Step by step - 06-09-2016

Qila Anaerobic Digestion plant
Step by step process

1 - Feedstock is taken from the clamps
and loaded into the feeder
Feedstock is gathered and delivered to the Anaerobic Digestion plant. It is
transported in pre-weighed trucks and driven onto weigh bridge, so that
each ton of feedstock is measured into the plant. The feedstock is then
stored in holding clamps, ready to be inserted into the plant.
The plant operator will then move the feedstock into hoppers, which
holds and weighs the amount added.
The plant will usually require 40,000 - 46,000 tonnes of feedstock each
year, this will be approximately 60 - 120 tonnes time it is fed, depending
on the density of material being added. Feeding is required every 1-3
days.
The feeder mixes and grinds the feedstock into small pieces, using a
series of screw conveyors, it then transfers the solid matter towards the
Energy Jet and Rotacut units.

2 - The feedstock is moved to the Energy Jet
and RotaCut units
The EnergyJet unit is an efficient solution, which continually feeds the digester with well-mashed
feedstock which has been mixed with liquid Digestate (which is circulated from the storage tank.)
The process of mixing, mashing and masticating reduces the time needed in the tanks and
increases the gas yield of the plant. To maintain the correct bacterial population in the Turbo Tank,
the substrate must have a dry matter content of 11%.
Initially, the feedstock is moved along a conveyor to a screw, which removes any extra moisture and
forms a dry plug. A high flow of liquid digestate, combined with a rotating screw blade breaks up
the plug to form a mashed liquid suspension. This is further blended with a screw which enables
liquid feedstock feeding into the Turbo Tank. The previously extracted moisture is collected and fed
into the system, ensuring that no nutrients are wasted.
The RotaCut macerator unit, further breaks up the material before it is pumped into the Turbo
Tank. At the same time, it separates foreign bodies, such as stones and pieces of metal, preventing
them from damaging parts of the system downstream and from collecting in the digester.

3 - The liquid feedstock is moved to the
Turbo Tank
The liquid feedstock suspension is pumped into the Turbo Tank, via the pump room. The first stage
of the AD process, Hydrolysis, occurs in this tank as the matter is broken down by the bacteria. The
tank is maintained at 50 -55 °C, using hot water circulated in steel coils around the inner
circumference of the tank. The water is heated to temperature by harnessing the heat produced in
the Combined Heat and Power Engine (CHP), as it burns some of the biogas that is produced on
site.
The feedstock in the Turbo Tank is broken down by the microorganisms in approximately 2–3 days.
During the hydrolysis phase, the gas that is produced, contains a high percentage of Carbon
Dioxide.
The tank is made of concrete which is cast in-situ, this method provides a stronger more durable
tanks than those constructed from concrete panels or stainless steel, and therefore offers a more
robust design. The tank is then insulated to retain the heat and covered with aluminium cladding.
This tank has a flat gas-tight concrete roof and the net volume of the tank is 1,407m 3 .

The Qila design uses three mixers in the Turbo Tank. These high quality
mixers have a 5.75m long shaft and 840mm propeller and are designed to
prevent the development of settled and floating layers in the tank. The
mixers mounted, at different heights, which enable mixing at all levels of
the tank. This ensures a homogenised substrate and good circulation of
the liquid thus maximising gas production. Efficient mixing, which is
controlled via the control panel, avoids solidification and stratification of
the substrate in the tank.
This unit offers a powerful mixing technology, with low-noise transmission
by means of a toothed geared belt drive. The agitator mounting has
anti-vibration bearings and the drive shaft has multiple ball bearings
running in an oil bath. It offers a user friendly hydraulic adjustment system.
The mixers gear boxes are mounted externally to enable access for
maintenance.
The movement of the liquid feedstock and substrate between the various
tanks on the plant is via the pump room where the lobe pumps and
controls are located. If the Turbo tank is unavailable, for example down for
maintenance, there is an option to bypass the tank and move any liquid
feedstock directly to the main digester.

4 - The liquid feedstock is then transferred to
the digester tank – main biogas production
After 2-3 days, the liquid feedstock is moved, via the pump room, to the digester tank. There the
methanogenic bacteria continue to break down the organic matter and produces the majority of the biogas.
The gas then rises and collects at to the top of the digester and is then moved to the CHP engine, the
upgrader unit or into the final storage tanks. The digestate is moved to the separator unit, where it is
separated into its liquid and solid forms.
The main digester tank is maintained at 41-44 o C using heating coils to circulate hot water around the tank.
The Turbo Tank heat is taken from the CHP engine which burns biogas produced by the plant. The retention
time in the main digester tank it about 15-20 days. This gives an average retention time of processing
feedstock through both tanks of 18.3 days. The net volume of the digester tank is in the region of 7,200 m 3 .
The digester tank has walls made of concrete, which is cast in-situ, insulated and then covered with cladding.
The tanks have a double membrane dome roof. The inner shell is the integrated gas storage and the outer
membrane is a durable weather protection foil. The tanks are also interconnected with pipes, so gas
pressures can be monitored and biogas moved around the plant. The biogas is also stored in the storage
tanks. The pressure of the gas is monitored on all the domes and if this becomes too high there is a hydraulic
pressure release valve to release biogas, if needed.

