AC0410 ANNEX 5: CASE STUDIES

A5.1.160 kWe Dairy AD: Lodge Farm (Fre-Energy)

The farm

Lodge Farm is an organic dairy farm with 650 dairy cows producing c.6,000 L milk each, fed with

grass, imported wheat, peas and soya. The farm area comprises 445 ha, mainly grassland with red

and white clover used to fix nitrogen. Organic rules require >60% dry matter feed to be from forage,

and have an N spreading restriction of 170 kg N ha-1 yr-1 maximum. However, the stocking rate (3.6

cows per ha) and milk productivity has been maintained since the transition to organic farming 13

years previously.

Cows graze from April to November, with a morning top-up ration of concentrate feed during the

grazing season. Indoors, cows are bedded on lime ash and woodchip (sand is the ideal bedding

material as it is clean and maintains animal health).

Anaerobic digestation feedstock

Lodge Farm’s standard agricultural AD licence includes permission to feed milk-based waste and

vegetable peelings that originate directly from the field, in addition to animal manures. Currently for

Lodge Farm, accepting food waste would require £15,000 plus annual costs, plus a lot of

administration. An experimental licence allowed 50 t/day food waste to be digestated on a research

and development basis (no problems were observed with this).

Feedstock comprises:

30 t/day cattle slurry

4 t/day broiler muck bought in @ £10/t (includes wood chip bedding material)

NB: broiler muck c.60% DM compared with c.20% DM in laying-hen muck

Of the approximately 13,000 tonnes per year inputs, approximately 5 % is converted to biogas.

Slurry previously went directly to an anaerobic lagoon (below).

Now, it is stored for 2-3 weeks directly adjacent to the cow shed, then pumped 1 km in batches (to

maximise efficiency) to the large storage tank (below) next to the digester and stored for a further 2-

3 weeks.

It is then mixed with imported broiler manure (from neighbouring farm) in a mixing pit (below) prior

to being fed in to the digester.

Previously a long-fibre chopper was used (foreground, picture below), but this is unnecessary and

was expensive to buy and operate (50 kW for 2 hours per day, plus pumps requiring 22 kW and 7.5

kW = additional 159 kWh per day). Farm yard manure is not fed into the digester, but is composted

for one year prior to spreading.

Fermenter

There is a 30 day retention time in the fermenter (above). The mixing is achieved via sequential

unconfined gas mixing. Compressed gas taken from the top of the tank is blown through individual

pipes mounted on a de-gritting arm rotating slowly around the base of the digester. Large gas

bubbles rise up through the digester, expanding as they rise. This is the most energy efficient mixing

system available and is widely used in the wastewater treatment industry in the UK. Using the

rotating de-gritting arm for the compressed gas enable the whole area of the tank to be covered by

the mixing, and avoids 72 separate fixed supply pipes. The gas pumps draw 3kW for 80 seconds out

of every 8 – 12 minutes under normal operation.

The de-gritting arm is hydraulically operated and is fitted with “sweeps” that can be activated on the

bottom arm. The hydraulic operation is highly efficient, drawing 1 kW compared with a 15-20 kW

draw for a conventional motor. Inverted motors are used throughout the system to optimise

efficiency and to enable monitoring of system performance via current monitors – an increase in the

draw of a motor can be an early indication of vibration and wear, etc. Temperature data are also

monitored. Consequently, parasitic load is just 7 kW (4% peak output). Critical components are

installed in replicate; redundancy minimises downtime and requires a relatively small additional

capital investment.

The digester has a single-skin fibreglass roof with an estimated 30-yr lifespan. A leak test was

procured from a German company to check for leaks in the fibreglass panel seals, and those found

were sealed using silicone injected below the leak. A few subsequent leaks developed around the

pipe-work and the rotating joint at the top of the digester. Such leaks can be detected through

periodic inspections using washing-up liquid.

Digestate becomes stratified within the fermenter, facilitating separation and syphoning-off of

mature digestate (often the mid-layer is the most digested). Feedstock can be fed in and digestate

removed from different levels, enabling optimisation.

CHP generators

Vent gas is routed through a conventional boiler to be combusted off. Hydrogen sulphide gas

scrubbing is achieved using ferric chloride and injection of air into head-space. 65 ppm H2S

achievable with air-only. CHP generators run fine with 120 ppm, and up to 250 ppm with more

frequent oil changes and maintenance.

Two generators are based on six cylinder Perkins diesel engines with spark ignition for the gas

(below). Maximum output from the two engines is 160 kWe @35% electrical efficiency. 40-45%

thermal output for a combined efficiency of c. 80%. The goal is for the generators to run for 8000 per

year at 85% of their capacity, generating 1 088 000 kWhe per year though output has so far been

below this level.

