Poultry farms with floor-standing poultry generate large amounts of poultry litter waste. The direct application of this waste as an organic fertilizer does not ensure sustainable and cost-efficient utilization of all waste fractions, and can also be linked to environmental hazards. Therefore, the development of new technologies is required for processing poultry litter into a safe product with higher added value. In this work, the characteristics of activated carbon derived from hydrochar, along with the liquid products obtained from hydrothermal carbonization (HTC) and the wet torrefaction (WT) of poultry litter, were investigated. Poultry litter (PL) was applied in a mixture with sawdust (SD) in the following ratios: 1:0 (PL/SD 1:0), 1:1 (PL/SD 1:1), 1:2 (PL/SD 1:2), and 2:1 (PL/SD 2:1). WT processing took place in an innovative fluidized bed system in a superheated steam medium with low overpressure (less than 0.07 MPa) at 300 °C and 350 °C for 30–45 min. Conventional HTC processing was performed in a water medium at 220 °C for 1–4 h. The hydrochar produced in the experiments was activated with steam for 1 h at 450–750 °C. The porosity characteristics of activated hydrochar were measured, including pore size, pore volume, and specific surface area, in view of potential industrial applications as an adsorbent. Additionally, the contents of 5-hydroxymethylfurfural (HMF), as high-value product, were determined in the liquid products obtained from HTC processing, as well as in the condensate obtained after WT processing. Specific surface areas of the activated hydrochars may still be too low for application as adsorbent material. Hence, its use as a biofertilizer and soil improver should be preferred. Interestingly, the liquid fraction obtained from the innovative WT process displayed a significantly higher 5-HMF content compared to the conventional HTC process.

This work investigated the effect of experimental conditions of dry and wet torrefaction on the properties of olive leaves and olive pomace. Torrefaction improved the fuel properties of olive waste. According to Van Krevelen parameters (O/C and H/C ratios), torrefied biomass, tested as solid biofuel, achieved a similar quality threshold to lignite. For example, dry torrefaction conducted at 230 °C for 80 min reduced the O/C and H/C ratios of olive leaves from 0.51 and 1.51 for raw biomass to 0.25 and 1.17 for torrefied biomass, respectively. Under the same conditions, the O/C and H/C ratios of olive pomace were also reduced from 0.34 and 1.60 to 0.27 and 1.36, respectively. Calorific values of raw olive leaves and olive pomace amounted to 18.0 and 23.2 MJ/kg, respectively. Following dry torrefaction and biomass conversion into biochar, calorific values of olive leaves and olive pomace increased by 24% and 14% up to 22.2 and 26.3 MJ/kg through dry torrefaction, compared with 17% and 23% increments up to 21.1 and 28.5 MJ/kg through wet torrefaction, respectively. Interestingly, biomass processing through wet torrefaction performed in a fluidized bed powered by superheated steam could be completed 8- to 12-fold more rapidly than dry torrefaction. SEM analysis indicated a breakdown of the surface structure of olive waste following the torrefaction process. According to the Brunauer–Emmett–Teller (BET) method, total pore surface areas of biochar obtained from wet torrefaction of olive pomace and olive leaves amounted to 3.6 m²/g and 0.8 m²/g, with total pore volumes amounting to 0.0225 cm³/g and 0.0103 cm³/g, respectively. Maximal contents of 5-hydroxymethylfurfural and furfural in liquid by-products from dry torrefaction amounted to 1930 and 1880 mg/1 kg, respectively. Alternately, in liquid by-products from wet torrefaction, concentrations of these high-value compounds remained very low.

