• Energy Research

Hyperspectral imaging and multivariate image regression can replace expensive and time-consuming laboratory analyses of biomass and hydrochar polysaccharides.

Differences in ash behavior during hydrothermal treatment were identified based on multivariate data analysis of literature information on 29 different feedstock. In addition, the solubility of individual elements was evaluated based on a smaller data set. As a result two different groups were distinguished based on char ash content and ash yield. Virgin terrestrial and aquatic biomass, such as different types of wood and algae, in addition to herbaceous and agricultural biomass, bark, brewer's spent grain, compost and faecal waste showed lower char ash content than municipal solid wastes, anaerobic digestion residues and municipal and industrial sludge. Lower char ash content also correlated with lower ash yield indicating differences in chemical composition and ash solubility. Further evaluation of available data showed that ash in industrial sludge mainly contained anthropogenic Al, Fe and P or Ca and Si with low solubility during hydrothermal treatment. Char from corn stover, miscanthus, switch grass, rice hulls, olive, artichoke and orange wastes and empty fruit bunch had generally higher contents of K, Mg, S and Si than industrial sludge although differences existed within the group. In the future information on ash behavior should be used for enhancing the fuel properties of char based on feedstock type and hydrothermal treatment conditions.

The experimental design was used to determine the effects of the reaction temperature (180–260 °C), the reaction time (0–2 h) and the sawdust concentration (9.1–25 wt%) on the biomass conversion, the product yields and product properties during the hydrothermal liquefaction of pine sawdust. The determined conversion and the aqueous product (AP) yield were in the range of 23.1–57.2 wt% and 14.6–43.4 wt%, respectively. The pH of the slurry product was 2.54–3.44, while the higher heating values (HHVs) of the attained solid residue (SR) and the heavy oil (HO) were in the range of 21.3–28.3 MJ/kg and 21.6–29.4 MJ/kg, respectively. Comprehensive mass and carbon balance model predictions were built. They can be used for predicting the appropriate reaction condition to obtain the desired HTL product. The results showed that all experimental variables had a statistically significant effect on the conversion and the liquid product yields. An increase in the reaction temperature improved the conversion, the liquid product yields (except for the carbon yield of AP), the acidity of the slurry product and HHVs of SR and HO. The reduction of the sawdust concentration led to an increase in the conversion and the liquid product yields, while a very short reaction time favored the liquid product formation. The lignocellulosic degradation products in AP were qualitatively analyzed by means of the GC-MS analysis. The kind of furan, cyclic and one oxygen-containing species produced in AP depends on the reaction temperature and the reaction time.

Hydrothermal carbonization (HTC) of pure cellulose (CE) and birchwood (BW) samples was carried out at temperatures between 160 and 280 °C, 0.5 h residence time and biomass-to-water ratio 1:5, to investigate the reactivity of cellulose in lignocellulosic biomass. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) showed that the CE samples remained unaltered at temperatures up to 220 °C, but were significantly decomposed at 230 °C producing a thermal recalcitrant aromatic and high energy-dense material. FTIR showed that dehydration and aromatization reactions occurred at temperature equal or higher than 230 °C for the CE samples while a similar increase in aromatization for the BW hydrochars was evident only at temperatures equal or higher than 260 °C. Acid hydrolysis, TGA and FTIR suggested that a higher thermal resistance of naturally occurring cellulose in BW (when compared to CE sample) could be related to a ‘protecting shield’ offered by interlinked lignin in the plant matrix.

A face centered central composite design was used for determining the effects of reaction temperature (180-260°C), reaction time (0-2 h) and sawdust concentration (9.1-25 wt%) on conversion and recovered product yields during hydrothermal liquefaction of pine sawdust. The determined conversion and aqueous product (AP) yield were in the range 23.1-57.2% and 14.6-43.4%, respectively. The results showed that all linear model terms and some interaction terms were statistically significant for sawdust conversion and liquid product yields, whereas all quadratic terms were found statistically insignificant. In general, increasing reaction temperature increased sawdust conversion and AP formation, while increasing sawdust concentration led to a reduction in conversion and liquid product yields. The determined model revealed that the decomposition reactions of lignocellulosic constituents may have occurred competitively, thus resulting in a certain trend in recovered AP and heavy oil (HO) yields.

To address the growing need for economically viable sludge processing technologies and associated resource recovery, a novel convective high-velocity pilot cyclone was set-up at the Swedish University of Agricultural Sciences in Umeå, Sweden. In essence this process entails feeding (≤900kgh-1) a material to a heated high-velocity air flow (approx. 13×103m3h-1) allowing moisture removal at low temperatures. This equipment is expected to improve drying of challenging industrial sludge materials when used as a pretreatment method prior to further processing. Modeling results based on an experimental campaign on recycled paper mill sludge indicated that efficient drying with a specific energy consumption of ≤1-1.2kWhkg-1H2O can be achieved with inlet air temperature levels ≥40°C coupled with respective feeding capacity of ≤750kgh-1.

The generation of biomass ash (BA) is expected to increase in the future because biomass is generally recognized as carbon neutral fuel. Since BA is known, in many cases, to contain hazardous concentrations of trace elements, this will simultaneously produce more potentially hazardous ash. Soon this will need to be handled amidst stricter waste policies and a societal evolution towards a circular economy. In many cases, to allow recycling of BA, trace elements need to be removed for the protection of health and the environment. A better understanding of trace element origins in BA, and knowledge of which trace elements are most critical to advantageous recycling schemes are also needed. In this work available BA data were reviewed and processed for study by multivariate statistical analyses. This allowed for reorganization of the complex nature of BA data into simpler forms for interpretation. An established connection between peat fuels and As was thoroughly reinforced. Wood co-fired with peat would produce BA most advantageous for any recycling, while other biomass for forest recycling and wood, bark and wood waste split between forest recycling and needing treatment or disposal. Some trace elements were still an obstacle to the recycling schemes presented, therefore current state of the art ash treatments for targeting individual trace elements or for total treatment of ash were discussed. Additionally, treatment methods for ash were reviewed because their options are varied, and the goal of utilization will be achieved through matching treatment methods and recycling targets. In the future, the method of using legislative limit values and multivariate analyses to determine BA recycling routes could be replicated for other national limit values and other wastes.

Experimental design and response surface methodology are useful tools for studying, developing and optimizing a wide range of engineering systems. This tutorial provides a summary and discussion on their use in energy applications. The theory and relevant calculations are clearly presented and discussed along with model diagnostics and interpretation. This is followed by a review of recent reports within the energy field. Overall, this contribution will clarify many aspects of experimental design and response surface methodology that are often confusingly discussed in the academic literature and summarizes relevant applications where they have been found useful.