BMP testingEnergy crops and whole-crop plants (WCS)

Napier grass — BMP: 35 studies, weighted mean 253 m³ CH₄/t VS

Elephant grass (napier) is a tropical grass with a very high biomass yield, studied as an energy crop. The dataset covers different growth stages, leaf fractions and silage.

rating: good CH₄ ≈ 54 % (estimate)

Published 24 September 2020Updated 29 October 20263 min readFeedstockData Editorial Team

Photo Salil Kumar Mukherjee / Wikimedia Commons, licence CC BY-SA 4.0, source file; changes: cropping

Weighted mean: 253m³ CH₄/t VS
Range: 98–372m³ CH₄/t VS
Records: 35studies
Publications: 6sources
Temperatures: 35 °C, 37 °Cof the test

Methane in biogas

≈52–55% CH₄

Medium methane content · ≈ 5.3 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 2.1 % 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

good — editorial rating for biogas plantsStable, usually accepted without a fee

Potential per tonne of fresh matter: high (83 m³ CH₄/t); no particular threats to stability. Handling costs: fibres, floating layer.

Why? Rating factors

Potential per tonne +2

+2potential 83 m³ CH₄/t fresh matter (BMP 253 × DM 37.8 % × VS 87.2 %)

Process stability 0

0no factors

Handling and costs −0.5

−0.5fibres, floating layer — shredding and intensive mixing

Total points: 1.5 · thresholds: ≥ 2.5 very good, ≥ 1 good, below — neutral; negative with cumulative process risk.

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

Add to the co-digestion calculator CSV data

BMP curves from 35 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 (253); 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.

Show the remaining records in the legend (23)

Records and sources

Download CSV

Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Tong et al. (1990) DOI	288	35 °C · mesophilic	Napier grass · pretreatment: air dried	end point (model curve)
Chynoweth et al. (1993) · 1 DOI	191	35 °C · mesophilic	Napier grass · pretreatment: none (sample designation in the source: R2)	end point (model curve)
Chynoweth et al. (1993) · 2 DOI	221	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 3 DOI	231	35 °C · mesophilic	Napier grass · pretreatment: Dried to 20% moisture R9	end point (model curve)
Chynoweth et al. (1993) · 4 DOI	238	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 5 DOI	247	35 °C · mesophilic	Napier grass · pretreatment: Dried to 20% moisture R5	end point (model curve)
Chynoweth et al. (1993) · 6 DOI	248	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 7 DOI	249	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 8 DOI	251	35 °C · mesophilic	Napier grass · pretreatment: Dried 40% moisture -R8	end point (model curve)
Chynoweth et al. (1993) · 9 DOI	255	35 °C · mesophilic	Napier grass · pretreatment: Dried 40% moisture -R3	end point (model curve)
Chynoweth et al. (1993) · 10 DOI	257	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 11 DOI	258	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 12 DOI	260	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 13 DOI	264	35 °C · mesophilic	Napier grass · pretreatment: Ensiled	end point (model curve)
Chynoweth et al. (1993) · 14 DOI	274	35 °C · mesophilic	Napier grass · pretreatment: none (sample designation in the source: R3)	end point (model curve)
Chynoweth et al. (1993) · 15 DOI	291	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 16 DOI	294	35 °C · mesophilic	Napier grass	end point (model curve)
Chynoweth et al. (1993) · 17 DOI	310	35 °C · mesophilic	Napier grass · pretreatment: Ensiled	end point (model curve)
Dahunsi (2019) · 1 DOI	200	37 °C · mesophilic	grass silage Pennisetum puroureum	end point (model curve)
Dahunsi (2019) · 2 DOI	202	37 °C · mesophilic	grass silage Pennisetum puroureum	end point (model curve)
Gunaseelan (2004) · 1 DOI	342	35 °C · mesophilic	NapierGrass Sheath	end point (model curve)
Gunaseelan (2004) · 2 DOI	372	35 °C · mesophilic	NapierGrass Lamina	end point (model curve)
Kouas et al. (2017) DOI	255	35 °C · mesophilic	napier grass	end point (model curve)
Surendra i Khanal (2015) · 1 DOI	98	37 °C · mesophilic	Napier Grass growth stage 8	end point (model curve)
Surendra i Khanal (2015) · 2 DOI	103	37 °C · mesophilic	Napier Grass growth stage 8	end point (model curve)
Surendra i Khanal (2015) · 3 DOI	112	37 °C · mesophilic	Napier Grass growth stage 8	end point (model curve)
Surendra i Khanal (2015) · 4 DOI	121	37 °C · mesophilic	Napier Grass growth stage 6	end point (model curve)
Surendra i Khanal (2015) · 5 DOI	123	37 °C · mesophilic	Napier Grass growth stage 6	end point (model curve)
Surendra i Khanal (2015) · 6 DOI	150	37 °C · mesophilic	Napier Grass growth stage 6	end point (model curve)
Surendra i Khanal (2015) · 7 DOI	181	37 °C · mesophilic	Napier Grass growth stage 4	end point (model curve)
Surendra i Khanal (2015) · 8 DOI	189	37 °C · mesophilic	Napier Grass growth stage 4	end point (model curve)
Surendra i Khanal (2015) · 9 DOI	203	37 °C · mesophilic	Napier Grass growth stage 2	end point (model curve)
Surendra i Khanal (2015) · 10 DOI	217	37 °C · mesophilic	Napier Grass growth stage 4	end point (model curve)
Surendra i Khanal (2015) · 11 DOI	218	37 °C · mesophilic	Napier Grass growth stage 2	end point (model curve)
Surendra i Khanal (2015) · 12 DOI	236	37 °C · mesophilic	Napier Grass growth stage 2	end point (model curve)
Weighted mean	253	arithmetic mean 227, SD 65