The digester tank has four mixers. The variable angle of the mixers
is easily adjusted by a hydraulic unit, so the optimum positioning of
the mixers can be controlled by the operators who set the mixing
pattern, which will depend on the exact characteristics of the
substrate within the tank. The mixers have a 4.5m shaft and a
1600mm propeller. These are highly efficient mixers, even when
the dry substance content if high. The mixing provided is strong
and slow, but also gentle which is well suited to operating in the
digester tank.
As the gear mechanism is outside the tank, it allows for easy maintenance, as no electrical components are
situated within the tank. The mixer is mounted with anti-vibration bearings which are fixed to an instillation
bracket. This robust gear unit has low noise during operation. The drive shaft has multiple bearings and an oil-bath
lubrication.
The biogas produced is mainly methane, (50% and 65%), Carbon dioxide (48%-33%) some water, Hydrogen
sulphide and some other impurities in small quantities.
One of the impurities in the biogas is Hydrogen Sulphide (H 2 S), biological desulphurisation which removes this can
be facilitated by injection of small amounts of oxygen, or air, into the tanks. This occurs as the oxygen encourages
the growth of a certain type of bacteria, called thiobacilli, which occur on the surface of the digestate and as they
grow and use the H 2 S in the biogas and leave a yellow clusters of sulphur on surface of the digestate. This process
can also be performed in the storage tanks.

5 - Biogas is moved to the upgrader, where
it is cleaned to generate Biomethane
Biogas is moved from where is stored in the domes of the digester and storage tanks, to the
upgrader unit. In the upgrader, the gas is cleaned by removing the CO 2 and other impurities to
produce biomethane which is suitable to be injected into the national gas grid.
Biogas usually contains around 60% methane, 40% CO 2 , a small amount of water, hydrogen sulphide
and a few other impurities. In order to inject the gas into the national grid, the upgrader unit must
remove all the moisture, hydrogen sulphide and other impurities and most of the CO 2 . The
biomethane should contain about 97-98% methane to be entered into the grid.
The Qila design uses a high quality membrane technology from Pentair Haffmans to separate the
methane from the the CO 2 and other gasses. The gasses have different molecular size and varying
solubility thus permeate the membrane to differing degrees. The membrane acts as a molecular
sieve, letting some gasses pass through easily, such as Co 2 and water vapour whilst the methane
(CH 4 ) does not pass through the membrane. The gas that does not pass though the membrane
becomes enriched in methane.

The technology is an efficient and cost effective
method to upgrade biogas and requires a low energy
input compared to some alternatives.
As the gas enters the upgrader it is pressurised and the
methane content of the gas is analysed. The gas is
then scrubbed to remove water soluble impurities and
chilled. Active carbon filters are used to remove
Hydrogen Sulphide (H 2 S) by a process called
adsorption. The removal of the H 2 S protects the plant
and the membranes. The pressurised gas is then
heated and passed through the biogas separation
membrane unit.
The biogas separation membrane unit is contained
within stainless steel housing. The membranes split
the gas into CO 2 and a methane rich gas.
If there is a build-up of gas over and above the
amount that can be safely stored or processed by the
upgrader and the CHP unit, this can be securely
disposed off via the flare.

6 - Biomethane is moved to the Grid
Entry unit
Once the gas has upgraded to about 97 - 98% methane, the biogas moves
to the grid entry unit. This equipment monitors the quality of the gas
and ensures that it conforms to the specifications and pressure of the gas
required in the grid.
Natural gas is made up of several gases, some of which have a higher
calorific value than methane. To bring the biomethane up to the same
energy content, approximately 4% of propane must be added at the point
of injection. An odorant must also be added for health and safety reasons
– as this gives the gas a smell and should it escape it can be detected. The
pressure of the gas must also match that of the pipeline in which it will
be injected.
Prior to injection, the Grid Entry Unit adds propane (approximately 4%)
to the biogas so that it matches the calorific value of the gas in the
national grid. The Renewable Heat Incentive (RHI) subsidy is payable on
all eligible biomethane injected.

7 - Biomethane enters the National Gas
Grid and reaches the end user
Once the gas is in the grid, it is easy to arrange to sell the gas to
residential and industrial customers.
As well as, the benefits to the farmer, using biogas to displace gas
from fossil sources reduces levels of greenhouse gases in the
atmosphere. This is because all the CO2 released on combusting
the biogas has recently been absorbed from the atmosphere,
unlike for the fossil fuels.
Upgrading biogas to biomethane maximises these benefits, as far
less of the energy is wasted compared to using it to generate
electricity or power vehicles.

