Biogas produced from anaerobic digestion (AD) consists of 55-70% methane, which can be used as a source of renewable energy, but also contains carbon dioxide and traces of hydrogen sulfide (H2S). H2S is produced during AD by sulfate reducing bacteria and can corrode the AD vessels and electric generator sets that turn methane into electricity. H2S in biogas can be especially high on dairy farms due to the high sulfur content of the manure. Market available solutions to high H2S concentrations in biogas can have high capital and operating costs or unpredictable efficiencies. A desulfurization alternative is renewable biochar, a precursor to activated carbon produced via thermal degradation of organic material under limited oxygen. In this study, biochar will be tested as an AD additive for H2S removal from biogas. The project objectives are to investigate: 1) biochar addition into a dairy manure digester for in-vessel H2S desulfurization to eliminate the need for an additional biogas scrubber, 2) modification of biochar surface for enhanced desulfurization of biogas, and 3) desulfurization post-digestion in a biochar packed separate scrubber vessel. The results of this project will be published in a manuscript and incorporated into a Farmer’s Guide to Biogas Production, Scrubbing and Utilization that we are preparing. It is expected that the results will lead to the use of biochar as a low-cost H2S scrubbing alternative for farmers and AD practitioners.
Project objectives from proposal:
Objective 1: Application of biochar as an additive to anaerobic digestion of dairy manure for H2S reduction and enhanced methane production.
Hypothesis 1a: Smaller particle size and higher surface area of the biochar will lead to higher H2S removal due to the liquid phase reaction controlled by available adsorption sites (higher surface area), with higher methane production.
Hypothesis 1b: Biochar surface modification with sodium, magnesium and iron (III) salts will lead to enhanced H2S reduction and greater methane production when compared to unmodified biochar, with H2S removal efficiency dependent on the metal oxide charge [Iron (III) > Magnesium (II) > Sodium (I) > Unmodified]
Objective 2: Compare H2S removal efficiency between biochar addition into a digester and biochar used in a separate gas adsorption column for H2S scrubbing.
Hypothesis 2: Reactions in a biochar column are dependent on biochar pH and will have a higher adsorption capacity when compared to reactions in the liquid phase. In addition, the H2S sorption capacity in a gas adsorption column will follow the trend: corn stover biochar > maple biochar > activated carbon, due to corn stover biochar having the highest pH and activated carbon having the lowest pH.
The project will aim to identify the effect of particle size, surface area, and surface charge of biochar from two sources (maple saw dust and corn stover) as well as activated carbon on H2S production when added into a digester and packed into a separate scrubber vessel. It is hypothesized that the dissolved bisulfide (HS-) ions will be adsorbed to the biochar surface, resulting in a lower H2S partial pressure in the gas phase. The project will also aim to compare the H2S adsorption capacities of direct addition of biochar to a digester to H2S adsorption from the biogas when using an activated carbon or biochar filled column as a separate scrubber vessel. Other possible benefits of direct biochar addition could be ammonium N (NH4-N) and dissolved P removal, and possible enhancement of CH4 formation, due to the presence of important trace metals in the biochar required for methanogenesis.
It is also hypothesized that biochar addition will reduce P concentration in the digester effluent due to electrostatic forces, and ionic precipitation between the cations on the biochar surface and the dissolved P. The biochar could then act as a slow-release fertilizer for the bound P, when compared to chemical fertilizers (Sizmur 2017).