Typical composition

Dry matter (DM): 37.8% (range 16.1–81.5)
Volatile solids (VS): 87.2 % DM (range 80.2–94.9)
Nitrogen: 2.1 % DM
C/N ratio: 16.4

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: CROPGEN D4, MY-D); dry matter: 8 determinations, volatile solids: 9 determinations. C/N was calculated as the ratio of C and N content (% DM) from the same source. The DM range includes fresh samples as well as samples partly dried or dried before the test – for fresh material the lower part of the range applies, ideally your own measurement.

Sources: Tong et al. (1990); Dahunsi (2019); Gunaseelan (2004); Surendra et al. (2015).

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

seasonal
fibrous
editorial classification

Advantages

Methane potential in the database: weighted mean 253 m³ CH₄/t VS (median 238; range 98–372; 35 records from 6 sources).
The best-described tropical grass (35 records): Surendra 2015 shows that yield falls with maturity (2-month-old 219 vs 8-month-old 104 m³/t VS) and that a 6 mm sieve gives higher yields than 10 and 20 mm.

Drawbacks and risks

Late harvest drastically lowers yield (Surendra 2015).

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

combines well
group: Animal excreta (slurry, manure, poultry litter)
Co-digestion with excreta (slurry/solid manure) is the standard arrangement for plant biomass: they supply alkalinity and microflora.^[1][2]

Dosing

No source-based dose limits.

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

Elephant grass (napier grass, Pennisetum purpureum, now also Cenchrus purpureus) is a perennial tropical grass reaching several metres in height. It is grown as fodder in Africa, Asia and America, and in biogas research it is regarded as one of the highest-yielding energy crops. The dataset contains several dozen records from several studies, including an American programme from the 1990s, a study describing successive growth stages and fractions (leaf blade and leaf sheath) and silage, under conditions of 35 to 37 °C.

Properties relevant to digestion

The biomass of elephant grass is very variable depending on age: young shoots are succulent and easily digestible, and mature ones lignified and rich in lignin. An increase in fibre content reduces the rate of hydrolysis. The protein content is low, so the C/N ratio is high. The green biomass has a high moisture content and spoils quickly, so ensiling is used. It is a C4 grass with a lot of silica in the leaves, which increases the abrasiveness for equipment. Chopping is essential, and mixing with slurry improves the nitrogen balance.

How to read the spread of results

The greatest influence is the growth stage at harvest, visible in the study that compares successive stages. Further influences are the fraction (blades, sheaths, whole plant), the form of the material (fresh matter, silage), variety, fertilisation and climate. Some records come from one series, so they are not independent. Added to this are the test parameters: inoculum, substrate-to-inoculum ratio and incubation length. A single result therefore refers to a specific stage or fraction, and not to the whole category.

In biogas plant practice

In Polish conditions elephant grass is not available as a local substrate, because it does not overwinter. It is an important reference point for other energy grasses and a substrate in tropical regions, where its yields allow several cuts per year. It is harvested at the stage before full lignification, chopped and ensiled. In the ration, like other grasses, it requires mixing and nitrogen supplementation. The basis for a decision should be a BMP test with a description of the harvest stage.

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 = 0.085 d⁻¹ was assumed (median of rate constants k reported in the sources and/or fitted to the mean measured curves).

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

Results after pretreatment (4) — excluded from the mean

Source	BMP m³ CH₄/t VS	Pretreatment	Sample
Dahunsi (2019)	312	physical	grass silage Pennisetum puroureum
Dahunsi (2019)	320	physical	grass silage Pennisetum puroureum
Dahunsi (2019)	387	physical	grass silage Pennisetum puroureum
Dahunsi (2019)	366	physical	grass silage Pennisetum puroureum

Bibliography

All sources of the site: bibliography.