BMP testingBy-products of the agri-food industry

Almond shells — BMP: 3 studies, weighted mean 40 m³ CH₄/t VS

Almond shells are a hard, woody waste from nut shelling, rich in lignin and cellulose; they digest slowly and depend strongly on comminution.

rating: neutral CH₄ ≈ 52 % (estimate)

Published 11 May 2025Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Ivar Leidus / Wikimedia Commons, licence CC BY-SA 4.0, source file; changes: cropping

Weighted mean: 40m³ CH₄/t VS
Range: 20–55m³ CH₄/t VS
Records: 3studies
Publications: 3sources
Temperatures: 37 °C, 55 °Cof the test

Methane in biogas

≈51–52% CH₄

Low methane content · ≈ 5.1 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 0.6 % 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 (37 m³ CH₄/t); no particular threats to stability. Handling costs: fibres, floating layer, slow degradation.

Why? Rating factors

Potential per tonne 0

±0potential 37 m³ CH₄/t fresh matter (BMP 40 × DM 93.6 % × VS 99.3 %); low degradability of VS (BMP 40 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: medium · excluding market prices

Add to the co-digestion calculator CSV data

BMP curves from 3 studies

Weighted mean Individual records

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 (40); 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

Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Nitsos et al. (2015) DOI	20	55 °C · thermophilic	almond shells (untreated AS)	end point (model curve)
Oliva et al. (2024) DOI	55	37 °C · mesophilic	Almond shell (AS raw)	end point (model curve)
Shen et al. (2018) DOI	45	37 °C · mesophilic	almond shell · pretreatment: particle size reduction/sample preparation (MY-D: physical treatment)	end point (model curve)
Weighted mean	40	arithmetic mean 40, SD 18

Typical composition

Dry matter (DM): 93.6% (range 93.6–93.6)
Volatile solids (VS): 99.3 % DM (range 99.3–99.3)
Nitrogen: 0.6 % DM
C/N ratio: 83.4

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 40.1 m³ CH₄/t VS (SD 17.9; 3 records, 3 sources).
Almond 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

combines well
substrates: high alkalinity (buffer) — e.g. Cattle manure, Pig slurry, Cattle slurry, Duck slurry, Rumen and digestive tract content
Slurry supplies N and alkalinity, balancing the high C/N of the material.^[1][2]
combines well
substrates: protein-rich — e.g. Food waste, Municipal sewage sludge (mixed), Poultry manure, Organic fraction of municipal solid waste, Grass clippings
The high C/N of the material balances protein substrates, limiting the ammonia risk.^[1][3]

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

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

What it is and where it comes from

Almond shells are the hard coverings of the nuts, removed in almond shelling and processing plants. The largest almond producers are the United States, Spain and other countries of the Mediterranean basin. In Poland almonds are imported, and shells arise only to a small extent. As a waste they are used as fuel, bedding or a material for producing activated carbon.

Properties relevant to digestion

The shells are a lignocellulosic material with a very high content of lignin and cellulose and a low content of simple sugars, protein and fat. The hard structure greatly limits enzyme access, so without size reduction and pretreatment most of the material remains undigested. There is no risk of acidification, but there is also no rapid energy yield. The high C/N ratio requires supplementation with nitrogen from slurry. The shells are dry and durable, which makes them easy to store but difficult to mix with a liquid feed, because they are dense and sink.

How to read the spread of results

The dataset contains several records from several sources, including records at 37 °C and under thermophilic conditions at 55 °C. The spread of results is large. It results from particle size (coarse or ground), the almond variety, the duration of the test, which for woody material must be long, and from whether the material was pretreated. Untreated samples were used here, which means low availability of the lignin. The inoculum also has an effect.

In biogas plant practice

Almond shells are rarely used in biogas plants, because they are worth more as a solid fuel. However, if they do go into the digester, they must be ground and their share in the feed kept small. Pretreatment (steam, alkaline) can improve the result, but raises costs. The material stores well. Waste from food processing is subject to waste legislation or, when sold as a by-product, to separate rules.

In an economic balance, almond shells almost always lose out to use as a solid fuel, because their calorific value is high and their biogas potential low. For this reason, BMP test data serve more for comparing lignocellulosic materials than for planning the feed.

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

All sources of the site: bibliography.