Step by step

Qila Anaerobic Digestion plant
Step by step process

1 - Feedstock is taken from the clamps and
loaded into the feeder
Feedstock is stored on site in the clamps. The plant operator has to
move the feedstock into the two feeder 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 60 - 120 tonnes fed
each day, 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 and by
using a series of screw conveyors it transfers the solid matter towards
the Vogelsang Rotacut.

2 - The feedstock is moved to the
Vogelsang energy jet and RotaCut
The Vogelang EnergyJet unit is an efficient solution which continually feeds the digester with wellmashed
feedstock which has been mixed with liquid digestate re 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 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 about 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 BG500 Paul Michl 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.
The BG500 offers a powerful mixing technology, but 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. Here
the methanogenic bacteria continue to break down the organic matter and whilst doing so
produce the majority of the biogas. The gas rises and collects at to the top of the digester,
and then moved to the CHP engine, the upgrader unit or if into the final storage tanks. The
digestate is moved to the separator, where it is separated into its liquid and solid forms.
The main digester tank is maintained at 41-44 o C by using heating coils to circulate hot
water around the tank. As with 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 digester and storage tanks have a double membrane dome roof
provided by Baur. 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 Mammut RF3 mixers from Paul Michl. 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 Mammut 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 this
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.
Need to ensure consistency on 97/98%. Neither is ‘right’.

6 -Biomethane is moved to the Grid
Entry unit
Once it is upgraded, with about 97 - 98% methane, the biogas moves to the grid entry unit.
This is the equipment, which 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 pipline 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.
Need to use this term. RHI is not paid on everything injected, but there is a term in the
scheme of ‘eligible biomethane’ which takes off deductions for propane.

7 - Biomethane enters the National Gas
Grid and reaches the end user
Once 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 - Some of the biogas is used to generate
electricity using a Combined Heat and 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, this is harnessed to heat water that is circulated
to the Turbo tank and the digester tank to keep them at the required temperatures. Other
uses for the heat produced are to dry the solid digestate or to heat any buildings. The CHP
is expected to produce about 4,700,000 kWh of thermal energy each year that is used in
the plant.
There are subsidies for the production of the electricity in a CHP and also for using the heat
that is produced.
Before the biogas is moved to the CHP unit it is cooled down to condense and remove any
moisture that 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
The solid portion will typically contain 20% to 40% of the phosphorous and the liquid fraction will contain most of the nitrogen.
See the picture I will send separately

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
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

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