Anaerobic Digestion Demonstration of The Agri … Biogas.pdf · Anaerobic Digestion – Demonstration of The Agri Digestore – Final Report Driving Innovation in AD: Marches Biogas

Anaerobic Digestion – Demonstration of The Agri Digestore – Final Report

Driving Innovation in AD:

Marches Biogas Agri Digestore

Following a feasibility study undertaken by Marches Biogas, a private company

based in Ludlow, Shropshire the Agri Digestore at Hall Farm, Snitton, Nr Ludlow,

Shropshire was designed, built and commissioned. This Agri Digestore has been

used to demonstrate the system’s successful when fed on agricultural residues,

which are primarily on farm cattle slurry.

Project code: OIN001-512

Research date: November 2015-present Date: 25/04/2016

Anaerobic Digestion – Demonstration of The Agri Digestore – Final Report 2

WRAP’s vision is a world in which resources are used

sustainably.

Our mission is to accelerate the move to a sustainable

resource-efficient economy through re-inventing how

we design, produce and sell products; re-thinking how

we use and consume products; and re-defining what is

possible through re-use and recycling.

Find out more at www.wrap.org.uk

This report was commissioned and financed as part of WRAP’s Driving Innovation in AD programme. The report remains entirely the responsibility of the author and WRAP accepts no liability for the contents of the report howsoever used. Publication of the report does not imply that WRAP endorses the views, data, opinions or other content contained herein and parties should not seek to rely on it without satisfying themselves of its accuracy.

Document reference: OIN001-512 Driving Innovation in Anaerobic Digestion, small scale – Marches Biogas Ltd.,

Prepared by: Russell Mulliner & Jamie Gascoigne – Marches Biogas Ltd.

Written by: Russell Mulliner, Lucy Lewis and Zoe Ashbridge, contributions made by Joe Mann,

Jamie Gascoigne, Lucy Downes and Hope Baldwin, on behalf of Marches Biogas Ltd.

Front cover photography: Hall Farm, Ludlow – a Marches Biogas AGRIdigestore.

While we have tried to make sure this report is accurate, we cannot accept responsibility or be held legally responsible for any loss or damage arising out of or in

connection with this information being inaccurate, incomplete or misleading. This material is copyrighted. You can copy it free of charge as long as the material is accurate

and not used in a misleading context. You must identify the source of the material and acknowledge our copyright. You must not use material to endorse or suggest we

have endorsed a commercial product or service. For more details please see our terms and conditions on our website at www.wrap.org.uk

http://www.wrap.org.uk/


Executive summary

There is a demand for farm-fed anaerobic digesters, due to the many agricultural and

environmental benefits such as:

Waste management

Control of slurry and manures.

Nutrient rich digestate

Economic gain from the production of nutrient rich digestate on farm.

Recycling manures and slurries on site.

Increased self-sufficiency

Utilisation of existing wastes for energy that can be used on site, or

exported.

Eradicates the need for external fertilisers.

Income generation

Green credentials

The challenge can be meeting the specifications surrounding economic viability. The Agri

Digestore assists the farmer and provides an affordable, yet effective facility that can

store slurry to comply with NVZ regulations, whilst generating biogas and therefore

revenue from various tariffs, such as Feed in Tariff (FiTs) and Renewable Heat Incentive

(RHI).

The Agri Digestore was designed and constructed between 2013 and 2015, with

commissioned delayed until January 2016. This extended build time and delayed

commissioning was due to Marches Biogas receiving unprecedented demand for

commercial anaerobic digestion with constrained time frames due to pre-accreditation

dates which had to be met.

This project has demonstrated the successful construction and operation of the Agri

Digestore through the capture of greenhouse gases in the form of biogas. The biogas is

being produced and used to maintain the anaerobic process with slurry being fed to the

digester when the farmer requires it to be transferred from the reception pit. The

digester temperature is being maintained at 27.2oC with a gas production of 147m3 per

day at a methane quality of 64%. The lower than expected biogas production is due to

the high rainfall during the winter months which has diluted the slurry at the reception

pit lowering the dry matter of the slurry entering the Agri Digestore. The Agri Digestore

has allowed the farmer to keep the slurry in storage until it is required on the land and

therefore comply with NVZ regulations. The plant needs to operate for a full year before

complete conclusions can be drawn on the benefit of the enhanced slurry characteristics

together with a full economic assessment, Marches Biogas will be monitoring the plant

closely over the coming months to full fill these aims.

