The QUB ATBEST fellows were recently lucky enough to visit an AD plant under construction by ATBEST associate partners AgriAD. In this blog, researcher Fabio De Rosa describes the visit and what he learned from it.
Most of the jokes in my country start with “there were once a German, a French, a British and a guy from Naples…”. Things were slightly different during this trip, not only because of the company, but because when it comes to biogas we are not talking about jokes but about a great business instead.
Rawan (Lebanon), Liang (China), Joanna (Poland), Simon (Ireland) and I (the Italian guy) went to visit a 500 KW AD plant under construction located in Banbridge, County Down, Northern Ireland, owned by the directors of agriAD Ltd Thomas Cromie (Figure 1) (Figure 2).
Figure 1 – QUB fellows with Mr Thomas Cromie
Figure 2 – Aerial view and map of the AD plant in Banbridge, County Down, Northern Ireland from AgriAD
Thomas Cromie has 20 years’ experience in small and medium-sized enterprises active in the agricultural and energy industries. He has worked in collaboration with local government like the Department of Enterprise, Trade and Investment of Northern Ireland (DETI NI), for the development of regional policies in these sectors and on matters related to the exploitation and commercialisation of science, technology and R&D.
He is founder and partner in agriAD Ltd, which deals with the development and operation of joint-venture biogas projects, providing funding, technology and expertise in anaerobic systems. Thomas comes from a farming family, and graduated in geology at Queen’s University Belfast and after a working experience for an oil company in Saudi Arabia (back when the oil price was still 25$/barrel) he came back to the Emerald Island in the late 80s, recognizing what was raising in Germany during those years as a possible bargain for his family agricultural business: anaerobic digestion.
It took several years of travelling to the USA, Germany, Italy, France and Netherlands to understand the developing market, build contacts and meet politicians.
AgriAD’s purpose is to develop a network of standalone farm-based AD plants across Northern Ireland. It is proposed that the AD plants will be strategically located on large farms and operate in partnership with the farmer. Under the partnership, the farmer will provide a suitable site with planning permission for the AD facility, guaranteed feedstock (typically grass silage and animal manure) and the manpower to operate the facility. AgriAD will in turn provide the majority of finance, engineer, procure and construct (EPC) contractor, project delivery experience, project development and process management, biological support and plant maintenance.
The advantages from this network and the partnership between farmers and landowners include project experience and credibility, purchasing power, operational economies of scale and operational resilience (e.g. feedstock pooling, mitigating the key supply risk). The plant is currently under construction and will be operational in June 2015.
As Thomas says, “anaerobic digestion is a matter of opportunity”. Anaerobic digestion in Northern Ireland is a well-established technology and a great opportunity thanks to the perfect grassland conditions for AD and the most attractive financial regime in Europe. Indeed, Renewables Obligation Certificates (ROCs) represent the driving force of this economy.
ROCs are green certificates issued in UK to operators of renewable generating stations for the renewable electricity they generate and are basically used by suppliers to demonstrate that they have met their obligation . Every year the RO requires UK electricity suppliers to source a specified proportion of the electricity they provide to customers from renewable sources. ROCs are tradeable commodities that have no fixed price, i.e. the amount an electricity supplier pays for a ROC is a matter for negotiation between the supplier and generator  and changes according to the technology priority. Currently AD ( 500kW), Hydro ( 20kW), Onshore wind ( 250kW) and Solar Photovoltaic ( 50kW) technologies have all been assigned the highest values for 2014/2015 , which is equal to 4 ROCs. There is significant on the ground interest, with 86 AD project permissions granted in 2014  (Figure 2).
According to Mr. Cromie, AD brings more benefits to the rural communities. For example wind turbines have virtually no operating costs, whereas AD generates 200-300k£/year that can be spent to further develop the renewable energy sector. Moreover electricity generation through biogas has a constant load, whereas wind energy is more fluctuating.
It might seem odd, but also giant oil companies like Shell and its subsidiaries are very interested in renewable energies, photovoltaic and biogas on top . This is what looking ahead means.
The electricity grid in Northern Ireland was constructed to push the electricity from three large fossil fuel generating stations (Maydown, Kilroot and Ballylumford) around the coast into the interior of the country. Reconfiguring the grid to allow generation in other locations requires major investment.
Figure 3 – Projects with planning approval in Northern Ireland
The challenges for AD in NI are the funding scheme, small scale and individual projects as a barrier, the limited experience of project financing within the NI farming sector, the infrastructures and also the number of farmers in the UK that has been decreasing of the 4-5% per year in the last decade.
