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Qila Anaerobic Digestion plant<br />
<strong>Step</strong> <strong>by</strong> <strong>step</strong> process
1 - Feedstock is taken from the clamps and<br />
loaded into the feeder<br />
Feedstock is stored on site in the clamps. The plant operator has to<br />
move the feedstock into the two feeder hoppers which holds and<br />
weighs the amount added. The plant will usually require 40,000 -<br />
46,000 tonnes of feedstock each year, this will be 60 - 120 tonnes fed<br />
each day, depending on the density of material being added. Feeding is<br />
required every 1-3 days.<br />
The feeder mixes and grinds the feedstock into small pieces and <strong>by</strong><br />
using a series of screw conveyors it transfers the solid matter towards<br />
the Vogelsang Rotacut.
2 - The feedstock is moved to the<br />
Vogelsang energy jet and RotaCut<br />
The Vogelang EnergyJet unit is an efficient solution which continually feeds the digester with wellmashed<br />
feedstock which has been mixed with liquid digestate re circulated from the storage tank.<br />
The process of mixing, mashing and masticating reduces the time needed in the tanks and increases<br />
the gas yield of the plant. To maintain the correct bacterial population in the Turbo Tank the<br />
substrate must have a dry matter content of 11%.<br />
Initially, the feedstock is moved along a conveyor to a screw, which removes any extra moisture and<br />
forms a dry plug. A high flow of liquid digestate, combined with a rotating screw blade breaks up the<br />
plug to form a mashed liquid suspension. This is further blended with a screw which enables liquid<br />
feedstock feeding into the Turbo Tank. The previously extracted moisture is collected and fed into<br />
the system, ensuring that no nutrients are wasted.<br />
The RotaCut macerator further breaks up the material before it is pumped into the Turbo tank. At<br />
the same time, it separates foreign bodies, such as stones and pieces of metal, preventing them<br />
from damaging parts of the system downstream and from collecting in the digester.
3 - The liquid feedstock is moved to the Turbo<br />
Tank<br />
The liquid feedstock suspension is pumped into the Turbo Tank, via the pump room. The first stage of the AD process, Hydrolysis,<br />
occurs in this tank as the matter is broken down <strong>by</strong> the bacteria. The tank is maintained at 50 -55 °C using hot water circulated in steel<br />
coils around the inner circumference of the tank. The water is heated to temperature <strong>by</strong> harnessing the heat produced in the<br />
Combined Heat and Power Engine (CHP) as it burns some of the biogas that is produced on site.<br />
The feedstock in the Turbo Tank is broken down <strong>by</strong> the microorganisms in about 2–3 days. During the hydrolysis phase, the gas that is<br />
produced, contains a high percentage of Carbon dioxide.<br />
The tank is made of concrete which is cast in-situ, this method provides a stronger more durable tanks than those constructed from<br />
concrete panels or stainless steel, and therefore offers a more robust design. The tank is then insulated to retain the heat and covered<br />
with aluminium cladding. This tank has a flat gas-tight concrete roof and the net volume of the tank is 1,407m 3 .<br />
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<br />
propeller and are designed to prevent the development of settled and floating layers in the tank. The mixers mounted at different<br />
heights, which enable mixing at all levels of the tank. This ensures a homogenised substrate and good circulation of the liquid thus<br />
maximising gas production. Efficient mixing, which is controlled via the control panel, avoids solidification and stratification of the<br />
substrate in the tank.<br />
The BG500 offers a powerful mixing technology, but with low-noise transmission <strong>by</strong> means of a toothed geared belt drive. The<br />
agitator mounting has anti-vibration bearings and the drive shaft has multiple ball bearings running in an oil bath. It offers a user<br />
friendly hydraulic adjustment system. The mixers gear boxes are mounted externally to enable access for maintenance.<br />
The movement of the liquid feedstock and substrate between the various tanks on the plant is via the pump room where the lobe<br />
pumps and controls are located. If the Turbo tank is unavailable, for example down for maintenance, there is an option to <strong>by</strong>pass the<br />
tank and move any liquid feedstock directly to the main digester.
