Methane has been hitting the headlines of late as a significant greenhouse gas, even at concentrations of as little as 1,900 parts per billion.

The methane burden in the atmosphere has been calculated to be increasing by 22 million tonnes/yr due to the imbalance between production and natural removal.

Of the man-made sources ruminants are one of the big three culprits, along with mining and rice production, livestock being estimated at contributing between 80-115 million tonnes/yr to the input side of the equation.

Methane as a resource

Reducing this output is obviously desirable if the natural balance is to be restored, but so far little attempt has been made to address the issue.

However, that situation is changing, as a method to harvest the gas from livestock slurry is being rapidly developed by a Bennamann, a company based in Cornwall in the UK which has recently attracted significant investment from Case New Holland (CNHi).

FPT Methane gas
CNHi has already gained experience in using methane in the transport sector and is now bringing that expertise to agriculture

It is a natural match for CNHi, via its subsidiary Fiat Powertrain (FPT), which has been promoting methane as an alternative fuel, mainly in the transport sector, but that technology is now being transferred to agriculture.

Collecting methane from cattle and using it as a fuel has long been talked about, yet the two main hurdles have not, until now, been overcome.

The first is its collection while the second is its purification to enable it to be used in engines. Both have been addressed by Bennamann to create a working and financially viable on-farm system.

The Bennamann solution

The company has come up with three main collection systems. The first is for purpose-built lagoons, the second is for existing storage facilities, the third and latest method is for solid farmyard muck.

Cornwall county council
One of the pilot farms in north Cornwall is particularly impressed by the spreading properties of the remaining digestate

Cornwall is an ideal county in which to trial the methods for its council has 58 tenanted dairy farms as well as possessing a desire to reduce its consumption of fossil fuels. It has therefore invested £1.58 million (€1.82 million) in six pilot schemes around the county, alongside Bennamann and Cormac, a vehicle fleet management company, as partners in the project.

Working example

All three methods of gas collection ultimately rely on placing a lid over the lagoon and drawing out accumulated gas for cleaning and compression.

The 120-cow dairy unit run by Katie and Kevin Hoare is an example of of the purpose-built lagoon recovery system. This comprises a pit 6m in depth which has a primary liner between the soil and the slurry.

Kevin and Kate Hall
Kevin and Katie Hoare with the New Holland compressed natural gas (CNG)-powered T7

The slurry is then pumped into this from the reception pit via a macerator which reduces the fibre size, presenting a greater surface area for the bacteria to work on.

As the gases rise to the surface it is collected under a second liner which lies on the slurry surface. A third liner sits on top of this and is filled with air at a low pressure of around 10 millibars and it is this which squeezes the gasses out .

Slurry lagoon Bennamann Methane
The slurry is pumped from the reception pit and injected into the covered lagoon

Collecting the gas would appear straightforward enough, but it is its preparation for use that is the key to the whole operation – and this is the clever bit.

The clever bit

A major problem with methane harvesting is that the gas from the slurry is only around 60% methane, the rest being mainly carbon dioxide.

Bennamann Biocycle unit
The biocycle unit is portable and is taken to each farm by van where it may remain for a day or two before being moved on to the next

Investing in the processing plant to clean up this gas and compress it for storage would be prohibitively expensive for most farms. To get over this, Bennamann has designed and built a mobile processing plant that takes the raw product, extracts the moisture, separates the gases through filtration and and then compresses it into bottles.

It all sounds simple enough, yet it is this biocycle unit, as it is called, which enables the whole scheme to operate effectively and at an affordable cost.

Bennamann gas bottles
Once cleaned and compressed the methane is stored on farm in three banks of bottles

Bringing the purity of the gas up to around 97% methane is critical, for the cleaner the gas the more energy is stored and the better engines will run. The purity of the gas is the measure of the product’s quality and value to customers.

New Holland T6.180 CNG tractor
The T6.180 on loan to the farm is powered by compressed methane. Liquid methane tractors will have a much greater fuel capacity and will run for a full day on heavy work

Presently, the gas is compressed into three banks of cylinders on the farm for use by a New Holland T6.180 which is said to perform exactly as its diesel counterpart does, just a little more quietly.

Retrofit methane collection

If a fully functional slurry lagoon already exists on farm then instead of digging another large hole Bennamann has developed a system of gas collection which relies on a series of square covers, or tiles as they are called, that float on the surface of the slurry.

Tiles Benamann slurry
The tile system undergoing stress tests. The liners would normally flat but here gas has been allowed to accumulate beneath them as part of the development work. A pump would normally remove the rainwater for dairy washing etc.

These work in much the same way, although the gas is continuously pumped out and into storage bags to prevent the ballooning of the covers which could cause distortion to their shape.

Tiles rainwater Bennemann Methane
Gas collected by the tiles is stored at low pressure in the green bags

Farmers that are using straw bedding are also being catered for with an additional processing operation which adds either rainwater, pumped from the top of the lagoon covers, or dairy washings to the solid muck in a tub mixer.

The resulting wet mix is then fed through a slurry separator which extracts the fibre for use as green bedding while the liquid portion is fed into the slurry lagoon as normal. It may be thought of as a method by which the excreta is washed off the straw and rinsed into the lagoon.

Slurry gas storage
On one farm the old slurry lagoon is used as the reception pit. Shredded paper bedding is further reduced by the macerator and does not pose a problem to the system

Where other fibrous bedding materials are used, a similar operation will be required. Sawdust is a component of slurry that requires removal although shredded paper, as is used on one of the pilot farms, needs no prior separation.

Any surplus can be sold on to other users, one being Cornwall County Council which has a total vehicle fleet of over 1,000, a number of which are already powered by FPT engines that run on methane fuelling.

Bennamann as an energy company

While there is a strong emphasis on the reduction in methane emission it is in the role of methane as a fuel which is where the adoption of the Bennamann system will score commercially.

Methane will decompose naturally in the atmosphere to carbon dioxide and water, releasing energy as it does so. Collecting it at source captures that energy potential and utilises it to replace fossil fuels.

The range extender tank on the the T7
The range extender tank on the T7 may not be required due to the greater energy density of liquid methane, allowing the use of a front linkage

Presently, Bennamann is not discussing the cost of installing its equipment, yet the company is intending that the investment may be recouped within six years through savings on farm energy costs, plus the sale of any surplus.

CNHi does not claim that methane will be powering all our tractors in the future, it still sees a role for battery power in smaller machines, yet as a competitor to fossil-sourced diesel the company believes it has a bright future.

The Bennamann system is a leap forward in alternative fuels for it represents a step towards recycling carbon for energy storage, which is just what nature has done ever since photosynthesis evolved over three billion years ago.