Conclusions can be drawn from other Agri Digestore projects where the biggest benefit

to the farmer is the enhanced slurry characteristics in terms of greater availability of

nitrogen, increased concentration of nitrogen and consistency of product. Marches

Biogas can also conclude that despite the consistent interest in the Agri Digestore, due


to the degression of the Feed in Tariffs (FiTs) and Renewable Heat Incentives (RHI) for

anaerobic digestion, it is now very unlikely that projects of this nature are going ahead

to build unless small scale anaerobic digestion is supported by another means.


Contents

1.0 Introduction and background ............................................................................... 7

1.1 Marches Biogas ...................................................................................................... 7

1.2 The problems to be addressed ............................................................................ 7

1.3 Aims & Objectives .................................................................................................. 7

2.0 Technical appraisal of technology ........................................................................ 9

2.1 Purpose & Origins .................................................................................................. 9

2.2 Achievements of the Agri Digestore .................................................................... 9

2.3 Theory Behind The Agri Digestore Technology at Hall Farm ........................... 9

3.0 Method ................................................................................................................... 11

3.1 Construction & timeline ...................................................................................... 11

3.2 Demonstration explained ................................................................................... 13

3.2.1 Slurry Transfer .......................................................................................... 13

3.2.2 Digester Mixing ......................................................................................... 14

3.2.3 Biogas Storage .......................................................................................... 15

3.2.4 Digester Heating ....................................................................................... 15

3.2.5 Process Control ........................................................................................ 16

3.2.6 Digestate Removal ................................................................................... 16

3.3 Operation .............................................................................................................. 16

4.0 Evaluation & Results ............................................................................................. 17

5.0 Discussion .............................................................................................................. 18

6.0 Transfer to market ............................................................................................... 18

7.0 Conclusion .............................................................................................................. 20

Figures

Figure 1: Hall Farm Dairy Cows .................................................................................................. 12

Figure 2: Slurry Lagoon ............................................................................................................... 12

Figure 3: Completed Agri-Digestore at Hall Farm ................................................................... 13

Figure 4: Slurry Transfer Pump from Collecting Yard ............................................................ 14

Figure 5: Macerator and Feed Pump Figure 6: Feed Line into Digester and Biogas . 14

Tables

Table 1: Plant Readings Records ............................................................................................... 17


Glossary

Agricultural

Residues

Defined by Biogas Info1 as “slurries and manures”.

NVZ Nitrate Vulnerable Zone

CH4 Methane

CO2 Carbon Dioxide

H2S Hydrogen Sulphide

AD Anaerobic Digestion

Acknowledgements

The writers would like to thank all those involved for their contribution and support,

particularly those in WRAP and the DIAD project.

1 http://www.biogas-info.co.uk/about/feedstocks/#agricultural

http://www.biogas-info.co.uk/about/feedstocks/#agricultural


1.0 Introduction and background

1.1 Marches Biogas

Marches Biogas, a private company based in Ludlow, Shropshire, specialises in AD

technology, was founded by Russell Mulliner in 2009. Marches Biogas' acknowledged

strength is as an engineering company; we have in-house design, specification,

fabrication, installation and commissioning resources that allow us to offer a thoroughly

conceived, turn-key product over which we have complete control. This allied to the fact

that we already hold a market position at the forefront of AD technology in the UK gives

us a very strong on-going position.

Marches Biogas provides the industry with:

a proven track record of delivery: in 2014 Marches Biogas built 14 farm-based

anaerobic digestion plants that can generate over 7MWe. Many of which, have

or are in the process of doubling their generation output but still utilising only

the original assets other than additional CHP units.

maintenance and service contracts to assist plant owners and their projects.

Marches Biogas plants, with service contracts operate with up to 99%

efficiency.

expertise and advise for farm-based, industrial, foodwaste and waste water

anaerobic digestion facilities throughout the UK.

1.2 The problems to be addressed

Greenhouse Gas Emissions

The threat of carbon accounting has hung over agriculture for some years, in the simplest terms, as a ‘tax on cows’. The greenhouse effect of methane gas is 23 times that of carbon dioxide, almost a quarter of the methane produced by farming dairy cows is released from the slurry in storage and on application. This is estimated to account for 160,000 tonnes of methane emissions annually in the UK, according to IPCC data.

Nutrient Management

ADAS and DEFRA have estimated that in typical 6% dry matter cow slurry, 70% of the nitrogen content is lost/ not taken up after application. This is either through leaching unavailable nitrogen out of the soil before it becomes available to the plant or by losses of nitrogen in storage prior to application.