The fermenter uses 60 kW of heat, with a further 65 kW used in buildings in winter, reduced to 25

kW in summer with the remainder dumped to an outdoor swimming pool, a further 60 kW heat is

dumped throughout the year. Because of the excess heat produced relative to on-site demand, only

75 % of the heat recoverable from the CHP is actually recovered. Further exhaust heat recovery

would be possible from the second generator. Enough heat is available for a chicken house.

During the visit, the CHP generators were running on biogas with 62% methane and at 98% capacity.

The older engine had 22,000 hours operational time and the newer engine 6,200 hours operational

time and 361 000 kWh (average 58 kWe).

Large fans and gaps in the walls allow a through-flow of air to minimise the main safety issue of H2S

fumes. The main danger point for explosions is the air cooler (heat-exchanger) for the incoming gas,

where the gas could become present in explosive concentrations through any leaks. In case the CHP

generators are down and biogas is produced, it must be routed via a boiler to combust off the

methane (below).

Fre-Energy offer service contracts on the units they install for £6000/yr, with a minimum load factor

of 75%. Telemetry is used to relay information on system performance to Fre-Energy’s offices, where

performance is checked daily. Two thirds of maintenance costs are for the engines.

Digestate

Approximately 12,350 tonne digestate is produced annually, with approximately 2,500 tonnes as

solids (c.25 % - 27% dry matter) (pictured below). The solid fraction varies depending on feedstock.

For slurry entering the digester with 8 % dry matter content, approximately half the dry matter

remains in digestate. For chicken manure entering the digester with 60 % dry matter content,

approximately 40% of the dry matter remains in digestate.

The digestate is separated using the de-gritting mechanism in the digester (solid fraction removed

from the bottom). It would be possible to wash the grit, sterilise with waste heat and re-use for

animal bedding. Fre-Energy have plans to develop an auger extraction method to extract grit

without removing any liquid digestate.

The liquid digestate fraction is pumped into the storage lagoon, where Fre-Energy estimate a

methane loss of 5-10%. A further disadvantage of the uncapped lagoon storage is that 2.5 cm of rain

translates into £1,000 additional spreading cost. Covered storage would require a further £120,000

for a 20 m diameter tank with capacity for 2,500 m3 digestate plus 750 m3 head space, although this

could be reduced: most methane is likely to be emitted during first few days after leaving the

digester, as the temperature cools, so temporary storage could effectively capture more methane

and reduce fugitive emissions. This issue is less important for manure where emissions would occur

during uncovered storage anyway.

Digestate spreading rates limited by the organic cap of 170 kg N ha-1 yr-1 maximum, and digestate

now exported from the farm. Liquid digestate in pumped between the main storage lagoon and a

storage tank adjacent to the cow shed 1 km depending on where it is to be spread. The solid

digestate fraction is spread on fields reached by road that have lower historical loading with slurry.

As a rule of thumb, 1 kWe capacity requires 1.2 ha for spreading. Lodge farm now exports some

digestate to neighbouring farms, and aims to sell the liquid fraction for £7/t and the solid fraction for

£24 -£30/t. Richard notes that the PAS 110 standard with which digestate must comply if it is to be

spread on agricultural land is relatively loose in terms of residual gas in the digestate (completeness

of digestation) but is rigorous in the non-organic fraction of solids present, which can increase with

more efficient digestion. Even with PAS110 certification, solid digestate cannot be sold as a product

to the public, closing off a potentially valuable market for farmers (this product could retail in excess

of £ 100/t). However, this does mean that nutrients are maintained in the agricultural cycle.

Digestate is spread to maximise spreading fuel efficiency – i.e. three spreads at full equipment

capacity:

Mid April

Mid June

Mid August to September

Digestate has not yet been analysed since the inclusion of broiler muck in the feedstock. Soils are

about to be analysed for nutrient status, and the plan is to analyse three fields per year. So far,

digestate application has been prioritised on fields where nutrient demand is inferred through lower

yields.

Additional considerations

Chris estimates that the minimum likely size of AD plant is 80 kWe, on farms of at least 100 ha. They

recommend government backed loans as a cost-effective mechanism to encourage AD – the absence

of specified loan amounts would mean that plants more likely to be optimally sized according to

relevant technical criteria (feedstock availability, etc) rather that to fall below subsidy thresholds.

Fre-Energy are keen on the Hub and Pod concept of food waste distribution for AD in smaller farm

units, so that digestate can be spread efficiently. Gate fees are likely to be necessary for food waste

to be economically viable. They note that heat only AD units could be viable below 50 kW owing to

the low parasitic electrical demand and the synchronicity of heat generation from winter-manure

storage with demand .

The following rough estimates of capital investment costs for different-sized units were provided by

Fre-Energy:

500 kWe = £1.8 million

250 kWe = £1 million

160 kWe = £750,000

80 kWe = £500-600 k

Fre-Energy estimate that capital investment costs could be reduced by up to 40% if units were mass

produced, and that heat-only use could be a good option for small farms.

