Pig and cow manure close our energy circle

The Ten Have family runs a mixed livestock farm in the Achterhoek. Since the end of last year, the family business, with over 90 dairy cows and around 500 sows, has been producing energy from a mix of manure from all animal categories on the farm. Digesting, separating, and stripping manure yields a substantial list of benefits; although the process also presents challenges, especially in the start-up phase. Ten Have aims for a payback period of six years. This article is part of the Energy Special van Boerenbusiness.

The Ten Have partnership is run by brothers Bart and Stan and parents Erik and Imelda. At the kitchen table in Zieuwent, Erik and Stan talk about their experiences so far. The manure digester, with a capacity of about 450 m³, has been in operation since October last year. And that is not the only thing. The digestate after fermentation is separated, after which the liquid fraction of nitrogen is stripped. The produced gas is converted into electricity by two combined heat and power (CHP) units. The released heat can be used to maintain the temperature in the digester and the nitrogen stripper, and to heat the piglet barn.

High energy demand due to sows, milking robots, and manure processing
A few years ago, Ten Have began exploring energy generation options. The company has a substantial energy requirement due to the presence of air scrubbers and ventilation in the sow branch. Additionally, there are robots for the dairy cattle, and heating the piglet barns also consumes a lot of gas. Initially, options for solar panels or a wind turbine were explored, but these were rejected for various reasons.

The course is being shifted, and the focus is shifting to manure digestion and further processing of the manure on the farm itself. The constant energy production of a digester with CHP units aligns well with the farm's constant energy needs throughout the day. On an annual basis, the farm consumes approximately 275.000 kWh of electricity. The benefits of the current digestion and manure processing setup: lower energy, manure disposal, and gas costs.

Energy generation
The gas produced in the mono-digester is converted into electricity by two 16,5 kWh CHP units. Although the installation is not yet operating at maximum capacity, it is expected that approximately 280.000 kWh of electricity can be generated annually. This covers the company's needs exactly. No electricity is fed back into the grid, which is a deliberate choice. Ten Have does not see the added value in doing so. "Being self-sufficient in electricity needs is the goal; the manure currently closes our energy circle," says Erik ten Have.

Heat generation
The combustion of biogas in the CHP units generates heat. This heat is reused in three ways. First, the temperature of the fermentation process did not remain stable enough during the first few months. Now, part of the generated heat is used back into the digester to stabilize the process better. Additionally, heat is used in the nitrogen stripper, where the liquid fraction is 'stripped' of nitrogen after separating the digestate from the digester. The heat is also used to heat the pig barns. It is still difficult to estimate how much gas savings this will yield on an annual basis, but the initial results seem promising. "During the cold spell at the beginning of this year, we were able to heat the piglet barn almost entirely with heat from the digester."

Savings on fertilizer
In addition to energy production, separating and stripping the manure ultimately yields a significant advantage in manure disposal costs. Furthermore, efficient fertilization will soon be possible through the targeted application of the liquid Renure fertilizer produced after stripping. In total, the installation will process 5.500 m³ of manure annually. Approximately 55% of this is cattle slurry and approximately 45% is pig manure. Manure from both the pregnant sows and the piglet barn is utilized.

The residual streams after separation and stripping consist of Renure, expected to be around 100 m³ per year, solid fraction (700 m³), ​​and digestate from the stripper with lower concentrations than slurry (4.800 m³ per year). Given current fertilizer prices, this could result in significant savings if Renure is eventually permanently approved as a fertilizer substitute.

Payback period of 6 years
Ten Have's total investment in the entire process amounts to approximately €850.000. Of this, about €300.000 is for the stripper and €270.000 for the digester and the CHP units. In addition, there are investments of around €100.000 in the various manure storage facilities, as well as costs such as piping, paving, and other expenses.

Advisors from DLV Advies guided the ten Have family in the realization of the project. Securing financing was also part of this. Tom Baak, Financial Management Advisor at DLV Advies, explains the basic principles: "To realize the project, two subsidies were utilized: a provincial scheme covering 80% of the stripper costs, and the national SDE++ scheme. In this case, the latter is based on two tracks: the electricity generated by the CHP units, which falls under the category of renewable electricity, and the utilization of residual heat, falling under the category of 'renewable heat'."

SDE++ the basis for the financing application
The reuse of heat, which was highly applicable at this farm with sows and piglets, made the case interesting. "For dairy farmers without a heat requirement, it is often somewhat more difficult to make the numbers work on this scale. The project is financed based on the issued SDE++ subsidy (for electricity and heat) and the subsidy from the Province of Gelderland towards the purchase of the nitrogen stripper."

He adds: "Return that 'arises' in this case in addition to the SDE++ compensation, such as reduced fertilizer supply, reduced gas consumption, and lower manure disposal costs, is hardly included in the budget. We do include a provision for this as other revenues, but when viewed across the entire budget, it is somewhat marginal."

A question that is often asked is whether the investment will not be recouped sooner given current market conditions. According to Baak, that possibility exists, but is of course 'entirely dependent on the circumstances'. He does indicate, however, that SDE++ grants have increased considerably in recent years, meaning that even (smaller) projects can be made profitable somewhat faster anyway. "In the case of the Ten Have family, the SDE++ decision dates back four years; at that time, the grants were still significantly lower."

Puzzle pieces are falling into place
The Ten Have family is meanwhile satisfied with the investment made, but also indicates that the fine-tuning process takes time and energy. Ten Have: "The pieces of the puzzle are falling into place this year. Due to the abolition of the derogation, the mixed residual stream containing pig manure is now better applicable on the farm itself, and through the creation of a Renure stream, we are able to fertilize more precisely in the future, but also to utilize a larger proportion of the animal manure on our own farm. Furthermore, prices for manure disposal and gas are currently very high, which immediately increases the added value of the installations on the farm."

However, the benefits do not come easily, because running the digester in such a way that it achieves the projected production as much as possible does not happen by itself. "In the start-up phase, for example, it took some puzzling to get the manure robots properly adjusted. Getting the fresh cow manure into the digester efficiently and quickly is essential. The moment this isn't running smoothly, you see it reflected immediately in production. In addition, there were some teething problems to resolve regarding the technology of transferring the residual heat from the CHP units. All in all, a lot of technology is involved, and it is certainly not the case that things run smoothly in the first year. Nevertheless, we see significantly lower costs on the most recent energy bill; that is a great result, especially now that prices have risen sharply recently."

Currently, the entrepreneurs are exploring daily manure removal in the piglet barn, because with daily fresh piglet manure, the efficiency of the digester can be reached at its maximum capacity.

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