8 – Biogas is used to generate electricity using
a Combined Heat & Power (CHP) unit
Some of the biogas, which is stored in the main digester and the storage tanks, is moved to a Combined and
Heat Power (CHP) unit, which powers an alternator and generates electricity. The electricity is used to power
the plant and additional electricity is sold to the power grid. Annually a 499kw CHP unit is expected to
generate about 4,000,000 kWh/a if it runs at 92% availability which enables the plant to run on self-generated
power.
During the process, heat is also produced which is harnessed to heat water circulated to the Turbo Tank and
used to keep the digester tank at the required temperatures. Other uses for the heat produced are to dry the
solid digestate or to heat any buildings. The CHP unit is expected to produce about 4,700,000 kWh of thermal
energy each year which is used within in the plant. There are subsidies for the production of the electricity in a
CHP unit and using the heat that is produced.
Before the biogas is moved to the CHP unit, it is cooled to condense and remove any moisture content which is
in the gas. It is then reheated, to avoid further condensation and passed through a carbon filter to remove any
remaining H 2 S. The gas is then pressurised with a biogas blower and enters the CHP unit.

9 - Digestate is removed from the main
digester tank, separated and returned to
farm land
The process of converting feedstock into biogas leaves a residue - the digestate. This is then passed through a screw press which separates the
solid fibrous fraction from the liquid. The liquid digestate is piped to the storage tanks and the solid matter is collected under the screw press
in a dedicated skip below the separator – both are stored until they can be spread onto farm land.
The liquid digestate is transferred to the storage tanks via the pump room. The combined storage capacity of the tanks must be able to hold
the liquid digestate, and any contaminated rainwater, produced during for 6 months of operation this is estimated to be 14,500 m 3 . This is
important as there are times of the year when there are restrictions to liquid digestate spreading and the plant must be able to hold the
digestate produced over this time.
The storage tank has a gas tight dome and 3 mixers are installed with keep the digestate moving. There continues to be some biogas
production from the storage tank, and as there is controlled gas flow between the tanks and parts of the plant, the biogas produced is moved
to the upgrader or the CHP engine. As with the main digester oxygen is also introduced to into the gas storage to facilitate the biological
removal of the hydrogen sulphide.
The digestate is a nutrient-rich by-product of the AD process. It contains the leftover organic material and any dead microorganisms. It is
separated into solid and liquid fractions, both of which can be used as a biofertiliser. As a general rule the amount of digestate produced will be
90% of the feed stock. Of this digestate, about 25% is solid and the rest is in liquid form
The digestate can have a significant fertilizer value and also assists in returning organic matter to the soil. It should be applied to the land
according to a nutrient management plan. Using digestate in place of synthetic fertilisers saves energy, cuts consumption of fossil fuels and
reduces farm expenditure and management costs. This is an often underestimated benefit of an AD plant. Significant improvements in both
yield and crop quality are possible.
The typical values for the digestate are:
Nitrogen: 2.3-4.2 kg/tonne Potassium: 1.3-5.2 kg/tonne Phosphorus 0.2 1.5 kg/tonne

10 - The Qila control panel
Qila plants are managed by a bespoke control system. This provides central engineering, operation and
monitoring of field technology, as well as, a motor control centre and switchgear with one integrated
view of the production and electrical processes. It is user friendly and intuitive design allows
multifunctional controls and reporting.
Remote access login in allows off site monitoring of the site when required and a digital cloud-based
monitoring control system can be operated from smart phones and tablets whilst on the move. As the
system is designed by Qila, it can be made to cater for the individual needs of each plant.
An integrated view of the production and electrical processes is provided as the control centre monitors
the system. This provides:
• Increasing plant availability and reducing maintenance requirements through integrated asset
management functionalities and revolutionary online diagnostics.
• Low installation costs through the use of bus technology allows you to add devices quickly and easily
• Lower engineering costs due to comprehensive software tools
• Reduced commissioning time through flexible automation solutions
• Increasing transparency through automatic archiving and reporting of process and plant information

How we can help you?
Qila Energy is a British biogas engineering company building CHP and
biomethane gas to grid AD plants across the UK.
No matter what stage of your project, our team of UK based engineers can
help you throughout the throughout the whole process, from initial design,
construction and 24/7 ongoing maintenance. We can help you with:
• Planning permission
• Environmental Permitting Regulations
• Design of the AD plant
• Construction and Construction (Design and Management) Regulations
(CDM)
• Feedstock planning
• Grid connection
• FIT & RHI payments
• OFGEM sustainability

Qila Energy | Royal Institution of Great Britain | 21 Albemarle Street | London | W1S 4BS
020 3603 3016 | info@qilaenergy.com | qilaenergy.com