Regulations

The need to build slurry storage on farm is pressing, particularly for those farmers faced with NVZ regulations, dictating a prolonged closed period on spreading.

1.3 Aims & Objectives

The development of the Agri Digestore began through the desire to develop an

anaerobic digester for installation on farms that would deal with the problems


highlighted above and culminated in the Marches Biogas team designing a solution to the

integration of AD with farm slurry storage and management.

Reducing Greenhouse Gas Emissions

To demonstrate the use of a completely closed storage and anaerobic

digestion system effectively captures greenhouse gas emissions.

Increased Nutrient Management2

To demonstrate that by fully digesting fresh slurry and subsequently keeping it

in a covered anaerobic state up to the point of land application, the amount of

available nitrogen is increased, typically by 20 to 25% and the loss of volatile

nitrogen in storage is prevented by as much as 30%, maximising its fertilising

value when it reaches the field.

Regulations

The need for new storage vessels and the opportunity to make best use of that slurry while in enforced storage makes even more sense of incorporating the Agri Digestore concept into slurry storage plans and additionally by: The Agri Digestore Retrofit; the concept is to use an existing slurry system and turn it into an Agri Digestore, fulfilling both functions so not requiring new-build tanks resulting in a reduction in cost which is found by removing the need for additional new build process vessels.

The demonstration aims to monitor the AD plant and prove that the process works over

time by collecting key data. The project also aims to assess the economic viability of

investing in such a plant.

The plant will be monitored on

Feedstock quantity

Digestate quality

Biogas production

Biogas quality (methane, carbon dioxide, hydrogen sulphur, and oxygen levels).

Equipment running hours

Maintenance issues

As a result of this data we can calculate the monetary value of the biogas as heat, and

possibly electricity, together with other financial and environmental benefits of the Agri

Digestore such as the reduction in fugitive methane emissions and the subsequent

reduced carbon footprint of the farm. We can then use this data against the capital cost

together with the running costs to establish the long term economic viability of the Agri

Digestore.

2 For information on what happens to nitrogen in the digestate once it has reached the field please see DC Agri’s report titled:

Field experiments for quality digestate and compost in agriculture

http://www.wrap.org.uk/sites/files/wrap/DC-Agri_Work_Package_2_Digestate_Nitrogen_Supply_Environmental_Emissions_0.pdf


2.0 Technical appraisal of technology

2.1 Purpose & Origins

Anaerobic digestion is a natural biological process and has been used for many years.

There are thousands of digesters in China, India and Nepal where village-scale plants are

important for the recycling of nutrients in farm manure and night soil, as well as cooking

and lighting. The technology has been widely adopted across Europe.

The Agri Digestore is an affordable AD plant that aids the possibility of small scale

anaerobic digestion. It has been designed as an economic solution to integrate AD with

existing slurry storages. This has allowed for numerous benefits:

A purpose built or retrofit system.

No need for additional storage/process tanks.

Minimises methane and nitrogen emissions from the slurry storage.

Adheres to NVZ regulations.

Modular design.

Increased material residence time to ensure complete digestion of substrates.

Ease of operation and complete integration with slurry management.

2.2 Achievements of the Agri Digestore

Previous to Hall Farm, the Agri Digestore has aided Rainton Farm, Cream O'Galloway,

Dumfries3. Rainton Farm use the Agri Digestore to digest and store the slurry from the

dairy cows that produce Cream O’Galloway’s ice cream. This digester has proved very

successful and has found media acknowledgments in Scottish Farmer4, Anaerobic

Digestion and Bioresources Association (ADBA)5 and The National Non-Food Crops

Centre (NNFCC)6. More recently another Agri Digestore was built in Wales which is being

fed 2 tonnes/day of beef cattle slurry with additional chicken litter to increase gas

production, which is approx. 120m3/day at 52.4% CH4. With these feedstocks the Agri

Digestore heat requirement has been approximately 65% of the total heat produced.

There is now a plan to utilise any excess heat within the farm dwellings. At both the

above installations the biggest benefit has been the flexibility of the system and the

nutrient benefits and handling of the digested slurry.

