Effects of functional groups and species richness on biomass constituents relevant for combustion: results from a grassland diversity experiment

Abstract

Grassland biomass has been identified as a potential energy source. The combustion of mature and fibrous biomass, as occurs in extensive grasslands managed with low cutting frequencies, is one possible conversion technique. This study tested the relationship between plant diversity and biomass constituents relevant for combustion, as they determine energy content, energy yield and emission and corrosion risks. The biomass from a biodiversity experiment, with a species richness (SR) gradient of 1–60 species from Central European mesophilic grasslands divided into four functional groups (grasses, legumes, small and tall herbs), was harvested twice a year (in 2008 and 2009). The higher heating value (HHV) was estimated from carbon, hydrogen and oxygen contents to give insight into the energy potential of the species mixtures. The potential risk of emission and corrosion was assessed by analysing ash content, potassium, calcium, magnesium, nitrogen, sulphur and chloride content. HHV was independent of SR, and the overall mean was 18·13 MJ kg−1 DM. Biomass and gross energy (GE) yield were positively affected by SR. The presence of legumes in a mixture resulted in increased HHV, biomass yield and GE, irrespective of the level of SR. Annual GE varied between 59 (average of monocultures) and 152 GJ ha−1 year−1 (mixture of sixty species). The concentration of ash‐forming elements was generally high, suggesting a pre‐treatment of the biomass prior to combustion. Emission‐ and corrosion‐related constituents were clearly affected by the different functional groups, and sulphur and nitrogen both declined with SR. The results of this study show that high SR in experimental grassland communities is beneficial for the energy output and that legumes play a key role for the energy potential. However, identifying a functional group as being solely beneficial or disadvantageous for fuel quality was difficult.

Effects of species richness and functional groups on chemical constituents relevant for methane yields from anaerobic digestion: results from a grassland diversity experiment

“…If this were true, it would be reflected, among other factors, in the dominance structure of the plots; however, Khalsa et al . () showed no notable change in dominance structure with increasing SR. Furthermore, the independence of SR from functional‐group presence/absence could be proven by fitting each functional‐group term individually before SR in the hierarchy of the statistical model.…”

“…Calculated from the data presented in Khalsa et al . (), the C/N ratio of the all functional‐group mixtures (first cut: 27·6; second cut: 24·5) was near the optimum.…”

“…Crude protein, which was included as it is a major determinant of the CH 4 sub (even though it was previously published as nitrogen, N = CP/6·25) in Khalsa et al . ()), ranged from 60 to 228 g kg −1 DM with a mean of 115 g kg −1 DM. Differences between the two cuts were small, so that similar effects were found in the statistical analysis (Table ).…”

“…In general, area‐specific energy yield ( E area ) is calculated as the product of biomass yield and substrate‐specific energy yield ( E sub ) and is important for assessing the potential value of a grassland site for bioenergy production. In a study on solid‐fuel quality of grassland biomass for combustion and the effect of SR on E area , it was shown that E area increased with SR due to a biomass yield increase, while E sub remained constant (Khalsa et al ., ).…”

Foliage and Grass as Fuel Pellets–Small Scale Combustion of Washed and Mechanically Leached Biomass

“…Khalsa et al [32] found grass from extensive management to have an ash content ranging between 5.3% and 22% DM, depending on plant species composition and harvest date, with an average of 9.4% DM. Piepenschneider et al [6] observed the mean ash content in untreated urban leaf litter to be at 13.8% DM, while the range was between 6.7% and 19.4% DM, depending on tree species and soil contamination.…”