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Biogas Management
Sealing off, collecting and conveying the
biogas to the cogenerator are the most delicate aspects of the
system, both for optimisation of production yields and for
workers’ safety.
As described above, the amBiogas digester is designed to
ensure perfect airtightness: walls and flooring in
resin-treated reinforced concrete; hatch in AISI 304 STAINLESS
steel with inflatable gasket; forced aeration system and
irrigation plant equipped with airtight safety valves;
percolate evacuation system with hydraulic seal.
As regards collection of the biogas, a suitably gauged
overpressure valve “opens” when a determined pressure is
reached in the digester, favouring the flow of the produced
biogas into a gasometer. The latter accumulates the gas
produced by several digesters and is used as a “lung” with
view to ensuring a regular and constant supply to the
cogenerator after the gas has passed through a treatment
system for dehumidification and desulphurisation.
The cogenerator consists of an endothermic engine, running on
biogas, which produces electricity by means of the generator
to which it is coupled and produces heat derived from the
cooling of the engine itself.
The system is equipped with a safety torch which, in the case
of problems with the cogeneration plant, will provide for
combustion of the biogas produced.
On completion of the anaerobic phase the reactor must be
cleared of biogas before opening the hatch, since the entry of
oxygen in the presence of methane could trigger explosive
reactions. To this end a system of insufflation of inert gases
has been provided so that the hatch closure bolts open only
when methane is no longer present in the atmosphere of the
digester.
Process monitoring system
The amBiogas digester is equipped with probes and sensors
which monitor the main process parameters continuously:
temperature of atmosphere and of the mass (3 x PT 100 ATEX
probes) and pressure. At several atmosphere sampling points,
samples are extracted from the digester and conveyed to a gas
analyser that determines the concentration of CH4,
CO2, O2 and H2S.
The same parameters are moreover checked in the anaerobic
percolate collection tanks. All the data gathered are conveyed
to an electrical management switchboard and therefrom to a
dedicated PC, serving several digesters, where the ambiControl
management and control software that governs the whole system
is installed.
AmbiControl management and control software
The control system consists of a Personal Computer with LCD
monitor, at least 19", keyboard, mouse and dedicated software
for supervision of the process. The Personal Computer will be
located in the offices and hooked up by network cable to the
electrical management switchboard with which every digester is
equipped.
The procedure is managed by ambiControl software which
processes the data and controls: the irrigation system,
switching the valves, raising and lowering the false ceiling
(if present),
opening and closure of the front hatch (making this possible
only when the concentration of methane inside the digester has
fallen below the potentially dangerous threshold, eliminating
risks of accidental opening due to human error) and the
inflation/deflation of the gasket. It controls and manages the
temperatures of both the mass and the percolate, acting on the
floor heating system and the exchanger respectively.
The software controls and memorises all phases of the process
and signals any anomaly with regard to the preset parameters.
The progression of the process is displayed in flow diagrams,
tables and menus of available functions, also providing for
filing the data.
Once a reactor has been loaded no particular intervention by
the operator is required apart from periodic checks on the
state of the batch being worked, which is carried out by
viewing the information displayed: a table with all the
functioning parameters acquired by the programme and a diagram
showing the progress in time. In the case of problems, special
windows advise the operator of the anomalous state of the work
processes in course. The programme also notes electricity
consumption, hourly and total, for each work cycle. The
programme signals the end of a cycle with a light.
System of biological filtering of gaseous emissions
As described above, the process-air extracted from the
digesters in the loading and unloading phases is bio-filter
purified before being released into the atmosphere. Moreover,
in plant with Ambientalia technology there are other sources
of gaseous refluxes to be purified, which is to say the waste
pre-treatment structures and the aerobic stabilisation
bio-cells.
To this end a system of biological filtering is proposed,
sized on the basis of the Lombardy Regional Administration
guidelines which are the strictest in the sector.
The bio-filter will be structured in separate, independently
functioning modules that can go on running if one has to be
stopped for maintenance.
A suitably sloped flooring (about 2%) in concrete will be laid
to bear the filling load.
The basic components of the bio-filter will be as follows:
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containment structure in reinforced
concrete, built with modular unit elements or laid in
situ. Each element will be 2 metres high.
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removable grid to support the
bio-filtering material, consisting of modular grille-tiles
suitable for a correct passage of air at low speed and
capable of ensuring homogeneous distribution.
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percolate draining system equipped with
hydraulic seal, with discharge to trap and therefrom to
the percolate collection tank.
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filtering bed with sizing 25-120 mm,
consisting of a composite mixture of vegetable material.
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system of humidification of the bed,
governed by the management and control software.
Air flow management
The air sucked from the digesters, the aerobic bio-cells and
the work process sheds is transferred to the bio-filter by way
of a stainless steel collector mounted on a galvanised carbon
steel framework fixed to the flooring. The system is completed
by flexible tubing, joints, sleeves, nozzles, butterfly and/or
guillotine valves with actuators governed by the control
system.
Anaerobic digestion in amBiogas reactors
Organic waste (wet from differentiated collection, agri-food
industrial discards, zootechnical excrement etc.), suitably
mixed with a preset percentage of ligno-cellulose fraction
(chipped vegetable waste), is fed into the digester with a
mechanical shovel.
To facilitate development and bacterial activity, the
temperature of the process material is maintained around
35/38°C through heating the reactor and sprinkling previously
heated percolate.
Use of the percolate, produced by the materials in digestion
and collected in dedicated tanks, also has the function of
microbic inoculation. Under these conditions the biogas
production process (absence of oxygen and constant mesophilia)
evolves through a first phase, known as acidogenic, of
demolition of the organic molecules and their transformation
into fatty acids and acetate, and a second phase, called
methanogenic, of degradation of the products from the
acidogenic phase into methane, carbon dioxide and water.
The quantity of biogas produced depends on the biomasses
treated: for example, using the organic fraction of the waste
we can obtain between 120 and 150 m3 of biogas for every tonne
of waste. The percentage of methane in the biogas varies on
average between 55 and 65%.
The operational cycle is carried on while the quantities of
biogas produced are maintained at economically advantageous
values. On average each cycle lasts 4 weeks.
On completion of the anaerobic digestion phase an inert gas is
insufflated into the reactor to remove residual methane, this
in order to avoid the formation of potentially explosive
gaseous mixtures (oxygen/methane) at the moment of opening the
hatch.
The AmBiogas digester’s sophisticated technological equipment
permits automated management of all process phases, constantly
guaranteeing optimal ecological conditions for the
micro-organisms and safety for the operators.
The pilot plant created at the Agricultural College in San
Michele all’Adige.
> Aerobic
stabilisation of amBiocell digestate
>
Presentation and data sheet (PDF)
< amBiogas
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