**Preface to "Bioenergy and Biochar: Repurposing Waste to Sustainable Energy and Materials"**

The following summary of the Special Issue papers was kindly prepared by Thomas R. Miles, Executive Director of the United States Biochar Initiative. This Special Issue on "Bioenergy and Biochar: Repurposing Waste to Sustainable Energy and Materials"comprises 11 papers that explore creative pathways to zero waste, greenhouse gas (GHG) reduction, and circular economies through recycling of nutrients, feedstock production on marginal land and natural grasslands, and conversion of agricultural and wood residues. Renewable, low-carbon products include coffee cup lids made from starch-based thermoplastic and coffee waste biochar, fuels and biochars from coconut husks and shells, biochar-amended manure pellets for rice cultivation, hydrochars from sewage sludge and food waste, ethanol from sugarcane grown on marginal land, mineral soil amendments from biomass boiler ash, lipids for biofuels from meat wastes, low greenhouse gas (GHG) ethanol from wood, fermentable sugars from semi-natural grasslands, and methane from microalgae-enhanced anaerobic digestion.

Diaz et al. produced biodegradable containers that can be degraded through processes such as composting or bio-digestion at the end of life to demonstrate closed-loop systems for organic waste. They used thermoplastic starch to replace traditional plastic and thermoformed it with polycaprolactone. They then used coffee waste biochars as fillers. The properties of the materials were tested to show that the coffee waste biochars could be reused. They conclude that starch and biochar can be used for manufacturing thermoformed containers. This is a continuation of ongoing research.

Two papers evaluate the suitability of coconut wastes as fuels and biochar. Obeng et al. in Ghana gasified coconut husks and shells to make green charcoal. They sun-dried coconut residues, which constitute 62–65% of the whole coconut fruit, and gasified them in a simple top-lit updraft gasifier, or TLUD. TLUDs are widely used to produce biochars for soil but have not been used on coconut shells and husks. Heating values of the char increased by 42% compared with the uncharred wastes. Emissions from the TLUD exceeded WHO standards but can be optimized through design. The authors recommend a switch from open burning to carbonization in a controlled system and briquetting to maximize calorific value and minimize smoke emissions in domestic cooking.

Coconut waste is abundant in 90 countries. In spite of the extensive production of biochar from coconut shell for charcoal and activated carbon, there is limited literature on coconut shell pyrolysis. Sarkar and Wang obtained coconut shells from Bangladesh and conducted detailed studies to determine the product yields and characteristics at increasing pyrolysis temperatures. The authors found that increasing temperature between 400 and 600 C resulted in important changes in yield and physical and chemical characteristics of the char, oil, and gas. This will be useful for those wanting to promote solid, liquid, or gaseous products from waste coconut shell.

Methods to use biochars to recycle manures and mitigate greenhouse gas emissions in rice cultivation were tested by Shin et al. in Korea. They tested pelletized biochar, manure, and animal waste compost amended fertilizers as environmentally safe application methods to mitigate non-source pollution and to reduce nutrient loss from drift and surface losses. Supplemented biochar manure pellets (SBMP) were made from 40% rice husk biochar combined with 60% composted pig manure. Urea, phosphate, and potassium chloride were added in various combinations and applied in a neutral clay loam soil. Paddy water quality showed that the SBMP can mitigate the loss of nitrogen and phosphorous. Silicon increased and nutrient release was slower. Carbon sequestration was measured, and the cost targets for GHG reduction were established. Authors conclude that the application of SBMP fertilizers can contribute to reducing the agro-environmental impacts of runoff and enhance sequestration and rice yield.

Vardiambasis et al. analyzed existing research on hydrothermal carbonization (HTC) using advanced techniques to determine the focus of research and to determine correlations between feedstock and product qualities. They reviewed publications between 2014 and 2020, which indicated sewage sludge and food waste to be the most popular feedstocks for HTC. They conducted a statistical analysis of the key properties to establish correlations to guide analysis. They developed a series of modules using artificial neural networks (ANN) and used the models to predict higher heating values (HHVs) from carbon and other fuel elements. The work is a fascinating review of HTC research. It demonstrates a series of useful tools for literature review with useful outputs.