4 - The liquid feedstock is then transferred to the<br />
digester tank – main biogas production<br />
After 2-3 days, the liquid feedstock is moved, via the pump room to the digester tank. Here<br />
the methanogenic bacteria continue to break down the organic matter and whilst doing so<br />
produce the majority of the biogas. The gas rises and collects at to the top of the digester,<br />
and then moved to the CHP engine, the upgrader unit or if into the final storage tanks. The<br />
digestate is moved to the separator, where it is separated into its liquid and solid forms.<br />
The main digester tank is maintained at 41-44 o C <strong>by</strong> using heating coils to circulate hot<br />
water around the tank. As with the Turbo Tank heat is taken from the CHP engine which<br />
burns biogas produced <strong>by</strong> the plant. The retention time in the main digester tank it about<br />
15-20 days. This gives an average retention time of processing feedstock through both<br />
tanks of 18.3 days. The net volume of the digester tank is in the region of 7,200 m 3 .<br />
The digester tank has walls made of concrete, which is cast in-situ, insulated and then<br />
covered with cladding. The digester and storage tanks have a double membrane dome roof<br />
provided <strong>by</strong> Baur. The inner shell is the integrated gas storage and the outer membrane is a<br />
durable weather protection foil. The tanks are also interconnected with pipes, so gas<br />
pressures can be monitored and biogas moved around the plant. The biogas is also stored<br />
in the storage tanks. The pressure of the gas is monitored on all the domes and if this<br />
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<br />
easily adjusted <strong>by</strong> a hydraulic unit, so the optimum positioning of the mixers can be controlled <strong>by</strong><br />
the operators who set the mixing pattern, which will depend on the exact characteristics of the<br />
substrate within the tank.<br />
The Mammut mixers have a 4.5m Shaft and a 1600mm propeller. These are highly efficient mixers,<br />
even when the dry substance content if high. The mixing provided is strong and slow, but also gentle<br />
which is well suited to operating in the digester tank. As the gear mechanism is outside the tank this<br />
allows for easy maintenance, as no electrical components are situated within the tank. The mixer is<br />
mounted with anti-vibration bearings which are fixed to an instillation bracket. This robust gear unit<br />
has low noise during operation. The drive shaft has multiple bearings and an oil-bath lubrication.<br />
• The biogas produced is mainly methane, (50% and 65%), Carbon dioxide (48%-33%) some water,<br />
Hydrogen sulphide and some other impurities in small quantities.<br />
• One of the impurities in the biogas is Hydrogen Sulphide (H 2 S), biological desulphurisation which<br />
removes this can be facilitated <strong>by</strong> injection of small amounts of oxygen, or air, into the tanks. This<br />
occurs as the oxygen encourages the growth of a certain type of bacteria, called thiobacilli, which<br />
occur on the surface of the digestate and as they grow and use the H 2 S in the biogas and leave a<br />
yellow clusters of sulphur on surface of the digestate. This process can also be performed in the<br />
storage tanks.
5 - Biogas is moved to the upgrader, where it is<br />
cleaned to generate Biomethane<br />
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 <strong>by</strong><br />
removing the CO 2 and other impurities to produce biomethane which is suitable to be injected into the national gas grid.<br />
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<br />
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<br />
should contain about 97-98% methane to be entered into the grid.<br />
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<br />
have different molecular size and varying solubility thus permeate the membrane to differing degrees. The membrane acts as a molecular sieve, letting<br />
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<br />
pass though the membrane becomes enriched in methane.<br />
The technology is an efficient and cost effective method to upgrade biogas and requires a low energy input compared to some alternatives.<br />
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<br />
impurities and chilled. Active carbon filters are used to remove Hydrogen Sulphide (H 2 S) <strong>by</strong> a process called adsorption. The removal of the H 2 S protects<br />
the plant and the membranes. The pressurised gas is then heated and passed through the biogas separation membrane unit.<br />
The biogas separation membrane unit is contained within stainless steel housing. The membranes split the gas into CO 2 and a methane rich gas.<br />
If there is a build-up of gas over and above the amount that can be safely stored or processed <strong>by</strong> the upgrader and the CHP unit, this can be securely<br />
disposed off via the flare.<br />
Need to ensure consistency on 97/98%. Neither is ‘right’.
6 -Biomethane is moved to the Grid<br />
Entry unit<br />
Once it is upgraded, with about 97 - 98% methane, the biogas moves to the grid entry unit.<br />
This is the equipment, which monitors the quality of the gas and ensures that it conforms<br />
to the specifications and pressure of the gas required in the grid.<br />
Natural gas is made up of several gases, some of which have a higher calorific value than<br />
methane. To bring the biomethane up to the same energy content, approximately 4% of<br />
propane must be added at the point of injection. An odorant must also be added for health<br />
and safety reasons – as this gives the gas a smell and should it escape it can be detected.<br />
The pressure of the gas must also match that of the pipline in which it will be injected.<br />
Prior to injection, the Grid Entry Unit adds propane (approximately 4%) to the biogas so<br />
that it matches the calorific value of the gas in the national grid. The Renewable Heat<br />
Incentive (RHI) subsidy is payable on all eligible biomethane injected.<br />
Need to use this term. RHI is not paid on everything injected, but there is a term in the<br />
scheme of ‘eligible biomethane’ which takes off deductions for propane.