A5.2. Arable 1.4 MWe AD: Spring Farm (Future Biogas)

CHP Unit

1.4 MWe CHP unit (3 MWth input) commissioned August 2010, costing £4 million to build.

Neighbouring digester of 1.56 MWe commissioned more recently. Both have electrical efficiency of

42-43%. Currently no use for heat, but one option is to increase electrical efficiency by a further 9%

through addition of ORC engines (to 52% electrical efficiency – requires further £500,000

investment). Various options for heat such as storage and transport in salt containers. The

neighbouring AD unit provide all heating requirements for a local abattoir.

6% of electricity used for plant operation (parasitic load). System supplied by German company who

have built 70 other plants in Germany, and are therefore advanced with design optimisation (e.g.

fermenter stirrers operate in sequence to achieve slow gyre effect within the cylinder). System can

be operated remotely and data goes to German system providers (who guarantee operation and

provide technical support).

After fermenter and storage biogas production, biogas goes to booster and chiller. Plant needs to be

running >90% of time to be commercially viable.

Feedstock

Total feedstock 24,000 tonnes per annum. For 2013, 15,000 tonnes maize, 7,000 rye, 2,000 tonnes

grass. Supplied by approximately 20 farms, with an average supply distance of 5.2 miles. Silage has a

DM content of 32%. Leachate from the silage clamp is collected and fed into the fermenter. Empty

silage clamp can be used for storage of solid digestate fraction.

Nearby farms produce 1000 t of manure that could be fed into the digester (possible benefits of

maintaining bacterial composition, although biogas yields not so high). However, Future Biogas

would require a WAMITAB licence from the EA for this. The point was made that the EA could

provide exemptions for relatively small percentages of feedstock from manure. Rigid interpretation

of the EU Waste Directive is regarded as a barrier for AD implementation in the UK. France and

Germany appear to enforce waste regulations more flexibly.

Digestion

The fermenter contains a 14% DM porridge, slowly stirred by 4 large paddle stirrers (15 kW each),

avoiding aeration but with some compressed air bled through to reduce H2S production (H2S c.90

ppm in biogas). 90% of biogas is produced within the fermenter over the 40 day residence time; the

remaining 10% biogas is produced in the storage tank. Heating pipes run through the fermenter

from the CHP unit, but very little heating is required. Degritting is not envisaged through the plant

lifetime owing to the clean feedstock coming in.

Typically, manure digestate has a pH of over 8. The digestate from Spring Farm appears to be around

pH 7 from results of daily “VFA/TIC ratio” analyses.

The storage tank has a double skin, with positive pressure to maintain full inflation of outer skin and

reduce leaks. Future Biogas undertook a leak test (c. £1,500) with a CH4 leakage camera during

commissioning to ensure that the tanks and seals were leak-free. Reducing leaks avoids fuel

wastage; leak tests pay back quickly.

Digestate

80% of feedstock consumed in the plant comes out as digestate. Nutritional value of digestate:

fractions

N-3.7kg/t

P-2.75 kg/t

K- 4.82 kg/t

SO3 – 1.1kg/t

MgO – 0.37kg/t

Liquid digestate is Injected at rates of 20 to 45m3/ha, prior to drilling or as a topdressing to the rye.

Solid digestate is applied using a muck spreader at 2-40t/ha prior to drilling. All digestate is used in

the spring and the tanks are emptied again in Aug/Sept. Max storage capacity for digestate is around

4.5 months.

PAS110 certification of digestate is unnecessary because feedstock is not a waste. Digestate

returned to feedstock farms using 15 t tractor-trailer or 29 tonne converted milk tanker. The

digester storage tank has sufficient capacity to comply with the NVZ regulation (Nov-March storage).

Solid fraction can be stored in empty silage clamp and spread at any time.

Digestate concentration technologies are being developed (reverse osmosis) – e.g. GG Ecobox (also

applicable to slurries). In Germany, some plant operators dry digestate in greenhouses to produce

fertiliser pellets.

Spring Farm have undertaken some trials with Piadin (a nitrification inhibitor) showing N2O

reductions of approximately 60%. Currently there is no financial incentive for farmers to trial this.

A nearby AD farm is hosting experiments with strip-tilling (partial tilling of drill rows only) which are

being studied as an option for more sustainable maize cultivation (runoff, ammonia and GHG

balance being monitored).

Future prospects

Future Biogas predict that AD capital investment costs will not reduce much further (steel, concrete

and IC engines are the main components, and all are mature technologies). This differs from other

RE technologies such as PV where design and manufacturing innovations are leading to significant

capex reductions. FIT and RHI will reduce over time, and it will become challenging for new AD

plants to establish under lower subsidy schemes. Future Biogas make the case that efficient plant

operation and fugitive emissions avoidance through capped digestate storage, etc, requires

specialist operators and larger scale, and suggest that AD subsidies could be linked to rotational

cropping of any PGCs to avoid mono-cropping issues.