Ash and fly ash were traditionally applied on agricultural or forest lands as a mineral ash supplement. Today, a large proportion of fly ash from biomass plants is landfilled. Land application in Europe has been restricted by regulations that are based on contaminants in coal fly ash. Bubbling and circulating fluidized bed boilers have become the predominant biomass technologies in Europe. Jarosz-Krzeminska and Poluszynska show through extensive analysis that feedstock supplies and combustion technologies have improved biomass fly ash. They examine the physical and chemical properties of the fly ash, micro- and macronutrients, contaminants, non-essential elements, and the bioavailability of elements. They investigated the speciation of metals and acute toxicity of fly ash amendments to plant germination and growth. They show that fly ash from bubbling and circulating fluidized bed boilers have different characteristics due to process conditions and feedstocks. Circulating fluidized beds (CFB) operate at higher temperatures and recirculate the fly ash. Bubbling fluidized bed (BFB) fly ash was richer in potassium, phosphorous, carbonates, and micronutrients than fly ash from CFBs. The BFBs also have fewer contaminants. They attribute some differences to mixtures of feedstocks, with the BFBs firing higher percentages (20%) of agricultural residues including straws and sunflower husks. They did not find toxic effects on plant growth or germination from either technology, so they concluded that biomass fly ash should be used as soil amendments instead of landfilling.

Li et al. in China explore the potential conversion of meat wastes to biodiesel through the production of microbial lipids using strains of yeast. The team used amino acid (AA) blends, which represented sheep viscera and fish waste as carbon sources for lipid production with the oleaginous yeast *Rhodosporidium toruloides*. The lipid products and fatty acids compared favorably with those produced from vegetable oils from maize stover and palm. They concluded that further research is needed to identify cost-effective protein wastes, more robust oleaginous yeast strains, and advanced bioprocesses.

The demand for ethanol in the European Union for blending with petrol and diesel is expected to double to achieve 14% by 2030. Incentives are provided for processes that produce ethanol with lower GHG emissions. Sawdust from wood processing is an abundant lignocellulosic feedstock in Sweden. Haus et al. evaluate the economic competitiveness of lignocellulosic ethanol compared with agricultural-based ethanol fuels, which are imported. They found that the savings in GHG emission from the sawdust-based ethanol was 93% compared with 68% for the mainly crop-based ethanol, which could result in a 40% increase in price for sawdust-based ethanol. The authors modeled a 200,000 dry mt per year plant to see if the increased economic advantage was sufficient to promote large-scale commercial production. Various alternatives for energy recovery and feedstock procurement were analyzed. The authors determined that lignocellulosic ethanol could be viable and that the incentives could be useful in the long term but that they were insufficient to offset the high short-term risks of large-scale production.

Unmanaged, semi-natural grasslands are a potential biomass resource in Europe, but diverse species and highly variable factors challenge the conversion of these herbaceous species to fermentable sugars. Mezule et al. evaluate potential fermentable sugar yields and overall productivity from various grasslands habitats, which are common in a temperate climate and classified under European Union habitat codes. They used non-commercial enzymes from white-rot fungi as agents in hydrolysis. They evaluated habitat type, seasonality, cutting time, weather, and solids content in the biomass and other factors in two municipalities in Latvia. Of six habitat types, the highest yields of fermentable sugars were obtained from lowland grasslands and scrubland facies on calcareous substrates. The highest average yields were from lowland meadows and the lowest were from xeric sand calcareous grasslands. These correspond to yields from semi-natural grasslands in Estonia, central Germany, and Denmark. Average dry matter yields ranged from 1 to 6 tons per ha. The reasons for variations in yield are discussed. Additional research is needed to determine other factors that could impact production on these grasslands. Authors conclude that fermentable carbohydrate production can be used as an alternate strategy to grazing.

To address China's needs for additional sources of renewable fuels, Peng et al. simulate the production of sugarcane on marginal and cultivated land in the Southern province of Guangxi. They located potential lands through statistical methods while avoiding lands reserved for other uses. They then used a modification of the APSIM sugarcane model to simulate the growth in the selected areas. They verified model results through field testing and GIS techniques. The results allowed them to estimate the potential ethanol production, which resulted in opportunities to export to other provinces. They point out that additional study is needed to ensure that the lands they have identified are not subject to environmental hazards not considered in the simulation or GIS data.

Anaerobic digestion is an important conversion pathway for electricity, heat, and transportation in Europe. Debowki et al. investigate the effect of adding microalgae to common feedstocks such as cattle manure and maize silage for biogas production. Algal biomass is a source of nitrogen and microelements for the growth of microorganisms. Microalgae have a high growth rate and do not compete with crops for feed or food. They have a high photosynthetic efficiency, fast growth rate, potential to utilize CO2, and resistance to contamination and can be cultured in areas not suitable for other uses. Microalgae culture was raised in photobioreactors and mixed with cattle slurry and maize silage. Six different species were tested. Adding microalgae improved biogas yield and composition. Methane increased. They found the highest methane production when the ratio of microalgae to feedstock was added at 20–40% v/v. There was no change in efficiency or other parameters. They found a strong correlation between methane production and C/N ratios, Anaerobic digestion with microalgae was limited by high protein and low C/N ratios which can be aided by co-digestion with carbon-rich feedstocks.

#### **Dimitrios Kalderis, Vasiliki Skoulou** *Editors*