2.3 Theory Behind The Agri Digestore Technology at Hall Farm

Cattle slurry from the dairy is continuously collected and is scraped into the reception pit adjacent to the dairy. There is an option at this point in the process to add other feedstocks such as waste feed and or chicken litter. The preparation of the feedstock will require that a

3 http://marchesbiogas.com/cream_of_galloway_dumfries

4 http://www.thescottishfarmer.co.uk/mobile/renewables/news/a-breakthrough-in-small-scale-ad.27342375

5 http://adbioresources.org/wp-content/uploads/2014/02/AD-Biogas-News-February-2014_final.pdf

6 http://www.nnfcc.co.uk/news/innovation-in-on-farm-small-scale-ad?sectorKey=bioenergy&typeKey=news

http://marchesbiogas.com/cream_of_galloway_dumfries

http://www.thescottishfarmer.co.uk/mobile/renewables/news/a-breakthrough-in-small-scale-ad.27342375

http://adbioresources.org/wp-content/uploads/2014/02/AD-Biogas-News-February-2014_final.pdf

http://www.nnfcc.co.uk/news/innovation-in-on-farm-small-scale-ad?sectorKey=bioenergy&typeKey=news


predetermined quantity of waste feed and or chicken litter is administered and mixed into the raw slurry reception pit contents. The Agri Digestore feeding / filling process is conducted manually, the frequency of feeding recommended would be daily. An option exists to feed the Agri Digestore via a lobe feed pump and macerator operated automatically. The feed and discharge arrangements share the same low level tank pipework connection. The through wall tank connection at low level allows either for direct tanker discharge or connection to umbilical spreading equipment. The design of the gas holder roof allows for digestate to be withdrawn at any rate from the tank to the minimum operating level. A warning beacon to indicate that this level has been reached is fitted adjacent to the connection. Critical to the Agri Digestore design is the freedom to allow digestate use as and when required, this means that the digestate may be applied directly from the tank without any process interruption, the only difference from normal slurry store management is that the Agri Digestore is typically only emptied to a level of 15%. This means that the process is continuous, no downtime or re-commissioning is required after digestate application. The increased residence within the Agri Digestore when compared with a conventional one tank CSTR digester may be as much as 200%, this allows complete digestion, maximising the benefits of energy recovery and increased available nitrogen in the digestate. The prolonged residence time also increases the benefits of pathogen kill and weed seed suppression that AD offers. The increased residence time allows the feeding regime to be very flexible, the variation in feedstock will be due to the availability of slurry at different times of the year, the ease of operation allows a proportion digester contents to be returned to the reception pit where it can be used to make the feed mix with the waste feed and or chicken litter at times of the year when little or no raw slurry is available.

The Agri Digestore vessel at Hall Farm is a circular glass coated panel steel tank. The gross capacity is 1600m3, with a displaceable capacity of 1400m3. The tank kit comes complete with a standard Ø800mm galvanised mild steel low level access manway and a standard 1m x 1m galvanised mild steel viewing platform mounted at approximately handrail height below the top angle complete with a lockable hooped safety access ladder and intermediate rest platform with a viewing port. The tank is erected on a reinforced 5° coned concrete base to a point at the centre where the discharge pipework terminates. The digester is mixed by the recirculation of biogas through a series of thirty six (36) nozzles cast into the concrete base ensuring even temperature distribution and preventing stratification; each nozzle is mixed sequentially at a flow rate of 60m3/hr. The mixing can be operated continuously or intermittently depending of process requirements. The process heating is performed by pumping hot water through an internal heat exchanger fitted near the floor of the tank and connected to the heating water circuit and the biogas boiler. The heating connections are made through the tank wall at low level. The digester heat requirement is divided into two parameters, the first being the feedstock heat requirement that is the heat requirement to raise the daily feedstock from ambient temperature to the process temperature, in this case the average feedstock heat requirement is a continuous 25 kWth. The second being the process tank heat loss, this is minimised by insulating the Agri Digestore with 100mm of insulation and clad in profiled steel panels, the average heat loss is a continuous 15 kWth, resulting in a continuous heat requirement of 40 kWth. The biogas boiler will be rated at approximately 70 kWth which will consume all the biogas generated resulting in a surplus heat production which can be used elsewhere on the farm. The designed flexibility of the Agri Digestore allows for it to be heated at times when heat isn’t required elsewhere for example in the middle of the night. In the summer months the digester heat requirement will be considerably less than the winter months, the flexibility of the system allows enough high energy solid feedstock to be


mixed with returned digester contents to be fed to the Agri Digestore in the summer months to produce enough gas to keep process temperature.

A double membrane gas holder is installed to capture and store gas. The gas holder is a full coverage roof thus eliminating all rain water. The outer membrane is constantly inflated using a small ventilation fan, the inner membrane is much larger and floats on the contents of the Agri Digestore. The biogas is piped from the gas holder to the gas consumers via the mixing compressors which are positioned adjacent to the digester base. The gas holder is designed to work over a range of tank levels from within 500mm of the top of the tank to the minimum operating capacity, typically 20% of the tank volume. Air is injected through the gas mixing skids into the Agri Digestore to help reduce the levels of hydrogen sulphide within the biogas. Condensate is removed from the biogas at the lowest point in the gas pipework system.

