A look at grass as a dedicated energy crop for biogas

The British Department of Trade and Industry (DTI) has released an interesting report on the potential of using common rye grass as a dedicated energy crop for the production of biogas. We report on it here, because the results offer clues for a future scenario of large-scale grass-based biogas production in the tropics, where biomass productivities are considerably higher. Below, we make a very rudimentary comparison based on the results of the DTI study.

In Europe, biogas is being made more and more often from energy crops that are used as a single substrate, instead of manure which is traditionally used. Several research efforts and trials are underway, analysing the potential of specially bred biogas maize, exotic grass species such as Sudan grass and sorghum, or new hybrid grass types.

Compared to making liquid biofuels, biogas has the advantage that the entire crop can be utilized and not merely the starch-, sugar- or oil-rich parts which is the case with first-generation ethanol or biodiesel production. A biomethane digester can ferment a much wider range of biomass sources. This is why the green gas has a large potential (as was recently illustrated by a report showing that biogas can replace all Russian gas imports in Europe - previous post).

Once the biogas is produced, it can be used either directly in gas engines and generators or in more efficient cogeneration plants for the production of power and heat. It can be purified to natural gas grade standards, after which it can be fed into the NG grid and utilized like ordinary fossil methane, by households, industries, or in cars and fuel cells (and here). As an automotive fuel, used in CNG-capable vehicles, biogas has the highest well-to-wheel efficiency and the lowest carbon dioxide footprint of all biofuels (earlier post).

The DTI report compares the energy balance of the most efficient ethanol and biodiesel production paths using UK crops, with that of biogas based on grass. Results of this comparison can be found in the table. The same ratios, we think, are roughly valid for tropical crops. Sugarcane ethanol's current energy balance is around 1 to 8. If the grass crop were to be used for the production of biogas, it would be more positive still.

This is why we see the large-scale production of biogas in the tropics and subtropics as a promising bioenergy sector, because the technology is well understood and already has a foot on the ground in most developing countries (on a micro-scale, at the household level); it yields more energy per hectare than liquid biofuels, which implies a better use of resources and less land needed; and the variety of suitable feedstocks is much larger.

The DTI report analysing rye grass as a dedicated energy crop set out the following set of basic objectives:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: biogas :: rye grass :: sugarcane :: energy balance :: methane :: UK ::

• to achieve a minimum yield of 4060 cubic meters of methane per hectare per year, which, when converted to electricity on a commercial scale would generate 14MWh per hectare per annum
• to establish the relationship between the biogas yield and the harvesting cycle
• to confirm that through storage of the grass, it is possible to achieve a constant yield of biomethane throughout the year
• to assess the mass balance and energy balance of the entire process
• to estimate th economics of a commercial grass-to-biogas plant

Several experimental plots were used to grow the rye grass, and different harvesting schemes were implemented (cutting the grass in cycles of two, three, four, six and eight weeks). After each harvest, the organic fertilizer - a byproduct from biogas production - was applied to the plots to measure the efficiency of this fertiliser.

Soil analyses were carried out to measure the effects of nutrient depletion, as well as the effect of different fertilizer regimes on grass productivity.

Analysis of the application of digestate - the 'biofertilizer' from the fermentation of the grass - showed positive results: the biogas digestate considerably increases biomass productivity of rye grass.

Grass yields
In the 2003 trials (see table), the grass showed a high average dry matter yield of 8.6MT per hectare (when the grass was cut at 50mm height) and a low average yield of 6.8 MT of dry matter per hectare (cut at 100mm). The highest yield in a single plot was 11.1MT.

In trials held a year later (see table), the results were lower, with averages of 6.7MT and 5.1MT respectively and a maximum yield of 12.7MT/dry matter/ha.

It is here that we can already draw an extremely basic point of comparison with the situation in the (humid) tropics. There, biomass productivity is much higher and the yield of grass species like sugarcane and sudan grass easily reaches 28MT/dry matter/ha per year on average (85MT/wet weight), or roughly three to four times the productivity of rye grass in the UK. Sheer biomass productivity is the single biggest factor determining the final biogas yield and energy balance of the biofuel production system.

Small digestion trials and methane yields
Two very basic and small (0.3 cubic meter and 1.5 cubic meter) anaerobic digesters were used to analyse the methane yield of the grass substrate. They were run continuously and fed daily.

