Influence of sequential HTC pre-treatment and pyrolysis on wet food-industry wastes: Optimisation toward nitrogen-rich hierarchical carbonaceous materials intended for use in energy storage solutions

Due to elevated protein content, the food-industry bio-wastes are promising feedstock to produce hierarchical (micro-mesoporous) carbonaceous materials with the intended use as electrodes in the energy storage solutions. However, the high initial water content, makes their direct activation through high-temperature processes costineffective due to significant heat requirements. In this study, the influence of pretreatment with hydrothermal carbonization (HTC) on wet food-industry bio-wastes, further pyrolysed, was investigated. Selected wastes (brewer's spent grains, spent coffee grains and spent sugar beets) were pre-treated by HTC at 180 °C or 240 °C, and then pyrolysed at 500 °C or 700 °C. Obtained materials were examined using elemental analysis, gas adsorption (N₂ and CO₂) and FT-IR. Besides minor differences caused by the bio-composition of wastes, the general trends were similar for feedstock. The pre-treatment had a beneficial influence on the properties of all wastes. The HTC at 180 °C and pyrolysis at 700 °C for all wastes show the most promising total specific surface area 560 ± 10 m²/g and accessible specific surface area 96 m²/g. Those conditions simultaneously did not reduce the total solid yield in comparison to the one-step process. The pre-treatment at 240 °C led to elevated nitrogen incorporation in the carbonaceous structure compared to HTC at 180 °C. However, it formed a hierarchical structure that was not stable for the thermal treatment. Study proves the HTC pre-treatment at 180 °C is beneficial for the conversion of food-industry bio-wastes into hierarchical carbonaceous material for their use in the energy storage systems application.

Valorization of the poultry litter through wet torrefaction and different activation treatments

The increase in volume in bio-waste is inseparable from the production of biomass derived commodities. To reduce the use of conventional resources, the valorization of waste streams is gaining importance, and the valorisation of poultry litter fits perfectly into such scheme. This study shows a possible valorization of wet torrefied (300 °C) poultry litter (WTPL) through activation and its further use as a fertilizer, and as a wastewater micro-pollutant absorbent. The WTPL was activated thermally, physically (CO2) and chemically (KOH) at two different temperatures (600 °C and 800 °C) and 30 min residence time. The properties of ACs were evaluated based on results of the elemental and proximate analysis, suspension pH measurement, ICP-OES, FT-IR, N2 and CO2 adsorption and quantity of absorbed methylene blue (MB). The yields in thermal and physical ACs were comparable, but much higher than ACs from chemical activation (c.a. 50% and 15% at 600 °C and c.a. 47% and 6.5% at 800 °C). The thermal and physical ACs showed good suitability for application as a fertilizer due to their high macro- and micro-nutrients and low heavy metals concentration. Carbons activated with KOH proved their usefulness as wastewater pollutant absorbers through high MB's absorption (675.8 mg/g for 600 °C and 872.8 mg/g for 800 °C). Results state that the valorization of PL through activation is possible, and the selection of the activation method affects the final application of obtained material.

Metal sorption by biochars: A trade-off between phosphate and carbonate concentration as governed by pyrolysis conditions

Three feedstocks, pine wood, grass and cow manure, were pyrolyzed under various conditions and tested on their ability to sorb metals in aquatic systems. The feedstocks were pyrolyzed at 2 different temperatures (350 °C and 550 °C) and 2 different residence times (10 and 60 min) and resulting biochars were assessed on their capability to immobilize Pb, Cu, Cd and Zn. Manure-based chars, and to a lesser extent grass-based chars, featured high concentrations of phosphates and carbonates. These anions play an important role in metal sorption because they form insoluble complexes with the metals. Washing reduced the concentration of these anions, leading to a reduced sorption of metals by the biochar. The carbonate concentration on the biochars' surface increased at higher reactor temperature and longer residence times. The opposite trend was observed for the phosphate concentration and the cation exchange capacity. Accordingly, the optimal temperature-residence time combination for sorption was a trade-off between these properties. Biochar produced from cow manure and pyrolyzed at 550 °C for 10 min showed the best sorption for all metals considered.

Heat recovery during treatment of highly concentrated wastewater: economic evaluation and influencing factors

This paper assesses the economics of heat recovery from biological wastewater treatment plants (WWTPs) treating concentrated wastewater, as higher concentrations result in higher heat generation in the treatment basin. A heat balance model has been applied to calculate the amount of recoverable heat from the system and the effect of the heat extraction capacity on the economics of a heat pump installation, evaluated using the internal rate of return. A sensitivity analysis has been performed to evaluate the effect of several parameters on the economics of heat recovery in this type of WWTP: the electricity price, the price of the fuel substituted by heating savings, the investment costs, the coefficient of performance (COP) and the amount of heat extracted from the system. It was calculated that the heat pump capacity has to be high enough to recover a significant amount of heat, but low enough to improve the economics of the system. The economic performance of the system is very dependent on the energy prices of both electrical power to run the heat pump and the fuel (heat) cost substituted by the heat pump.

Mild hydrothermal conditioning prior to torrefaction and slow pyrolysis of low-value biomass

The aim of this research was to establish whether hydrothermal conditioning and subsequent thermochemical processing via batch torrefaction or slow pyrolysis may improve the fuel quality of grass residues. A comparison in terms of fuel quality was made of the direct thermochemical processing of the feedstock versus hydrothermal conditioning as a pretreatment prior to thermochemical processing. Hydrothermal conditioning reduced ash content, and particularly nitrogen, potassium and chlorine contents in the biomass. The removal of volatile organic matter associated with thermochemical processes can increase the HHV to levels of volatile bituminous coal. However, slow pyrolysis only increased the HHV of biomass provided a low ash content (<6%) feedstock was used. In conclusion, hydrothermal conditioning can have a highly positive influence on the efficiency of thermochemical processes for upgrading low-value (high-ash) biomass to a higher quality fuel.

Effect of foam on temperature prediction and heat recovery potential from biological wastewater treatment

Heat is an important resource in wastewater treatment plants (WWTPs) which can be recovered. A prerequisite to determine the theoretical heat recovery potential is an accurate heat balance model for temperature prediction. The insulating effect of foam present on the basin surface and its influence on temperature prediction were assessed in this study. Experiments were carried out to characterize the foam layer and its insulating properties. A refined dynamic temperature prediction model, taking into account the effect of foam, was set up. Simulation studies for a WWTP treating highly concentrated (manure) wastewater revealed that the foam layer had a significant effect on temperature prediction (3.8 ± 0.7 K over the year) and thus on the theoretical heat recovery potential (30% reduction when foam is not considered). Seasonal effects on the individual heat losses and heat gains were assessed. Additionally, the effects of the critical basin temperature above which heat is recovered, foam thickness, surface evaporation rate reduction and the non-absorbed solar radiation on the theoretical heat recovery potential were evaluated.