Torrefaction is a technology for the preliminary thermochemical treatment of biomass in order to improve its fuel characteristics. The aim of this work is to conduct comparative studies and select the optimal operating conditions of fluidized bed torrefaction for the processing of poultry litter (PL) into an environmentally friendly fuel. PL torrefaction was evaluated according to three different process configurations: (1) torrefaction of PL pellets in a fixed bed in a nitrogen medium at temperatures of 250 °C, 300 °C and 350 °C (NT1, NT2 and NT3); (2) torrefaction of PL pellets in a fluidized bed of quartz sand in a nitrogen medium at temperatures of 250 °C, 300 °C and 350 °C (NT4, NT5 and NT6); and (3) torrefaction of PL pellets in a fluidized bed of quartz sand in an environment of superheated steam at temperatures of 250 °C, 300 °C and 350 °C (ST1, ST2 and ST3). The duration of the torrefaction process in all experiments was determined by the time required for completion of CO₂, CO, H₂, and CH₄ release from the treated biomass samples. The gas analyzer (Vario Plus Syngaz) was used to measure the concentration of these gases. The torrefaction process began from the moment of loading the PL sample into the reactor, which was heated to the required temperature. After the start of the torrefaction process, the concentration of CO₂, CO, H₂, and CH₄ in the gases leaving the reactor initially increased and, subsequently, dropped sharply, indicating the completion of the torrefaction process. The chemical composition of the obtained biochar was studied, and it was found that the biochar contained approximately equal amounts of oxygen, carbon, nitrogen, hydrogen and ash, regardless of the torrefaction method. Furthermore, the biogas yield of the liquid condensate, obtained from the cooling of superheated steam used in the torrefaction process, was evaluated. The results highlight the efficiency of fluidized bed torrefaction, as well as the performance of superheated steam as a fluidization medium.

A numerical model for the wet torrefaction of poultry litter in a pilot unit was developed in this study. The model accounted for the following process steps: preheating biomass in a feed hopper, feeding biomass into the reactor, fluidized-bed generation using superheated steam, and the supply of additional heat by the electric heating of the reactor walls. Following a “black box” approach, a major assumption of the model is that the behavior of the fluidized-bed reactor is similar to a completely stirred tank reactor (CSTR). Under this assumption, the properties of the particles and gases do not depend on their location inside the reactor. During wet torrefaction, poultry-litter biomass was heated to a predetermined temperature and decomposed, generating biochar along with a gas phase (torgas), whose amounts depended on the content of inert ash in the biomass particles. Variable optimization in the model was performed using MATLAB software. The model successfully estimated the optimal duration required for the completion of wet torrefaction under various conditions: temperature, batch weight, reactor dimensions, etc. The model was validated using experimental data obtained from a series of wet torrefaction experiments performed in a fluidized bed, and provided reliable estimations of the duration of the process depending on material properties, reactor size and feedstock characteristics.

Biomass can be considered an alternative to coal in the production of heat and electricity. Many types of biomass are waste from agriculture and the food industry. This waste is cheap, readily available, and replenished annually. However, most agricultural and food industry wastes (sugar cane pulp, olive and sunflower oil production wastes, straw, etc.) have ash with a low melting point. This leads to a rapid growth of ash deposits on the heating surfaces of boilers; as a result, the actual efficiency of boilers in which waste from agriculture and the food industry is burned is 45–50%. Known biomass pre-treatment technologies that allow for the fuel characteristics of biowaste. For example, leaching of biowaste in water at a temperature of 80–240 °C makes it possible to drastically reduce the content of alkali metal compounds in the ash, the presence of which reduces the melting point of the ash. However, this biomass pre-treatment technology is complex and requires additional costs for drying the treated biomass. We proposed to use torrefaction for pre-treatment of biomass, which makes it possible to increase the heat of combustion of biomass, increase the hydrophobicity of biomass, and reduce the cost of grinding it. However, we are not aware of studies that have studied the effect of torrefaction on the chemical composition of ash from the point of view of solving the problem of preventing the formation of agglomerates and reducing the growth rate of ash deposits on the convective heating surfaces of boilers. In this paper, the characteristics of sunflower husk subjected to torrefaction in an environment of superheated steam at a temperature of 300 °C and in an environment of gaseous products at a temperature of 250 °C are studied. All experiments were conducted using fluidized bed technology. The resulting biochar has a calorific value of 14.8–23% higher than the initial husk. To assess the behavior of sunflower husk ash, predictive coefficients were calculated. Torrefaction of sunflower husks does not exclude the possibility of slagging of the furnace but reduces the likelihood of slagging by 2.31–7.27 times. According to calculations, the torrefaction of sunflower husks reduces the likelihood of ash deposits on the convective heating surfaces of the boiler by 2.1–12.2 times. According to its fuel characteristics, the husk, after torrefaction in an environment of superheated steam, approaches wood waste, i.e., can be burned separately without additives or mixtures with other fuels with refractory ash.