BDS-V DIGESTATE DRYER

BDS-evaporator digestate dryer as heat utilisation concept at a renewable resource biogas plant with 20,000 tons of digestate.

The recommended heat utilisation concept emanates from a preferable maximum utilisation of the available heat of the two CHP units with 566kWth (amount = 1,132kWth). In order to improve efficiency, the heat is used on two levels.

Level 1:

The two digestate evaporators receive the complete amount of waste heat of both CHP units with a supply temperature of 85-90°C of two heat exchangers.

Level 2:

The second level receives the waste heat from level 1 and is made up of the digestate belt dryer and the biogas plant, the ultimate consumers.

The heat emission occurs in direct emission from the outlet of the evaporator; the amount of heat for the ultimate consumers on the second level is approx. 90% and the supply temperature reaches approx. 70°C.

This results in the following advantages:

• production of a drainage capable condensate
• maximum reduction of the amount of digestate
• production of a storage-stable dry fertiliser (>85% DM)

The maximum available amount of heat in the projected solution is about 1,100kWh. The complete amount of heat is supplied by two heat exchangers. Thereby, the heat network of both of the CHP units and of the biogas plant is initially completely disconnected. The heat supply of the biogas plant takes place over the cold side of the heat exchanger at a temperature level of 70°C. The existing cooler of the CHP units remain as before integrated in the system.

As a result, with a power input of 1,100kW, approximately 1.3m³ of water can be evaporated and turned into condensate in both of the digestate evaporators. (This data applies to a supply temperature of 90°C. If the temperature is decreased to 80°C, the performance is reduced by about 10-15%. If the supply temperature drops to <70°C, the performance drops to <50%.)

The belt dryer has a planned output of 0.45m³ of dried material per hour, with an input dry matter contents of minimum 25% (non-drip). The drying performance of the digestate belt dryer is approx. 0.6-1l/kWhth, depending on temperature level, external influences, etc.

Accordingly, there should still be enough heat available to dry the 0.45m³ of dry material per hour.