Process for producing fertilizer from a biogas stream
A process for producing organic fertilizers involves using a waste ammonia stream to capture sulfur dioxide from the oxidation of sulfur or a gas containing sulfur dioxide to make an ammonium bisulfite/sulfite solution. The sulfite solution is used to scrub hydrogen sulfide out of a biogas or other gas stream. The resulting products contain ammonium thiosulfate and ammonium bisulfite/sulfite.
1 . A process for producing an organic fertilizer, comprising:
introducing a stream of sulfur dioxide into a first column;
routing an ammonia-containing gas stream into the first column, wherein the ammonia-containing gas stream is circulated through an aqueous solution containing bisulfite ions in the first column, and further wherein the ammonia-containing gas stream reacts with the sulfur dioxide and the aqueous solution containing bisulfite ions to produce an aqueous solution comprising ammonium bisulfite and diammonium sulfite;
wherein the first column contains at least two contact zones, and wherein the sulfur dioxide is introduced near a first contact zone and the ammonia-containing gas stream is introduced near a second contact zone, and
further comprising maintaining a higher pH in the first contact zone and a lower pH in the second contact zone;
routing a portion of the aqueous ammonium bisulfite and diammonium sulfite solution to a second column;
introducing a biogas stream into the second column, wherein the biogas stream comprises hydrogen sulfide; and
contacting the routed aqueous ammonium bisulfite and diammonium sulfite solution and biogas stream in the second column to substantially scrub the hydrogen sulfide from the biogas stream and produce an effluent stream containing aqueous ammonium thiosulfate.
2 . The process according to claim 1 , wherein the second column comprises a plurality of contact zones.
3 . The process according to claim 2 , wherein the second column includes three contact zones, and wherein the pH of a first contact zone in the second column is maintained at a higher value than the pH in the second and third contact zones to quickly scrub hydrogen sulfide out of the biogas stream.
4 . The process according to claim 3 , wherein chilled water is circulated into a third contact zone.
5 . The process according to claim 4 , wherein the chilled water has a low chloride content in the range of 0 to 250 ppm to prevent corrosion of the column.
6 . The process according to claim 2 , wherein the pH of each contact zone in the second column is around 4.5 to 6.3.
7 . The process according to claim 1 , wherein a portion of the aqueous ammonium thiosulfate is recirculated through the second column.
8 . The process according to claim 7 , wherein another portion of the aqueous ammonium thiosulfate is routed to a storage unit.
9 . The process according to claim 1 , wherein the ammonia-containing gas stream is obtained from an anaerobic digestate.
10 . The process according to claim 1 , wherein the ammonia-containing gas stream is produced from a sour water stripper or steam stripper.
11 . The process according to claim 1 , wherein the pH of the first and second contact zones is in the range of 5.5 to 6.5.
12 . The process according to claim 1 , wherein another portion of the aqueous ammonium bisulfite and diammonium sulfite solution is circulated back into the first column.
13 . The process according to claim 1 , wherein the biogas is produced during an anaerobic digestion reaction, and wherein the biogas further comprises methane and carbon dioxide.
14 . The process according to claim 1 , wherein a stoichiometric excess of ammonia to hydrogen sulfide is maintained in the second column.
15 . The process according to claim 1 , wherein the process utilizes an anaerobic digester produced ammonia-containing gas stream and a non-hydrocarbon sourced gas to remove hydrogen sulfide from the biogas.