Nowadays only the 7% of the AD projects in NI is operational, 10% are under construction, 54% have been approved but seeking for funding while 29% is waiting for planning. Only 16% of the total fundings are secured. Regarding the source, 50% are provided by banks, 23% by contracts, 14% by leasing and 9% are unsecured .
The total project cost of the plant in Banbridge is around £2.5m and two thirds of the heavy concrete construction is already done. Usually it takes 9 months to build the plant, under and EPC contractor: fixed price and time and penalty in case of late delivery of the plant.
At the beginning the whole feedstock will be grass silage secured within Cromie family landholdings.
Later industrial wastes from a near milk factory will be integrated, representing the 25% of the feedstock. The advantage is that the feedstock composition is known (there is no need for pre-processing) and in any case these wastes have to be pasteurized before disposal, so that using them in an AD plant makes sense.
The biogas yield will be equal to around 2Mm3/year. The combined heat and power system creates the opportunity to transport heat to a number of major heat users nearby (Figure 4). It will work for 8000 h/year in order to produce 4MKWh of electricity with a 91.3% operational efficiency. There are operation responsibilities and should the electricity generation be less than planned a penalty has to be paid. Knowing the outputs is paramount with respect to the funding.
Statistically 55-60% of the problems in an AD plant arise from CHP inefficiencies and only the 10-15% is due to human operators.
Figure 4 – Combined heat and power (CHP) unit
Figure 5 – CHP engine
The CHP engine is basically a modified Diesel engine, factory made and tested, operating with a biogas consisting of 55% of methane (Figure 5).
There are radiators for the excess heat developed in the CHP unit, activated carbon filters and a gas boiler to keep the biological temperature during the start up of the plant and in case of problems with the CHP unit (Figure 6). It takes 4-5 weeks to heat the system up to the biological temperature of the process.
Figure 6 – Gas boiler, radiators for excess heat and activated carbon filter.
There is a primary digester, which receives the feedstock by gravity, and a secondary digester (Figure 7). The tanks are factory-made, which means that the concrete wasn’t poured on site, but each slab was shaped singularly elsewhere and put together in loco. This allows for a faster construction.
Figure 7 – Primary and secondary digester and control room (on the left)
The control building is on the left in Figure 7, with the pumps in the cellar.
Usually the retention time depends on the feedstock and it is around 40 days. It will be around 100 for this plant, because of the grass silage.
The pumps and the pipes going to the second digester from the control room cellar offer a high operational flexibility to this plant (Figure 8).
Figure 8 – Pump room
There are 3 points in the tank at three different heights where the temperature is monitored (Figure 9). The optimal value is around 35-40°C and when the variation between the three points is no more than 0.1°C then it means that there is a good mixing inside the tank.
The sampling point is at 3-4 feet, while the other 2 holes below are the drain points.
In both tanks there is a propeller mixing system. When you have to fix the mixing system in case of faulty, large tanks have to be emptied first. These two tanks have a particular sleeve which allows for fixing the propellers without emptying the tank and venting the gas.
Figure 9 – Inside of the digester
Figure 10 – Primary and secondary digester from the inside
The concrete tank lifetime is equal to 25 years, also because of the black coating in the top part. It protects the concrete from the biogas, which is corrosive (Figure 9 and Figure 10).
Small amount of oxygen can be fed in the overhead in order to oxidize hydrogen sulphide, producing elemental sulphur. This is a way to get a grass fertilizer which is even better than the commercial ones (11% more yield).
The digestate will be first collected in a storage tank and then spread in the surrounding fields after September (Figure 11). This is because NI is a nitrates and phosphorous vulnerable zone and spreading is not allowed through the winter.
Figure 11 – Digestate storage tank
Operation should start in 2 months, with slurry at the beginning and a little of digestate from another AD as inoculum. Before that the two domes have to be put in place and a pressure test, using just water, has to be carried out.
As a conclusion, this trip was extremely interesting and stimulating. We had the chance to see first-hand how an anaerobic digester plant is constructed and moreover to discuss with a professional in the sector.
1. [Online] https://www.ofgem.gov.uk/environmental-programmes/renewables-obligation-ro.
2. [Online] https://www.gov.uk/government/policies/increasing-the-use-of-low-carbon-technologies/supporting-pages/the-renewables-obligation-ro.
3. [Online] http://www.detini.gov.uk/existing_and_confirmed_roc_per_mwh_levels_from_1_april_2013.pdf.
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