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BMP testingBy-products of the agri-food industry

Peanut shells — BMP: 2 studies, weighted mean 50 m³ CH₄/t VS

Peanut (groundnut) shells are a light, fibrous and woody waste from shelling plants, with very similar results in several studies, but slow hydrolysis.

rating: neutralCH₄ ≈ 52 % (estimate)

Published Updated 3 min readFeedstockData Editorial Team

Photo Judgefloro / Wikimedia Commons, licence CC0 1.0, source file; changes: cropping

Weighted mean
50m³ CH₄/t VS
Range
48–51m³ CH₄/t VS
Records
2studies
Publications
2sources
Temperatures
37 °Cof the test

Methane in biogas

Low methane content · ≈ 5.2 kWh/m³ of biogas

estimate from the substrate type The collected studies contain no measurement. The interval follows from the approximate composition (protein from total nitrogen (N 0.7 % DM × 6.25)) and the yields of compound groups according to FNR.

How we calculate it and what the zones mean

Rating for biogas plants

neutral — editorial rating for biogas plantsNo clear advantages — worth charging a fee for accepting it

Potential per tonne of fresh matter: low, but limited by the poor degradability of the organic matter (44 m³ CH₄/t); no particular threats to stability. Handling costs: fibres, floating layer, slow degradation.

Why? Rating factors

Potential per tonne 0

  • ±0potential 44 m³ CH₄/t fresh matter (BMP 50 × DM 89.8 % × VS 98.5 %); low degradability of VS (BMP 50 m³ CH₄/t VS) — points capped

Process stability 0

  • 0no factors

Handling and costs −1

  • −0.5fibres, floating layer — shredding and intensive mixing
  • −0.5slow degradation — long retention time

Total points: −1 · thresholds: ≥ 2.5 very good, ≥ 1 good, below — neutral; negative with cumulative process risk.

Editorial rating per the methodology · confidence of the rating: low · excluding market prices

Add to the co-digestion calculator CSV data

BMP curves from 2 studies

Weighted mean Individual records

BMP curves: Peanut shellsCumulative methane production over time for 2 records and the weighted mean of 50 m³ CH₄/t VS. 2 of the curves are model curves (first-order kinetics).02040600510152025303540time [d]m³ CH₄/t VSOliva et al. (2023): 48 m³ CH₄/t VS (model curve)Shen et al. (2018): 51 m³ CH₄/t VS (model curve)mean50

Dashed lines are a model curve (first-order kinetics, B(t) = B₀·(1 − e−kt)); the end point comes from the publication. All curves are model curves.

Cumulative methane production over the test [m³ CH₄/t VS]. The thick line is the weighted mean of all records (50); dots mark the end values given in the publications. Point at the chart to read values; click a legend item to hide or show a record.

Records and sources

Download CSV
SourceBMP
m³ CH₄/t VS		TemperatureSampleCurve
Oliva et al. (2023) DOI48—Peanut shells (PS), rawend point (model curve)
Shen et al. (2018) DOI5137 °C · mesophilicpeanut shell · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)end point (model curve)
Weighted mean50arithmetic mean 50, SD 2

Typical composition

Dry matter (DM)
89.8% (range 89.8–89.8)
Volatile solids (VS)
98.5 % DM (range 98.5–98.5)
Nitrogen
0.7 % DM
C/N ratio
73.2

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 1 determination, volatile solids: 1 determination. C/N was calculated as the ratio of C and N content (% DM) from the same source. Samples were tested dried/air-dry – the DM refers to the material in the test, not to fresh material.

Sources: Shen et al. (2018).

The composition of the raw material varies between batches — for plant calculations use an analysis of your own sample. The calculator inserts typical values automatically.

Process properties

  • fibrous
  • slowly degrading
  • low in nitrogen
  • carbon-rich
  • high dry matter
  • editorial classification

Low confidence: the description is based on general characteristics of the substrate group, not on studies of this material. Treat it as a hint to be verified.

Advantages

  • Methane potential in the Substraty.pl BMP set: weighted mean 49.6 m³ CH₄/t VS (SD 2.3; 2 records, 2 sources).
  • Peanut shells: a dry and durable material with no acidification risk, which does not load the system with VFA (Hendriks and Zeeman 2009).

Drawbacks and risks

  • No additional inhibitors, but also no rapid energy effect (Hendriks and Zeeman 2009).

Co-digestion — what to combine it with, what to watch out for

Dosing

No source limits on the dose for this substrate.

Introduce gradually, starting with a small dose and monitoring the process (e.g. FOS/TAC, pH, gas composition).

