• Energy Research

The carbonization of biomass residuals to char has strong potential to become an environmentally sound conversion process for the production of a wide variety of products. In addition to its traditional use for the production of charcoal and other energy vectors, pyrolysis can produce products for environmental, catalytic, electronic and agricultural applications. As an alternative to dry pyrolysis, the wet pyrolysis process, also known as hydrothermal carbonization, opens up the field of potential feedstocks for char production to a range of nontraditional renewable and plentiful wet agricultural residues and municipal wastes. Its chemistry offers huge potential to influence product characteristics on demand, and produce designer carbon materials. Future uses of these hydrochars may range from innovative materials to soil amelioration, nutrient conservation via intelligent waste stream management and the increase of carbon stock in degraded soils.

AbstractOilseed rape is one of the leading feedstocks for biofuel production in Europe. The climate change mitigation effect of rape methyl ester (RME) is particularly challenged by the greenhouse gas (GHG) emissions during crop production, mainly as nitrous oxide (N2O) from soils. Oilseed rape requires high nitrogen fertilization and crop residues are rich in nitrogen, both potentially causing enhanced N2O emissions. However, GHG emissions of oilseed rape production are often estimated using emission factors that account for crop‐type specifics only with respect to crop residues. This meta‐analysis therefore aimed to assess annual N2O emissions from winter oilseed rape, to compare them to those of cereals and to explore the underlying reasons for differences. For the identification of the most important factors, linear mixed effects models were fitted with 43 N2O emission data points deriving from 12 different field sites. N2O emissions increased exponentially with N‐fertilization rates, but interyear and site‐specific variability were high and climate variables or soil parameters did not improve the prediction model. Annual N2O emissions from winter oilseed rape were 22% higher than those from winter cereals fertilized at the same rate. At a common fertilization rate of 200 kg N ha−1 yr−1, the mean fraction of fertilizer N that was lost as N2O‐N was 1.27% for oilseed rape compared to 1.04% for cereals. The risk of high yield‐scaled N2O emissions increased after a critical N surplus of about 80 kg N ha−1 yr−1. The difference in N2O emissions between oilseed rape and cereal cultivation was especially high after harvest due to the high N contents in oilseed rape's crop residues. However, annual N2O emissions of winter oilseed rape were still lower than predicted by the Stehfest and Bouwman model. Hence, the assignment of oilseed rape to the crop‐type classes of cereals or other crops should be reconsidered.

Hydrochars were prepared by hydrothermal carbonization (HTC) of maize silage previously treated at 55 °C in a two-stage solid-state reactor system. The HTC was carried out in a 1-L stirred pressure reactor with pH regulation by citric acid. The treated silage carbonized at relatively mild conditions (190 °C, 2 h), and the hydrochars showed mainly amorphous macro-size features with a carbon content of 59-79% (ash-free, dry) and a higher heating value of 25-36 MJ kg⁻¹. Temperature was the main influencing factor. The surface area according to Brunauer-Emmett-Teller (BET) analysis was highest at 190 °C (12.3 m²) g⁻¹). Based on these results, the hydrochars are potentially interesting for applications such as an alternative fuel or a soil conditioner.

Abstract This study was designed to consider all nitrogen fertilizer-related effects on crop production and emission of greenhouse gases on loamy sandy soils in Germany over a period of nine years (1999–2007). In order to set up a CO 2 balance for the production of energy crops, different nitrogen pathways were investigated, such as direct N 2 O emissions from the soil and indirect emissions related to NO 3 leaching and fertilizer production. Fluxes of N 2 O were measured in an experimental field using closed chambers. Poplar ( Populus maximowiczii × P. nigra ) and rye ( Secale cereale L.) as one perennial and one annual crop were fertilized at rates of 0 kg N ha −1 yr −1 , 75 kg N ha −1 yr −1 and 150 kg N ha −1 yr −1 . The mean N 2 O emissions from the soil ranged between 0.5 kg N ha −1 yr −1 and 2.5 kg N ha −1 yr −1 depending on fertilization rate, crop variety and year. The CO 2 fixed in the biomass of energy crops is reduced by up to 16% if direct N 2 O emissions from soil and indirect N 2 O emissions from NO 3 leaching and fertilizer production are included. Taking into account the main greenhouse gas emissions, which derive from the production and the use of N fertilizer, the growth of poplar and rye may replace the global warming potential of fossil fuels by up to 17.7 t CO 2 ha −1 yr −1 and 12.1 t CO 2 ha −1 yr −1 , respectively.