Vacuum evaporation is an efficient method for reducing the volume of liquid phase digestate (LPD) from biogas plants (BGP). Furthermore, thickening the LPD in biogas plant contributes to the efficient utilization of waste heat. it also reduces fossil fuel consumption. That is a valuable asset needed for reducing the cost of transporting liquid phase digestate off-site.
However, the utilization of vacuum evaporation must be at a reasonable cost, For that reason, we recommend a comprehensive study should precede the integration of evaporation technology in any particular BGP. For this purpose, this study compares selected parameters of the types of industrial evaporators which may be suitable for LD thickening.
Evaporators in AD Plants
Industrial evaporators are offered for biogas plants, where evaporator systems treat digestate that is inappropriate for use as a crop fertiliser. The operators of anaerobic digestion facilities that run on the organic component of municipal solid waste are in great demand and highly appreciative of digestate evaporators.
In MBT plants, the digestate is the liquid part after the organic fraction has been removed (Mechanical Biological Treatment Plants).
Digestate evaporators, such as Scraped Plate Evaporators, are available which provide reliable performance, compact design, optimum low maintenance and long functional life.
These products are designed and manufactured in accordance with the international quality standards and norms using cutting-edge technology to ensure their reliability and cost-effectiveness.
Digestate evaporation for biogas plant
Increased digestate availability is linked to an increase in the number of biogas plants.
Digestate is an anaerobic fermentation byproduct that is mostly utilised as a fertiliser in agricultural areas around biogas plants.
Digestate in liquid form contains nutrients that are rather dilute. Furthermore, its storage and application in a crop field usually come at a significant expense.
If long-distance transportation is required, this type of export is just uneconomical.
Biogas digestate evaporation technology
Biogas digestate evaporation technology can help in this situation.
When the digestate is classified as an animal by-product, the same technology is applied.
The same technology is also used when the digestate is classed as an animal by-product, or in some way toxic, and therefore unsuitable for land application for agricultural fertiliser use
In Switzerland, a digestate evaporation system has been successfully integrated into the existing biogas plant.
Why Use Biogas Evaporators?
Evaporation is done to lower the volume of digested material.
Furthermore, because of the nutritional balance of evaporation, the nitrogen concentration in the fertiliser can be reduced. That way it is possible to tune the nitrogen/phosphorus ratio per hectare of application area.
In comparison to artificial fertilisers, digestate contains a considerable number of nutrients in low concentrations. The cost of storing and transporting digestate is a substantial financial burden for biogas plant operators. This is especially true in locations where agricultural production is extensive.
Vacuum evaporation is a method that allows AD plant operators to lower the amount of the digestate in an efficient manner.
Vapogant Digestate Evaporator
The Vapogant Digestate Evaporator processes biogas digestate to generate a useable, concentrated fertiliser with a lower water content.
The device uses waste heat from the biogas plant's CHPs to extract the majority of the water from the digestate via vacuum evaporation. Simultaneously, volatile nitrogen is bound, reducing any emissions during spreading. This makes nitrogen available as the Ammonium Sulphate Solution (ASS).
The “Ammonium Sulphate solutions and fertiliser Trading Group” reports:
“We already had an AD biogas plant on our farm running successfully, but we wanted to reduce the volume of substrate needed to be transported away from the ad plant. Victus energy invested with us, providing capital and technology by way of the Vapogant Digestate Evaporator. The Vapogant system removes water from the digestate through vacuum evaporation using waste heat from the CHP.”
Why Vacuum Evaporate the Digestate Output from Anaerobic Digestion Plants?
One of the most promising paths toward completely utilising green energy within a circular economy is biogas generation. Because of its low energy consumption and reliance on bacteria, anaerobic digestion is the industry-standard technology for biogas production.
Liquid digestate is still created from the leftovers of digested bio-feedstock and will require treatment even in such an environmentally beneficial procedure.
The bulk of bio-feedstock could potentially escape the circular supply chain within the economy if an improper treatment process for liquid digestate is used.
Biogas Evaporators Reduce the Cost of LPD Storage and Transportation
The cost of LPD storage and transportation rises with increased production of the liquid phase of digestate (LPD), which has a low dry matter content.
LPD handling also results in severe nitrogen losses. Biogas plants also necessitate a considerable amount of infrastructural water, which is a precious resource.
The approach of LPD nitrification and subsequent thermal thickening was examined as a possible alternate solution for overcoming such issues.
In a recent technical publication, the characteristics of thermally thickened nitrified LPD and the distillate generated from vacuum evaporation of nitrified LPD were investigated.
Evaporator design makes the best use of waste heat
The Arnold evaporator was created for the purpose of processing liquid fractions with low dry matter content. Clean water and a thick fraction are the final products. The high-quality heat exchanger's unique design eliminates clogging and makes operation simple. Waste heat can be used to power the process, but an electric-only variant is also available.
A multi-coil helical evaporator for a high-temperature organic Rankine Cycle plant powered by biogas waste heat was designed and tested.
Clean water from liquid manure
The Evaporator technique reduces the volume of liquid manure by concentrating it. Under vacuum, evaporative systems often utilise multi-stage thermal and/or electrical inputs.
They then condense ‘clean water' from manure liquids, leaving a concentration of residual manure. The majority of the nutrients are captured in the residual concentrate stream, hence evaporative systems are a viable choice for volume reduction.
Biogas Evaporators – A Conclusion
The boiling of a liquid substrate under negative pressure at a temperature lower than the average boiling temperature under atmospheric conditions is known as vacuum evaporation.
The condensate is represented by the condensed vapour, whereas the concentrate is represented by the residual substrate.
This technique comes from other industries and is mostly used to recover chemicals from industrial waste. While it has not yet been widely adopted in the field of agricultural digestate treatment, we anticipate that it will be in the not-too-distant future.
