Passionate dairy and biogas pioneers

Located in Marmorta di Molinella in the province of Bologna, the Agricola Antonio Farm is a dairy cattle farm owned and run by passionate dairy farmers, the Pasini family. The 500 ha farm has around 400 head of cattle of which 200 are milk cows. Apart from milk production, the farm grows its own feed and fodder. Furthermore, it owns and operates two manure and silage-based BTS Biogas biogas power plants, each with 1 MWe capacity supplying into the power grid under the feed-in tariff (FIT) scheme.

The Pasini’s are pioneers and great ambassadors for us at BTS. Their farm is in one sense a showcase of our technical developments over the last decade, said Björn Blankespoor, International Sales Manager for BTS Biogas Srl/GmbH.

Both biogas plants are of conventional Continuously Stirred Tank Reactor (CSTR) types that operate under mesophyll process conditions, around 40 °C and both produce biogas from manure and silage feedstock that is used directly for onsite power-to-grid production in two Jenbacher JGS 320 biogas gensets, one for each plant respectively. Though located next-door to each other on the farm, each biogas power plant including feedstock storage, substrate feed-in and genset is its own separate administrative unit, in compliance with Italian feed-in tariff (FIT) regulations.

Most of the feedstock is produced on the farm with silage as a part of crop rotation whereas an amount of chicken manure and vegetable waste is sourced locally from neighbouring farms.

The CSTR concept has several advantages such as uncomplicated construction and low operating costs in terms of labour. However, the most important advantage in this context is that it runs at a steady state with continuous operation as opposed to a batch process, commented Blankespoor.

It is a fair point. The substrate composition for both of the plants is well defined and relatively homogenous in its composition throughout the year as is the output, biogas for power production.

Same but different

The first plant commenced operations in 2009 and consists of two 1 900 m³ digesters, a 4 700 m³ post-digester. In 2012 the plant was retrofitted with a “BIOaccelerator^s” pre-treatment unit to accommodate a higher share of ligno-cellulosic material. A screw-type extruder, it assists with input substrate breakdown with a thermo-mechanical treatment process by means of two juxtaposed, interlocked screws.

As a result, the substrate for this plant currently consists of cattle slurry, maize- and ryegrass silage and chicken manure. The hydraulic retention time (HRT) is around 80 days and the biogas has around 55 percent methane content.

The second plant commenced operations in 2012 and is quite different. Like the first plant, it too has mechanical pre-treatment unit, a “BIOaccelerator^r” though this is a different type of pre-treatment unit – a mechanical size-reduction macerating unit. Furthermore, the plant differs in that it has a hydrolysis step with a 470 m³ tank and a single 3 000 m³ digester. The substrate for the second plant consists of cattle manure, maize silage, straw, and stalks. The hydraulic retention time (HRT) is shorter, around 70 days and the biogas has around 55 percent methane content.

The substrate for the second plant consists of cattle manure, maize silage, straw and stalks. The hydraulic retention time (HRT) is shorter, around 70 days and the biogas has around 55 percent methane content.

Heat utilisation

Apart from heating the digesters, the excess heat from the gensets of both plants is used onsite. From the first plant excess heat is used to dry around 5 000 tonnes per annum of forage and hay for the cattle as well as provide space heating for the 250 m² farmhouse. In energy terms around 1 200 MWh per annum of heat is used in this way.

The heat from the second plant was originally used to in a manure management and valorisation project to dry and pelletize digestate using another technology from BTS, its BIOdry. With a grant from the Rural Development Programme of the Emilia-Romagna region, a digestate drying and pelleting line was set up in 2013 to produce digestate fertiliser pellets as well as liquid fertiliser.

The concept is to use the residual heat from the genset to produce two fertiliser products from the digestate, a solid that can then be pelletised and a liquid ammonium sulphate fertilizer. The advantage is a reduction of the volume of digestate and a pasteurised final product, said Blankespoor.

The system uses the residual heat of cogeneration and is able to also exploit the residual heat potential of the drying air to thicken a part of the liquid fraction of the digestate resulting from the solid-liquid separation.

The air, rich in water vapour, ammonia, and malodorous compounds, resulting from drying of the digestate, is treated in a sulphuric acid scrubber that gives rise to an with production of an aqueous ammonium sulphate solution that can be used to increase the nitrogen content in the organic fertilizer or can be marketed as liquid nitrogenous fertilizer.

From digestate to wood to…

Although successfully demonstrated, the market for organic fertilizer was lacking and the farm has sufficient acreage to utilize all the digestate from both of the biogas plants without having to upgrade it. Instead, it was decided to repurpose the digestate pellet plant to produce wood pellets for the local market and so now the heat from the second biogas plant is used to dry sawdust and woodshavings for the 15 000 tonne per annum Greenergy Società Agricola S.r.l. pellet plant.

What is next for the pioneering Pasini’s remains to be seen, however, the FIT is for a 20-year period which means 2019 is the expiry for the first biogas power plant. Perhaps biogas upgrading to biomethane though the farm is not located within a reasonable distance of a gas pipeline for grid injection.

There is no shortage of ideas though I am not at liberty to disclose anything at this moment in time, ended Björn Blankespoor.

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