Combustible gas and fire detectors are installed where biogas is brought into a building. In addition H2S detectors monitor Hydrogen Sulphide levels. Fire or gas detection will raise visual and audible alarms and initiates shutdown of the gas supply to the biogas boiler. Marches Biogas provides on-going remote plant monitoring via an HMI/SCADA control system.

The following instruments are provided, all with the capability of measuring and recording levels:

Digester level control; Sensor for temperature of digester; Gas holder level control and; Water circuit temperature control.

The signals from the instrumentation are interpreted in the control module, which automatically controls the digestion plant. Information can be displayed on a computer and the outputs from the instrumentation can be recorded and stored on a computer. This information can also be available for onward transmission to a remote computer.

The operation of the Agri Digestore is automated from the plant control panel which interprets information transmitted from instrumentation around the plant.

The automatic operation includes:

Maceration and pumping of the Agri Digestore feedstock; (optional) Agri Digestore mixing; Agri Digestore heating; Agri Digestore and gas holder level monitoring; Diverting heat to the Agri Digestore/ on farm heat consumers according to priority; Boiler control on gas availability.

3.0 Method

3.1 Construction & timeline

Marches Biogas designed and project managed the development of the Agri Digestore

at Hall Farm, Ludlow. Hall Farm has a 240-head cross bred dairy herd comprising of

Holstein, Jersey and Ayrshire cattle which are housed for 5 months of the year, and, for

the remaining 7 months the majority of the herd graze on land near to the main farm

site.

At present, the dairy cattle at Hall Farm are fed a mixture of maize, grass and brewers

draft. The cows are bedded on a mixture of sawdust and sand and are milked twice per

day.


Whilst cattle are housed on the farm, the slurry from the cubicles is scraped twice a day

into an existing earth banked below ground slurry lagoon which has an approximate

capacity of 450m3. During 2011, the lagoon was emptied six times by a local agricultural

contractor who applied the slurry using a splash plate system, all the slurry was applied

to land owned or rented by H J Watkins to maximise beneficial nutrient use from the

slurry however due to the size of the existing lagoon this isn’t always possible, leading to

spreading outside the useful growing season.

Figure 1: Hall Farm Dairy Cows

Figure 2: Slurry Lagoon

The detailed design work commenced in March 2013 with work starting onsite in

October 2013 once the planning process had been approved. When the site of the tank


and associated equipment was stripped the site had to be inspected by the County

Council Archaeological Inspector. The tank base was completed by February 2014 and

the construction of the glass coated steel tank commenced. Once the tank construction

was complete, Marches Biogas was able to progress with the mechanical and electrical

installations. This was complete other than the client responsibilities by Autumn 2014,

with only the gas holder remaining. The design of the gas holder, which is one of the

most critical parts of the Agri Digestore, had been continuously reviewed since the first

similar installation in Dumfries, the changes made were incorporated into gas holder

installed on the Agri Digestore project in Wales. Once that digester was operational the

decision was made to incorporate all the designs used into the gas holder for Hall Farm.

The gas holder was installed by Marches Biogas in November 2015.

The timeline for building the Agri Digestore has been significantly extended primarily

due to the unprecedented demand for much larger scale anaerobic digesters having to

be built on a very tight timescale due to the enforced Government deadlines for

completion. Over the course of 2 years between June 2013 and December 2015,

Marches Biogas built and commissioned over 15 biogas plant which put significant

pressure on the business to meet pre-accreditation deadlines, and therefore the

commissioning of the Agri Digestore has suffered. During Winter 2015/16 the Agri

Digestore was ready to be commissioned.

3.2 Demonstration explained

Marches Biogas has demonstrated the successful construction of the Agri Digestore.

While it has taken longer than expected to complete the project, due to other

commercial pressures on the business, it is expected that were this project to be

constructed in perfect conditions then it could be completed in 16 weeks to be ready for

commissioning. This is also based on Marches Biogas’ experience of constructing much

larger plug flow digesters.

Figure 3: Completed Agri-Digestore at Hall Farm

3.2.1 Slurry Transfer


The slurry is pumped from the collecting yard, via the feed pump and macerator, directly

into the digester.

