Performance of different condensing setups and operating parameters in ammonia recovery pilot plant with biogas digestate

Technical nitrogen (N) recovery from biogas digestate via vacuum degasification (VD) of ammonia is an important task with regards to environmental issues and economic reasons. There has not yet been much research on VD, but compared to conventional stripping methods energy costs might be reduced. In the frame of the EU project ”Circular Agronomics” a VD pilot plant for N recovery from biogas digestate was built. This study aims on optimizing the pilot plant with special regards to the scrubber, where the N fertilizer is formed. The plant was operated at 310 mbar absolute pressure, 150 L · h−1 recirculation rate and 35 kg total water mass. Two different conditions were examined: condition 1) 70◦C, gas to liquid ratio (G/L) 20:1, pH 9 and condition 2) 50◦C, G/L 33:1, pH 10. A total ammonium nitrogen (TAN) elimination of 93% for condition 1 and 73% for condition 2 was achieved. Conducting the experiments a high amount of water was evaporated (25% for condition 1 and 5% for condition 2). The high water evaporation leads to a low TAN/water ratio in the gas stream of 0.4−1.2 (condition 1) and 1.6−2.8 mol · L−1 (condition 2) respectively. A low TAN/water ratio is disadvantageous, as it results in a dilution of the N fertilizer, that is being formed in the subsequent scrubber. Besides, it was found, that the plant loses high amounts of energy in form of latent heat due to water evaporation and requires more favorable energy recovery.

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