BMP testingEnergy crops and whole-crop plants (WCS)single study (18 samples)

Pennisetum hybrid grass — BMP: 18 studies, weighted mean 266 m³ CH₄/t VS

Hybrid pennisetum (a P. americanum × P. purpureum hybrid) is a tropical grass with a very high yield, studied in China. The records concern the fractions of stems, leaves and the whole plant.

rating: neutral CH₄ ≈ 52 % (estimate)

Published 15 July 2020Updated 29 October 20263 min readFeedstockData Editorial Team

Photo John Robert McPherson / Wikimedia Commons, licence CC BY-SA 4.0, source file; changes: cropping

Weighted mean: 266m³ CH₄/t VS
Range: 194–361m³ CH₄/t VS
Records: 18single study (18 samples)
Publications: 1source
Temperatures: 35 °Cof the test

Methane in biogas

≈51–53% CH₄

Medium 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 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

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

Potential per tonne of fresh matter: moderate (40 m³ CH₄/t); no particular threats to stability. Handling costs: fibres, floating layer.

Why? Rating factors

Potential per tonne +1

+1potential 40 m³ CH₄/t fresh matter (BMP 266 × DM 17.5 % × VS 85.2 %)

Process stability 0

0no factors

Handling and costs −0.5

−0.5fibres, floating layer — shredding and intensive mixing

Total points: 0.5 · 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 18 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 (266); 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 (6)

Records and sources

Download CSV

Source	BMP m³ CH₄/t VS	Temperature	Sample	Curve
Zhang et al. (2019) · 1 DOI	194	35 °C · mesophilic	Pennisetum hybrid fresh grass stem	end point (model curve)
Zhang et al. (2019) · 2 DOI	200	35 °C · mesophilic	Pennisetum hybrid fresh grass leaf	end point (model curve)
Zhang et al. (2019) · 3 DOI	226	35 °C · mesophilic	Pennisetum hybrid fresh grass whole	end point (model curve)
Zhang et al. (2019) · 4 DOI	233	35 °C · mesophilic	Pennisetum hybrid fresh grass stem	end point (model curve)
Zhang et al. (2019) · 5 DOI	236	35 °C · mesophilic	Pennisetum hybrid fresh grass whole	end point (model curve)
Zhang et al. (2019) · 6 DOI	238	35 °C · mesophilic	Pennisetum hybrid fresh grass whole	end point (model curve)
Zhang et al. (2019) · 7 DOI	241	35 °C · mesophilic	Pennisetum hybrid fresh grass leaf	end point (model curve)
Zhang et al. (2019) · 8 DOI	256	35 °C · mesophilic	Pennisetum hybrid grass silage leaf	end point (model curve)
Zhang et al. (2019) · 9 DOI	258	35 °C · mesophilic	Pennisetum hybrid fresh grass leaf	end point (model curve)
Zhang et al. (2019) · 10 DOI	263	35 °C · mesophilic	Pennisetum hybrid grass silage leaf	end point (model curve)
Zhang et al. (2019) · 11 DOI	270	35 °C · mesophilic	Pennisetum hybrid grass silage whole	end point (model curve)
Zhang et al. (2019) · 12 DOI	276	35 °C · mesophilic	Pennisetum hybrid grass silage whole	end point (model curve)
Zhang et al. (2019) · 13 DOI	277	35 °C · mesophilic	Pennisetum hybrid grass silage stem	end point (model curve)
Zhang et al. (2019) · 14 DOI	284	35 °C · mesophilic	Pennisetum hybrid fresh grass stem	end point (model curve)
Zhang et al. (2019) · 15 DOI	298	35 °C · mesophilic	Pennisetum hybrid grass silage stem	end point (model curve)
Zhang et al. (2019) · 16 DOI	316	35 °C · mesophilic	Pennisetum hybrid grass silage leaf	end point (model curve)
Zhang et al. (2019) · 17 DOI	356	35 °C · mesophilic	Pennisetum hybrid grass silage whole	end point (model curve)
Zhang et al. (2019) · 18 DOI	361	35 °C · mesophilic	Pennisetum hybrid grass silage stem	end point (model curve)
Weighted mean	266	arithmetic mean 266, SD 46

Typical composition

Dry matter (DM): 17.5% (range 10.6–28.5)
Volatile solids (VS): 85.2 % DM (range 80.4–92)
Nitrogen: 1 % DM
C/N ratio: 42.3

Median and range of the determinations reported in the publications behind this substrate's BMP records (read from: MY-D); dry matter: 18 determinations, volatile solids: 17 determinations. C/N was calculated as the ratio of C and N content (% DM) from the same source.

Sources: Zhang et al. (2019).

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 266 m³ CH₄/t VS (median 261; range 194–361; 18 records from 1 sources).
The hybrid P. americanum x P. purpureum studied with regard to plant height and ensiling (Zhang 2019).

Drawbacks and risks

Yield depends on the height of the plant at harvest (Zhang 2019).

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

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

Hybrid pennisetum is a hybrid of pearl millet (Pennisetum americanum, today also Cenchrus americanus) and elephant grass (P. purpureum). It was bred as a plant with very high biomass production and is grown in the warm zone, including in China. The compilation covers several records from one publication in which fresh matter was examined in different fractions: stems, leaves and the whole plant, in several variants of the experiment.

Properties relevant to digestion

The biomass of this hybrid consists mainly of stems rich in cellulose and hemicelluloses, while the leaves are more digestible. The lignin share rises with the age of the plant, which slows hydrolysis. The green mass is moist and spoils quickly, so it must be ensiled quickly or fed to the digester. Typical of C4 grasses are low protein and a high C/N ratio, so it is beneficial to combine it with a nitrogen-rich substrate. The fibre requires size reduction, because without it a floating crust forms. The plant does not tolerate frost, which limits cultivation.

How to read the spread of results

All records come from one study, so the spread does not include differences between laboratories, only between fractions and variants within the experiment. Stems and leaves have different digestibility, and their proportion in the whole plant changes with age. The harvest date, fertilisation, cutting height and sample preparation also have an effect. The description of the records refers to fresh matter, so the results should not be applied directly to silage. Test parameters, such as the inoculum and the S/I ratio, are common here to the whole series.

In biogas plant practice

Under Polish conditions hybrid pennisetum is not available as a local substrate. In countries where it is grown it is used in cyclical harvests, and the biomass is ensiled and mixed with other substrates. Feeding requires size reduction and good mixing. Because all the data come from one study, conclusions about behaviour in other conditions must be formulated cautiously and confirmed with a BMP test on local material. In interpreting the fraction results it must be remembered that in practice a biogas plant receives the whole plant, not stems and leaves separately, so the record for the whole plant is the closest to a real feed. The fraction records show rather where the difference in digestibility within a single plant comes from.

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

Bibliography

Zhang, Y.; Li, L.; Kang, X.; Sun, Y.; Yuan, Z.; Xing, T.; Lin, R. (2019). Improving methane production from Pennisetum hybrid by monitoring plant height and ensiling pretreatment. Renewable Energy 141, 57–63. DOI: 10.1016/j.renene.2019.03.084 DOI

All sources of the site: bibliography.