Figure 4: Slurry Transfer Pump from Collecting Yard

Figure 5: Macerator and Feed Pump Figure 6: Feed Line into Digester and Biogas

in Control Building Mixing System

3.2.2 Digester Mixing

The digester is mixed by the Marches Biogas gas mixing system which comprises of 2

mixing compressors and 2 mixing manifolds. Biogas is drawn from the gas holder and

compressed and released through a series of pipes cast in the base of the digester

which rises to the surface of the digester and in doing so mixes the contents and breaks

the surface of the slurry.


3.2.3 Biogas Storage

The double membrane gas holder captures the biogas and stores it ready to be used

within the boiler. The outer membrane is kept inflated using an air fan which creates a

pressure on the inner membrane and maintains a positive pressure within the tank. The

inner membrane is of a unique design that allows the digester contents to be withdrawn

continually without creating a vacuum and drawing air into the vessel.

Figure 7: Double Membrane Gas Holder

3.2.4 Digester Heating

The boiler is a natural gas boiler converted by Marches Biogas to run on biogas. As

biogas is produced within the process it is drawn into the boiler where it is burnt to

transfer heat into the digester heating circuit which comprises of water being pumped

from the boiler through the digester internal heating pipework.

Figure 8: Biogas Boiler Figure 9: Plant Control Panel


3.2.5 Process Control

The mixing and heating systems are operated automatically from a plc within the control

panel. The small HMI readout screen allows the operator to see the temperature,

digester volume, and gas holder level. Daily readings are taken of the hours run meters,

the gas quality, digester temperature and gas holder level. Gas quality is currently

monitored using a hand held Geotech 5000.

3.2.6 Digestate Removal

Digested slurry is removed from the digester via a tankering point. The end of the pipe

sits in the centre of the base of the digester which ensures that any heavy materials (grit,

stones, sand) are sucked out of the digester. The digester contents can be taken down

to 15% of total tank volume, which is approximately 210m3. This ensures that there is

always some inoculum in the digester on which to digest raw slurry pumped into the

tank.

During the demonstration of the technology, Marches Biogas was able to demonstrate

the simple commissioning of the digester using 250m3 of pre-digested agricultural

digestate and propane gas as an initial source of heat that was burnt through the biogas

boiler to raise the temperature in the Agri Digestore until the process started producing

burnable biogas. Within 3 weeks, even with low ambient and slurry temperatures, the

digester was producing biogas with a methane quality >45%. The boiler was then

converted to run on biogas and the plant became self-sufficient, this was at a process

temperature of 24.2oC as the temperature continued to rise the biogas quality rose to

over 60% CH4.

3.3 Operation

Since the commissioning period which was completed in early January 2016, the plant

has run in automatic mode, other than Hall Farm feeding the Agri Digestore by manually

running the feed pump to manage the slurry production and storage pit level at the

dairy. In the period since 18th January the Agri Digestore has been fed with an average of

8.2 tonnes of slurry, wash water and rainwater a day, a total of 803 tonnes. This feeding

has been on a batch basis as and when Hall Farm needed to pump slurry over from their

underground storage facility as seen in Figure 4.

The digester has been sitting at a temperature of approximately 28oC and has been

producing biogas from which to provide process heat. The digester has been in full

operation for 12 weeks, up until the point of writing this report no digestate has yet

been removed from the Agri Digestore and applied to land. At that point a new period of

analysis provided by Marches Biogas will commence regarding the benefits of the use of

digested cattle slurry.

Plant readings were initially taken several times per week and entered into a

spreadsheet.


The data taken includes the following:

Date and Time

Weather

Drive Description Units

CH4 %

Co2 %

O2 %

H2s ppm

M1 Slurry Pit transfer Pump hrs/day

M2 Macerator hrs/day

M3 Feed Pump hrs/day

M4 Compressor No.1 hrs/day

M5 Rotary Value No.1 hrs/day

M6 Compressor No.2 hrs/day

M7 Rotary Value No.2 hrs/day

M8 Gas Holder Air Fan hrs/day

M9 Biogas Boiler hrs/day

M10 Gas Boiler No.1 hrs/day

M11 CHP hrs/day

M12 Boiler No.1 Circulation Pump hrs/day

Observations

Yard Area- Clean and Free From Spills

Odour- No strong smell detected

Table 1: Plant Readings Records

Since the 20th March 2016 the readings have taken daily and entered into a spreadsheet.

This data allows the operator to assess the length of time drives have run for and

highlight an issue should they have run for less or more time than expected.

