Raw biogas, scrubbed biogas or diesel only?
How a generator that runs on raw biogas compares with the usual alternatives. The last column is our design data; the other two are general facts, with sources.
| Question | Diesel-only generator | Engine on scrubbed biogas | Our designRawWattraw biogas and diesel |
|---|---|---|---|
| H2S scrubbing and cleaning cost | Not applicable. No biogas is used. | Not needed. No scrubber media to buy, replace or dispose of.2 | |
| Engine service | Standard diesel service. | Oil and filters every 200 operating hours.2 | |
| Status | Established technology. | Established practice, widely used.1 | Concept stage. Final design due Q3 2027. Not yet for sale.2 |
- Advantage
- Drawback
- Neutral, or depends on the setup
Sources (6)
- Cornell University, Dairy Environmental Systems: Hydrogen sulfide removal from biogas: basics (2017).
- Design data and real-biogas test results. Not independently verified.
- Bertinatto et al. (2017), Analysis of lubricant oil contamination and degradation and wear of a biogas-fed Otto cycle engine, Acta Scientiarum. Technology 39(4): 409–416, DOI 10.4025/actascitechnol.v39i4.29458. Finds that H2S affects lubricant oil performance and engine wear.
- Potter and Chambers (2002), Gas utilization from sewage waste: H2S levels in biogas and what engines and turbines tolerate.
- Ryckebosch, Drouillon and Vervaeren (2011), Techniques for transformation of biogas to biomethane, Biomass and Bioenergy 35(5): 1633–1645.
- Shelford T, Gooch C (2017), Hydrogen sulfide removal from biogas, Part 3A: Iron sponge basics, Cornell University Dairy Environmental Systems Program. Source of the media price, iron content and H2S capacity used in the calculator estimate.
What could you save?
Tell us how much diesel you burn for power today. We will show you a rough saving.
Example numbers
You could save about
$120a month
About $1,440 a year
A rough estimate. The unit gets half of its fuel energy from biogas, so you would buy half the diesel you burn in a diesel-only generator today. Your real saving depends on your generator, your load and how much biogas you have.
Cleaning the gas instead? An engine on scrubbed biogas would spend about US$20–40 a year on iron-sponge media for this unit. That is only the media, before the vessel and the work of changing it.6
Assumptions
The unit runs at 3.1 kW for 6 hours a day, every day, with a 25% efficient engine and half of the fuel energy from biogas at 6 kWh per m³. That is about 2,300 m³ of biogas and 6.4 kg of H2S a year at 2,000 ppmv. Iron sponge at the US retail price of $12 per bushel holds 15 lb of Fe2O3, which takes up 0.56 lb of H2S per lb, used between fully and by half.6 The engine and gas figures are our assumptions. Local prices in sub-Saharan Africa may differ.
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