Syngas biomethanation process and anaerobic digestion system
An anaerobic digester is fed a feedstock, for example sludge from a municipal wastewater treatment plant, and produces a digestate. The digestate is dewatered into a cake. The cake may be dried further, for example in a thermal drier. The cake is treated in a pyrolysis system to produce a synthesis gas and biochar. The gas is sent to the same or another digester to increase its methane production. The char may be used as a soil enhancer.
1. A process comprising steps of,
a) producing a synthesis gas from pyrolysis of a feedstock;
b) producing a mixture of gasses in the headspace of an anaerobic digester;
c) withdrawing a portion of the mixture of gasses from the headspace of the anaerobic digester;
d) withdrawing a stream of digestate from the anaerobic digester;
e) mixing the withdrawn portion of the mixture of gasses and the synthesis gas into the stream of digestate to produce a stream of digestate comprising the withdrawn portion of the mixture of gasses and the synthesis gas; and
f) returning the stream of digestate comprising the withdrawn portion of the mixture of gasses and the synthesis gas to the anaerobic digester.
2. The process of claim 1 wherein the feedstock comprises one or more of a raw biomass, wood, municipal yard waste, municipal solids waste, primary sludge from a wastewater treatment plant, waste activated sludge from a wastewater treatment plant or an agricultural waste or residue.
3. The process of claim 1 wherein the feedstock comprises dewatered digestate.
4. The process of claim 3 wherein the dewatered digestate is produced from the digestate withdrawn from the anaerobic digester.
5. The process of claim 1 wherein the anaerobic digester is coupled with or part of a municipal wastewater treatment plant, or an agricultural or industrial digester.
6. The process of claim 1 wherein the synthesis gas is cooled before it is mixed with the digestate.
7. The process of claim 1 wherein condensable organic compounds are removed from the synthesis gas and returned as feedstock to step a).
8. The process of claim 1 wherein the mixing in step e) comprises aspirating the withdrawn portion of the mixture of gasses into the digestate by an ejector.