Samples of the digester contents are taken by Marches Biogas and analysed for dry

matter, organic dry matter, pH and FOS:TAC ratio. This indicates the stability of the

digestion process and allows any changes in health to be seen early.

4.0 Evaluation & Results

The data taken shows that even at a digester temperature of approximately 27.2oC

biogas is being produced with a quality of 64% CH4. The gas production has not yet

reached the anticipated figure of 300m3/day, this is due to the lower dry matter content

of the slurry which has included rainwater, the average monthly rainfall at Hall Farm

during the period of December 2015 to March 2016 was 93.3mm, this is 16mm greater

than previous average rainfall and is diluting the slurry at the reception pit. The current

gas production is 147m3/day due to the reason explained above and the lower than

average full throughput feed rate. Both have had an effect on the heat available to heat

the digester along with low ambient temperatures, however the biogas production is

enough to sustain a healthy temperature and process. As expected the hydrogen

sulphide levels are high at circa 1200ppm, however the biogas is being burnt through a

boiler and is therefore less of an issue than if consumed via a CHP unit.


5.0 Discussion

In the short time that the Agri Digestore has been operational it has proved that the

process is self-sufficient in terms of providing its own heat, at a time of year when heat

will be at its highest demand with low winter temperatures and higher heat losses.

The Agri-Digestore has provided Hall Farm with greater slurry storage capacity & a

particular benefit is that it does collect rain water unlike an open topped slurry store.

The area has had significant rain over the winter with the following rainfall recorded.

Over a surface area of 100m2 this would equate to an additional 45m3 water collected in

the lagoon which reduces the effective storage volume and also needs applying to the

land when conditions allow .

From other further developed Agri Digestore projects, including the one in North Wales,

Marches Biogas conclude that one of the biggest benefits to the farmer is the

enhancement of the digested slurry as a fertiliser and the ease of application to land.

There will be a higher N content and the nutrients will be more readily available as the

anaerobic bacteria have broken down the nutrients into easily available forms for plant

take up. This has not yet been proved at Hall Farm as no digestate has been applied to

land however Marches Biogas will be assessing this over the coming months as

digestate is applied to the land.

The maintenance requirement of the plant is low with no parts consumed to date. The

daily checks take a few minutes to note down - the hours run times of pumps, air fan

and mixing compressors, check the operation of the gas boiler, and top up oil in the gas

mixing compressor reservoirs.

The total project cost was projected to be £270,000. Marches Biogas have additionally

incurred costs related to obtaining planning permission, the Environment Agency permit

and considerable other mechanical and electrical works that were originally the

responsibility of Hall Farm. It is too early within the process to complete the full

economic assessment as we require a full 12 months operational data as consideration

needs to be given to seasonal variations in feedstocks and Hall Farm operations.

The slurry alone does not produce enough biogas all year around, therefore it is not

feasible to produce electricity at this scale unless other high energy feedstocks are used.

There are small CHP units available but they are expensive in terms of capex and opex

and additional biogas scrubbing will be required to reduce the levels of hydrogen

sulphide within the biogas. The use of biogas in boilers is well established so where

there is a heat demand this is the logical way to use the biogas in a project of this size.

6.0 Transfer to market

In summary the Agri Digestore is a concept of integrating slurry and farmyard waste storage with AD technology in a single stage. This allows numerous benefits:

May be purpose built or retrofitted to existing slurry storage.


No need for additional storage/ process tanks.

Minimises the fugitive methane and nitrogen emissions from slurry storage.

Eliminates the loss of nitrates from uncovered digestate storage.

May be incorporated with NVZ compliant slurry store construction.

Modular design allows several Agri Digestores to be linked.

Increased material residence time to ensure complete digestion of substrates and high specific gas yields.

Ease of operation and complete integration with existing on farm slurry management.

However, despite the consistent interest in the Agri Digestore, due to the degression of

the Feed in Tariffs (FiTs) and Renewable Heat Incentives (RHI) for anaerobic digestion, it

is now very unlikely that projects of this nature are going ahead to build unless small

scale anaerobic digestion is supported by another means.

The digester has not been running for a full 12 month period it is therefore difficult to

produce a full economic assessment, however we have carried out a desk top

assessment that has been done on an optimistic basis and the conclusions remain the

same. The system should be eligible to receive income from RHI and if the excess heat

could be utilised without further expense the annual revenue would now be less then

£5,000. This is based on a biogas production of 300m3/day for the 4 winter months the

cows are fully housed; 75m3/day for the remaining months; 5% heat losses within the

system and all the biogas being burned in the biogas boiler. With no additional costs

such as maintenance this would produce a payback of over 50 years which is clearly not

viable.

