
gas reduction credits. The European Clean
Vehicles Directive now requires public authorities
to use alternative fuels, and according
to Königs, “With Bio-CNG, we are
far ahead of other technologies as a diesel
substitute.”
Author
Christian Dany
Freelance Journalist
Gablonzer Str. 21 · 86807 Buchloe
00 49 82 41/911 403
christian.dany@web.de
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