Syngas biomethanation process and anaerobic digestion system

Abstract

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.

We claim: 
     
         1 . A process comprising steps of,
 a) producing a synthesis gas from pyrolysis of a feedstock; and,   b) adding the synthesis gas to an 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 digestate. 
     
     
         4 . The process of  claim 3  wherein the digestate is produced by 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 fed to the digester. 
     
     
         7 . The process of  claim 1  wherein condensable organic compounds are removed from the synthesis gas and returned as feedstock to step a). 
     
     
         8 . An apparatus for transferring syngas to digestate comprising,
 a) a pipe between a source of digestate and a digester;   b) a pump to create a flow of digestate in the pipe;   c) an ejector, microbubble pump or gas transfer membrane in communication with pipe having an inlet connected to a source of the syngas.   
     
     
         9 . The apparatus of  claim 8  wherein the source of digestate is the digester. 
     
     
         10 . The apparatus of  claim 8  wherein the pipe is connected to multiple outlets in the digester. 
     
     
         11 . The apparatus of  claim 8  wherein the inlet is further connected to the headspace of the digester. 
     
     
         12 . The apparatus of  claim 8  further comprising a heat exchanger between the ejector and the source of the syngas. 
     
     
         13 . The apparatus of  claim 12  wherein the heat exchange provides heat to a digestate cake drier. 
     
     
         14 . A process and apparatus for treating wastewater comprising,
 a) treating the wastewater to generate a sludge;   b) feeding the sludge to an anaerobic digester to produce biogas and digestate;   c) dewatering the digestate to produce a cake;   d) thermally drying the cake to produce a dried cake;   e) pyrolysing the dried cake to produce char and syngas;   f) feeding the syngas to the anaerobic digester.   
     
     
         15 . The process of  claim 14  further comprising extracting heat from the syngas and using the extracted heat in step d). 
     
     
         16 . The process of  claim 14  comprising using the char as a soil enhancer. 
     
     
         17 . The process of  claim 14  comprising steps of extracting a condensable gas from the syngas, pyrolysing the condensable gas to produce additional syngas, and feeding the additional syngas to the digester. 
     
     
         18 . The process of  claim 14  wherein step e) is conducted at a temperature of 250 degrees C. or more. 
     
     
         19 . The process of  claim 14  wherein the syngas includes one or more vapors and step f) comprises condensing the one or more vapors on contact with a digestate from the anaerobic digester.