Biogas is produced by anaerobic digestion that breaks down a biodegradable feedstock (such as wastewater treatment plant sludge, industrial food processing waste or slurry) into gas, water, and a solid waste termed digestate. The digestate is in the form of sludge sometimes called liquor. It is typically high in valuable nutrients such as nitrates and phosphates, but has a high water content that originates both from the moisture content of the original waste that was treated but also includes water produced during the microbial reactions in the digestion process.
A consequence of the high water content is that transportation costs for removal of digestate are often enormous; companies may have to pay up to 25 Euros per tonne of digestate. Reduction of transport costs by reducing the water content is, therefore, the key economic driver. However, there is a further potential benefit because in many cases the digestate contains sufficient nutrients for it to be converted into fertilizer. For biogas plants, therefore, investing in digestate management technology and promoting the production of fertilizer will make a major contribution to improving their profitability.
HRS Heat Exchangers propose a novel thermal process for managing digestate based on its patented scraped surface evaporation technology. The process heat required for the digestate management system can typically be derived from the waste heat produced by engines in biogas cogeneration plant. This heat, being essentially a free energy source, further improves the economic and environmental performance of the installation.
HRS Digestate Management Technology
Digestate is a fouling substance that contains colloids and other components that foul surfaces, reducing thermal efficiency and causing downtime in conventional heat exchangers. At the core of the HRS digestate process is an evaporation plant using HRS patented scraped surface evaporator technology that is self-cleaning and guarantees uninterrupted operation for periods of over 6 months.
The design is based on a traditional shell and tube heat exchanger but with the addition of scraping elements inside each interior tube. The scrapers are moved back and forth by hydraulic action. The scraping action has two very important advantages: any fouling on the tube wall is removed and the scraping movement introduces turbulence in the fluid increasing heat transfer.
HRS Heat Exchangers have studied the digestate evaporation process in detail, using a pilot plant developed specifically for this application, to ensure that this technology is effective and appropriate. HRS evaporation can concentrate digestate into a high-density viscous concentrate of up to 25% dry matter; a far greater degree of concentration than can be achieved with any competing technology.
The digestate treatment process consists of four main stages. The first stage is mechanical separation of the raw digestate, separating it in a liquid and solid fraction. In a second stage, the free ammonia produced in the biogas process and present in the liquid fraction of step 1, is fixed by adding acid. This way the nitrogen present in the ammonia is kept in the product adding value to the final product (fertilizer). The patented HRS evaporators are then used to concentrate the liquid waste fraction by removing the water. Finally, in the final fourth stage, the concentrate from the digestate evaporator is combined with the solid fractions from the mechanical separator into a highly concentrated waste that may either be transported as it is, or optionally, dried to produce fertilizer or solid fuel. Because an HRS multi-effect evaporation plant is far more energy efficient in reducing digestate water content than feeding digestate directly to a dryer, the evaporation plant reduces the energy used to produce dried fertilizer.
Process Flowsheet of HRS Digestate Management System
The critical and innovative evaporation process involves applying thermal energy at high temperatures typically using heat derived from cogeneration exhaust gases and engine cooling water. Multiple Effect Evaporation technology is used at this stage to allow maximum evaporated water output with minimum of energy input required. This is achieved by operating each evaporation stage at a lower pressure than the previous one, so reducing the temperature required to achieve evaporation to a value that can be achieved by the exhaust steam from the previous stage. The proposed process is easily adaptable to other available energy sources. Each biogas plant is different, and is considered separately when deciding how to integrate digestate management process into the biogas production process.
Apart from the obvious benefits of the large reduction in volume of digestate and the corresponding savings due to lower transport costs, the process can run on essentially free waste thermal energy from cogeneration plants. In addition, the clean water (condensate) obtained in the evaporation process can be used as make up water at the starting point of the biogas process, with a resulting reduction in utility costs. Where the nutrient content of the digestate is appropriate, further drying to produce dried fertilizer for sale further improves the ROI of the biogas plant.
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