There are other operational benefits for the farmer (i) additional nutrient value and (ii)

cheaper cost of spreading. However, these are also relatively small when compared to

the high capital expenditure. In order to make a project like this viable the government

would need to incentivise the reduction of or punish carbon emitters. Carbon taxes, for

example offer a potentially cost-effective means of reducing greenhouse gas emissions.

They help to address the problem of emitters of greenhouse gases not facing the full

social cost of their actions. Carbon taxes could be used as an incentive to reduce

greenhouse gas emissions from agriculture.

If 50% of the manures and slurry produced by the UK dairy cow population were treated

through anaerobic digesters, the resulting biogas yield would have the generating

capacity of over 90MWe, with a high rate of availability due to predictable supply.

Although as discussed above there is a potential to generate 90MWe the average UK

dairy farm of 125 cow herd size would have 39kWf continuous gas production from the

slurry alone to use on site resulting on either a potential 11 kWe and 22kWth via a CHP

unit or 33kWth through a boiler only. The parasitic heat requirement for a plant of this

size 24kWth therefore it is apparent that the CHP option is not viable as it will not

produce enough heat to sustain the anaerobic process.

https://en.wikipedia.org/wiki/Cost-effectiveness_analysis

https://en.wikipedia.org/wiki/Social_cost


The primary market for the Agri Digestore concept are cattle farmers, predominantly

dairy, who either have existing slurry storage facilities for adaptation or are able to

incorporate the technology in new-build slurry storage.

There are an estimated 10,400 dairy farmers in England and Wales, approximately 1.81

million cows producing over 36.3 million tonnes of slurry per year. Our aim with this

design is to open the anaerobic digestion market to those farmers who, until now, have

seen AD technology as unobtainable through high capital cost or as something that is

not applicable to their business. On farm AD is widely known to offer valuable benefits

to the farm and farmer, it is the barrier of affordable but robust technology that is

holding it's expansion back.

The increase in the availability of the fertiliser value of slurry and manure, particularly

nitrogen, bought about by AD is important in that it allows farmers to offset or even

replace their use of chemical fertiliser, which is a significant source of carbon dioxide in

it's production. While difficult to quantify the carbon value, except on a case by case

basis, the increased availability of nitrogen after digestion is known to be approximately

22%.

This is further augmented by this design particularly as volatile nitrogen losses are

significantly cut due to the covered anaerobic nature of the slurry throughout storage

and it's conversion to digestate. These losses, from cow to field have been measured at

over 60% where storage is prolonged in conventional systems.

7.0 Conclusion

The Hall Farm project, together with other demonstration Agri Digestores, have shown

that the technology works very well using a simple concept of a slurry tank with a double

membrane cover over it to capture the biogas. The biogas, comprising of harmful green

house gases, is captured and utilised through a biogas boiler, preventing the release of

methane into the atmosphere.

The resulting Agri Digestore analysis has shown to date that there has been a reduction

of dry matter of 53% and organic dry matter of 56% resulting in a digestate that is more

manageable, easier to spread and has a greater fertiliser potential; that there has been

an increase in the digestate pH resulting in less requirement to spread lime to control

low pH soils. The farmer can utilise the Agri Digestore like he would a normal slurry

store without damaging the digestion process. The only exception to this is that there

must be a minimum of 15% digester volume in the tank at all times to ensure that there

is always a culture of anaerobic bacteria. This will be demonstrated at Hall Farm over

the coming months as digestate is removed from the digester.

The Agri Digestore has provided Hall Farm with valuable slurry storage capacity which

enhances the value of the slurry in multiple ways as well as allowing the farm to become

compliant with NVZ regulations. The slurry is no longer subject to dilution from rain

water within storage, the anaerobic digestion process will increase the availability of

nitrogen and increase its concentration through the degradation of organic dry matter


and overall reduction in volume. The farm can now become compliant with NVZ

regulations as the slurry is applied to the land when it is required rather than when the

lagoon is full.

In the current economic climate and with the existing Government incentives, the Agri

Digestore, as a new product is unlikely to develop. The conversion of an existing slurry

store to an Agri-Digestore is more economically attractive but the returns will not be

enough to attract significant number of farmers from taking this option.

www.wrap.org.uk/relevant link

Documents

Anaerobic Digestion Demonstration of The Agri … Biogas.pdf · Anaerobic Digestion – Demonstration of The Agri Digestore – Final Report Driving Innovation in AD: Marches Biogas