7 - Biomethane enters the National Gas<br />
Grid and reaches the end user<br />
Once in the grid, it is easy to arrange to sell the gas to residential and<br />
industrial customers.<br />
As well as the benefits to the farmer, using biogas to displace gas from<br />
fossil sources reduces levels of greenhouse gases in the atmosphere.<br />
This is because all the CO2 released on combusting the biogas has<br />
recently been absorbed from the atmosphere, unlike for the fossil<br />
fuels.<br />
Upgrading biogas to biomethane maximises these benefits, as far less<br />
of the energy is wasted compared to using it to generate electricity or<br />
power vehicles.
8 - Some of the biogas is used to generate<br />
electricity using a Combined Heat and Power<br />
(CHP) unit<br />
Some of the biogas, which is stored in the main digester and the storage tanks, is moved to<br />
a Combined and Heat Power (CHP) unit, which powers an alternator and generates<br />
electricity. The electricity is used to power the plant and additional electricity is sold to the<br />
power grid. Annually a 499kw CHP unit is expected to generate about 4,000,000 kWh/a if<br />
it runs at 92% availability which enables the plant to run on self-generated power.<br />
During the process, heat is also produced, this is harnessed to heat water that is circulated<br />
to the Turbo tank and the digester tank to keep them at the required temperatures. Other<br />
uses for the heat produced are to dry the solid digestate or to heat any buildings. The CHP<br />
is expected to produce about 4,700,000 kWh of thermal energy each year that is used in<br />
the plant.<br />
There are subsidies for the production of the electricity in a CHP and also for using the heat<br />
that is produced.<br />
Before the biogas is moved to the CHP unit it is cooled down to condense and remove any<br />
moisture that is in the gas, it is then reheated to avoid further condensation and passed<br />
through a carbon filter to remove any remaining H 2 S. The gas is then pressurised with a<br />
biogas blower and enters the CHP unit.
9 - Digestate is removed from the main digester<br />
tank, separated and returned to farm land<br />
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.<br />
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<br />
spread onto farm land.<br />
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<br />
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<br />
liquid digestate spreading and the plant must be able to hold the digestate produced over this time.<br />
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<br />
controlled gas flow between the tanks and parts of the plant, the biogas produced is moved to the upgrader or the CHP engine.<br />
As with the main digester oxygen is also introduced to into the gas storage to facilitate the biological removal of the hydrogen sulphide.<br />
The digestate is a nutrient-rich <strong>by</strong>-product of the AD process. It contains the leftover organic material and any dead microorganisms. It is separated into solid and liquid fractions, both<br />
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<br />
form<br />
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.<br />
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<br />
benefit of an AD plant. Significant improvements in both yield and crop quality are possible.<br />
The typical values for the digestate are:<br />
Nitrogen: 2.3-4.2 kg/tonne<br />
Potassium: 1.3-5.2 kg/tonne<br />
Phosphorus 0.2 1.5 kg/tonne<br />
The solid portion will typically contain 20% to 40% of the phosphorous and the liquid fraction will contain most of the nitrogen.<br />
See the picture I will send separately
10 - The Qila control panel<br />
Qila plants are managed <strong>by</strong> a bespoke control system. This provides central engineering, operation<br />
and monitoring of field technology, as well as, a motor control centre and switchgear with one<br />
integrated view of the production and electrical processes. It is user friendly and intuitive design<br />
allows multifunctional controls and reporting. Remote access login in allows off site monitoring of<br />
the site when required and a digital cloud-based monitoring control system can be operated from<br />
smart phones and tablets whilst on the move. As the system is designed <strong>by</strong> Qila, it can be made to<br />
cater for the individual needs of each plant.<br />
An integrated view of the production and electrical processes is provided as the control centre<br />
monitors the system<br />
Increasing plant availability and reducing maintenance requirements through integrated asset<br />
management functionalities and revolutionary online diagnostics<br />
Low installation costs through the use of bus technology allows you to add devices quickly and<br />
easily<br />
Lower engineering costs due to comprehensive software tools<br />
Reduced commissioning time through flexible automation solutions<br />
Increasing transparency through automatic archiving and reporting of process and plant information
Qila Energy is a British biogas engineering company building CHP and<br />
biomethane gas to grid AD plants across the UK.<br />
No matter what stage of your project, our team of UK based engineers can<br />
help you throughout the throughout the whole process, from initial<br />
design, construction and 24/7 ongoing maintenance. We can help you<br />
with:<br />
• Planning permission<br />
• Environmental Permitting Regulations<br />
• Design of the AD plant<br />
• Construction and Construction (Design and Management) Regulations<br />
(CDM)<br />
• Feedstock planning<br />
• Grid connection<br />
• FIT & RHI payments<br />
• OFGEM sustainability
Qila Energy | Royal Institution of Great Britain | 21 Albemarle Street | London | W1S 4BS<br />
020 3603 3016 | info@qilaenergy.com | qilaenergy.com