Two types of feedstock were used: freshly cut grass and silaged grass. The overall average yield for silage was 342m³ of methane per ton of dry matter, whereas for fresh grass it was lower at 229m³.

There were large yield differences during the trials, with low monthly yields of 134m³ to maxima of 429m³.

The methane content of the biogas varied, with most months showing consistent CH4 contents of over 50%, while some dropped below that level and reached 40%.

An additional set of factors was analysed for their effects on methane yields, such as the retention time of the substrate, the effect of temperature changes and stabilisation rates (the time it takes for the biogas production to become 'consistent').

The most important conclusion of these trials was that ensiled grass clearly yields higher amounts of methane in the digester. This is good news, because it means more efficient management of feedstocks becomes possible. Grass can be harvested and stored, and then continuously feed a digester of a particular size, using optimal quantities of the feedstock for that size. This would be impossible if the digester (with its fixed scale) were to be fed fresh grass, the yield of which varies greatly per (bi-weekly or monthly) harvest.

Large-scale trials
In the second phase of the project, a large grass plot was established the biomass of which was used to feed a 20 cubic meter digester. Again, both silage and fresh gas was fed and methane yields compared.

The digester consisted of a reception tank for preparing the feedstock, a storage tank for the digestate and bell-over-water gas holder.

The feedstock was prepared as a liquid slurry using the recirculated digestate.

After first trials and modifications to the design of the digester, a very efficient plant was build that resulted in a consistent methane yield of 250 cubic meters per ton of dry matter.

Economic analysis and comparison with tropical feedstock
An economic analysis based on a system utilizing the biomass from 100 hectares of rye grass, showed that, despite promising biomass and methane yields, a large commercial biogas production system utilizing the grass as a single substrate would not be commercially viable.
Three scenarios were created each with different added value streams: (1) a system in which only the value of direct electricity production is taken into account (Case A1), (2) one in which biofertiliser as the byproduct from the digestion is given a commercial value (Case A2) and (3) one in which both the byproduct and useful excess heat is sold (Case A3).

None of these scenarios proved commercially viable. (It must be said that values for the electricity and heat are based on commercial prices as they stood at the time of the creation of the report - in late 2005 - meanwhile, they have increased considerably as all fossil fuel prices have risen.)

The basic table below shows costs versus income (some entries in the table reading '0' are the result of a later comparison of the pure rye grass system with one in which pig manure is added).

We want to re-write this table and include some guesstimated numbers for tropical biomass feedstocks, using sugarcane and its average yields (80 tons of dry matter per hectare per year) in Brazil as the grass substrate. Research on utilizing sugarcane as a single substrate for biogas is scarce. Some studies point to a considerably higher methane yield than that of rye grass (342m³/ton), but we limit it here to 350m³ of methane per ton.

For a more detailed calculation of these yields and ratios, see appendix 5 [*.pdf] of the DTI report.

Land prices in the developing world differ considerably from those in the UK (see our previous data on land prices in Africa), but we take an average of US$200/ha (£100 using the exchange rate at the time the study was produced) versus the £150/ha used in the DTI analysis.

We keep the production costs for both crops equal; labor costs are assumed to be half of those in the UK. Finally, costs for heating the digester (a factor falling under 'operating costs') reduced by a third because of higher and more consistent ambient temperatures in the tropics.


The table then looks as follows:

Concluding, we can say that biogas production from a single grass substrate in the UK will not be viable without subsidies. Given far higher biomass (and biogas and biofertiliser) yields of a tropical energy crop like sugarcane, large-scale biogas production based on such crops may be viable.

In Europe, a lot of research is being undertaken in this field, and in contrast with the rye grass study, some analyses do show that biogas from dedicated energy crops can be competitive at current market prices for energy. A recent PhD dissertation by Annimari Lehtomäki, which compared different potential biogas crops, showed that at least for specially bred maize varieties, large scale production is feasible and commercially viable.

Similar research on the potential of tropical energy crops as dedicated biogas feedstocks is scarce and would be very welcome.

More information:
DTI: Rye grass as an energy crop using biogas technology - page with links to the documents and appendices.
DTI: Rye grass as an energy crop using biogas technology - Main Report [*.pdf]
DTI: Rye grass as an energy crop using biogas technology - Appendix 5 [*.pdf]
Annimari Lehtomäki, Biogas production from energy crops and residues [*.pdf], Jyväskylä Studies in Biological and Environmental Science, PhD thesis, Jyväskylä University, Finland, 2006