Practical requirements

  • Shred: shredding increases the surface available for degradation and speeds it up, but does not necessarily raise the final gas yield (FNR 2010, Ch. 3 “Triturado”).
  • Control floating crust and sediment: when solids are fed directly into the digester, the risk of their formation depends on the substrate (FNR 2010, Ch. 3).

Sources of the process data

  1. Hagos et al. (2017). Anaerobic co-digestion process for biogas production: progress, challenges and perspectives. Renew. Sustain. Energy Rev.
  2. Weiland (2010). Biogas production: current state and perspectives. Appl. Microbiol. Biotechnol.
  3. Rajagopal, Masse, Singh (2013). A critical review on inhibition of anaerobic digestion process by excess ammonia. Bioresour. Technol.
  4. Hendriks, Zeeman (2009). Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour. Technol.
  5. FNR (2010). Guía sobre el Biogás – Desde la producción hasta el uso (hiszp. wydanie „Leitfaden Biogas”, GIZ 2013)

The co-digestion calculator will assess a specific mixture containing this substrate.

What it is and where it comes from

Peanuts (Arachis hypogaea) grow in pods underground, and after harvest the pods are shelled. The shells make up a considerable part of the mass of the pod and are the main waste of shelling plants. The largest producers are China, India, Nigeria and the United States. In Poland peanuts are imported, so shells occur only sporadically, mainly in processing and trade.

Properties relevant to digestion

The shells are a woody material with a large share of cellulose, hemicellulose and lignin and a low content of protein, fat and soluble sugars. Hydrolysis is slow, and without size reduction and pretreatment the yield is limited. The risk of acidification is small. The material is very light and bulky, which makes dosing difficult and raises transport costs per unit of energy. The high C/N ratio requires the addition of nitrogen. The shells may contain dust and mineral impurities from the field.

How to read the spread of results

The dataset contains several records from several sources: raw shells with no temperature stated, and shells under mesophilic conditions. The results are very similar, which suggests agreement between methods, but the amount of data is small. The result is affected by comminution, moisture, test duration, peanut variety, impurities and the inoculum. For woody material, a short test underestimates the result.

In biogas plant practice

Groundnut shells can be an addition to co-digestion after grinding, at a small share. Usually, however, they are used as fuel, bedding or a raw material for producing pellets, which limits their availability to biogas plants. The material stores well in dry conditions. Waste from processing is subject to waste legislation, and when sold as a by-product other rules apply.

Like other shells, this material is more interesting as a feedstock for pretreatment and for research on lignocellulose processing than as a routine addition to the feed. However, if it arises in large quantities on site, its use after grinding and in combination with slurry can be considered.

In any implementation, the dustiness of dry, ground material and the risk of dust explosion during its handling must also be taken into account, which requires safety rules to be observed and adequate ventilation.

Methodology in brief

BMP is the amount of methane obtained from volatile solids (VS) in a batch test, given in m³ CH₄ per tonne of VS (numerically the same as NmL CH₄/g VS in the publications) and converted to normal conditions (0 °C, 101.325 kPa). The weighted mean gives each publication a total weight of 1, split equally between its results for the substrate — a paper testing twenty varieties does not outweigh a paper with a single sample. In inter-laboratory programmes the unit is the laboratory (total weight 1), and within it the weight of a test grows with the number of bottles (replicates). Results after pretreatment (e.g. thermal, chemical, enzymatic) are shown separately and are not included in the mean.

Solid lines are curves measured in the tests. Where a publication gives only the final result, we draw a first-order kinetic model, B(t) = B₀·(1 − e−kt): only the end point comes from the publication, and the shape of the curve serves purely as visualisation — do not read the production on a particular day from it. For this substrate k = 0.091 d−1 was assumed (no kinetic data for the group – the median of all available rate constants k (93 values) was used).

The statistics describe the spread of published results — a descriptive compilation, not a meta-analysis or a guarantee of plant yield.

Full methodology and correction policy

Bibliography

  1. Oliva, A.; Papirio, S.; Esposito, G.; Lens, P.N.L. (2023). Impact of Chemical and Physical Pretreatment on Methane Potential of Peanut Shells. Energies 16(12), 4698. DOI: 10.3390/en16124698 DOI
  2. Shen, J.; Yan, H.; Zhang, R.; Liu, G.; Chen, C. (2018). Characterization and methane production of different nut residue wastes in anaerobic digestion. Renewable Energy 116, 835–841. DOI: 10.1016/j.renene.2017.09.018 DOI

All sources of the site: bibliography.

Other substrates in the group “By-products of the agri-food industry”