Essential Summer Biogas Digester Overheating Prevention for Profit Stability and Fugitive CH4 Emissions Avoidance

Summer biogas digester overheating prevention is essential for small AD Plant profitability, process stability, and fugitive CH4 emissions avoidance.

Summer Biogas Digester Overheating Prevention

In a number of nations, governments are beginning to provide financial incentives to farmers to install biogas plants in large numbers, and to do that in regions where the average farm size is small involves the construction of small AD plants. These biogas facilities will unfortunately be hampered by poorer financial viability and their biological stability will generally be less than for than larger plants.

In addition, many of these nations are hot countries and although heating of the digester tanks may be needed in winter, and is easily provided by using waste heat from the generator, cooling systems are less easily provided. If cooling was to be provided it would reduce plant efficiency by adding a substantial sacrificial load on the output of saleable electricity.

Great Care Needed to Design Biogas Plants for Resilience Against High Summer Temperatures

In order to ensure adequate performance and a reasonable payback period, it is therefore essential that every aspect of the operation of the new government-incentivized biogas capacity is carefully designed. If the biogas industry follows the latest best practice there will be little difficulty in doing that, apart from such aspects as rapid summer heating during heat-waves.

In this respect, the summer overheating of anaerobic digesters due to solar gains must be prevented. It is accentuated for small AD plants which have a lower thermal capacity and warm up quicker than larger digesters.

Hence, a significant problem to carefully monitor in anaerobic digesters is summer overheating due to solar gains through the gas storage dome.

High Solar Reflectance Covers to Improve Profitability of Smaller Biogas Plants

This would potentially undermine the profitability of biogas plants because of :

  • raising the temperature above their optimum for growth, can bring with it a damaging effect on the rate of methane production for the methanogenic bacteria accustomed to being held within the mesophilic temperature range, or
  • of causing premature venting of biogas due to overpressure induced in the digesters by the expansion of the stored biogas during excess heating.

A recent research study has suggested that a smart solution can be to finish the gasometer dome using cool roof materials. Suitable materials would be those with high solar reflectance. Matteo Bavutti et al. Energy Procedia 45 ( 2014 ) 1344 – 1353 1353 has completed a study, mainly performed by means of a unique calculator app. This app, validated on the basis of experimental data, has allowed assessing the effectiveness of the proposed solution.

Emergency Venting of Biogas Must be Avoided if the AD Industry is to Avoid Losing its Reason for Existence

It was shown that the adoption of a cool roof gasometer dome can greatly reduce (and even completely eliminate) summer overheating and the emergency venting of biogas. Venting of biogas (methane) during hot gasometer venting must be avoided due to the high greenhouse gas (GHG) potential of methane. Methane emissions are far worse as GHG, as opposed to the products of burning the biogas which have a much lower greenhouse gas warming effect.

The authors of the above research paper estimate that using “cool roof” technology can provide a useful saving in terms of biogas loss and, consequently, economical income up to 9200 €/year for a 205 kWe plant.

Cost Benefit of Summer Biogas Digester Overheating Prevention

They estimate that this compares with about 6000 € of the estimated cost for the cool roof finishing, if this is adopted at the design phase. via

They do issue a word of warning though that their result, was obtained by assuming that the gasometer dome is new and its solar reflectance is at its initial value.

In order to limit the performance drop due to aging of the digester cover dome surface, true the roof materials selected must be chosen which will only exhibit a very minimal drop-off in performance after weathering.

Choosing the Optimal Digester Temperature

I define the digester target temperature from the conditions of the location. Thereby two extremes can be distinguished, for example, liquid manure mono-fermentation and silage mono-fermentation. If I intend to ferment liquid manure exclusively and have good heat utilization I decrease the digester temperature as far as possible. Thus a higher proportion of usable heat remains.

This additional usable heat is quite relevant. In a liquid manure mono-fermentation at least half of the CHP waste heat is needed for the warming of the liquid manure to a fermentation temperature of for example 37 °C. At a digester temperature of 30 – 32 °C and a sufficient time for the adaptation the biology works as good as with 37 °C, however the heat requirement for the digester heating decreases considerably and about30 % more usable heat remains available.

Avoiding Thermal Shock on Operating Digesters

Uncontrolled heating of the digester is harmful whenever the temperature rise is too fast and strong. An increase of 3 – 4 °C within two weeks can damage the whole biology to an extent that the plant “crashes”. An especially low resilience is shown by AD plants, which for example suffer from a lack of icronutrients or which are biologically overloaded. That’s the importance of summer biogas digester overheating prevention. via

Factors to Consider About AD Plant Heating

The mean surrounding temperature and it’s seasonal variations are very important. Biogas plants without heating system work, therefore, only in warmer regions for the whole year. In regions with extreme temperature variations, for instance in Turkey (hot summer, cold winter), the biogas plant should be built under the ground for natural insulation to reduce the risk of damaging thermal shock. Hence, the biogas plant would run constantly all-year. Before implementation, at least an approximated average temperature profile and expected extremes over the year should be available for the site.

Conclusion to Summer Biogas Digester Overheating

A biogas plant with heating system and co-generation can be operated with process energy. Nevertheless, the dimensioning of such a heating system is difficult, as the substrate, which has to be heated up, is not homogenous. via

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