Waste to Energy: Solid Fuel Production from Biogas Plant Digestate and Sewage Sludge by Torrefaction-Process Kinetics, Fuel Properties, and Energy Balance

Is the Production of Agricultural Biogas Environmentally Friendly? Does the Structure of Consumption of First- and Second-Generation Raw Materials in Latvia and Poland Matter?

The importance of biogas in the energy mix in Poland and Latvia is very low. In Poland, 306 million m3 of biogas is produced annually, and in Latvia, 56 million m3. The share of energy from agricultural biogas in Latvia is 1.6%, and in Poland, only 0.12%. This study analyzed the impact of the structure on CO2 emissions from agricultural biogas production in Latvia and Poland. The emission was determined in accordance with the EU directive. The structure of substrates was dominated by those from the second generation, i.e., manure and food waste. In Latvia, it was 70%, and in Poland, 78%. The manure share was 45% and 24%, respectively. The anaerobic digestion of manure guarantees high GHG savings thanks to the avoided emissions from the traditional storage and management of raw manure as organic fertilizer. The level of emissions from the production of agricultural biogas was calculated for the variant with the use of closed digestate tanks, and it was about 10–11 g CO2/MJ, which is comparable to the emissions from solar photovoltaic sources. When using open tanks, the emission level was twice as high, but it was still many times less than from the Polish or Latvian energy mix. Such a low level of emissions resulted from the high share of manure. The level of emission reduction reached 90% compared to fossil fuels. The use of second-generation feedstock in biogas production provides environmental benefits. Therefore, if wastes are used in biogas generation, and the influence on the local environment and overall GHG emissions is positive, authorities should support such activity.

Medical Peat Waste Upcycling to Carbonized Solid Fuel in the Torrefaction Process

Peat is the main type of peloid used in Polish cosmetic/healing spa facilities. Depending on treatment and origin, peat waste can be contaminated microbiologically, and as a result, it must be incinerated in medical waste incineration plants without energy recovery (local law). Such a situation leads to peat waste management costs increase. Therefore, in this work, we checked the possibility of peat waste upcycling to carbonized solid fuel (CSF) using torrefaction. Torrefaction is a thermal treatment process that removes microbiological contamination and improves the fuel properties of peat waste. In this work, the torrefaction conditions (temperature and time) on CSF quality were tested. Parallelly, peat decomposition kinetics using TGA and torrefaction kinetics with lifetime prediction using macro-TGA were determined. Furthermore, torrefaction theoretical mass and energy balance were determined. The results were compared with reference material (wood), and as a result, obtained data can be used to adjust currently used wood torrefaction technologies for peat torrefaction. The results show that torrefaction improves the high heating value of peat waste from 19.0 to 21.3 MJ × kg−1, peat main decomposition takes place at 200–550 °C following second reaction order (n = 2), with an activation energy of 33.34 kJ × mol−1, and pre-exponential factor of 4.40 × 10−1 s−1. Moreover, differential scanning calorimetry analysis revealed that peat torrefaction required slightly more energy than wood torrefaction, and macro-TGA showed that peat torrefaction has lower torrefaction constant reaction rates (k) than wood 1.05 × 10−5–3.15 × 10−5 vs. 1.43 × 10−5–7.25 × 10−5 s−1.

Municipal Solid Waste Thermal Analysis—Pyrolysis Kinetics and Decomposition Reactions

In this study, 12 organic waste materials were subjected to TG/DTG thermogravimetric analysis and DSC calorimetric analysis. These analyses provided basic information about thermochemical transformations and degradation rates during organic waste pyrolysis. Organic waste materials were divided into six basic groups as follows: paper, cardboard, textiles, plastics, hygiene waste, and biodegradable waste. For each group, two waste materials were selected to be studied. Research materials were (i) paper (receipts, cotton wool); (ii) cardboard (cardboard, egg carton); (iii) textiles (cotton, leather); (iv) plastics (polyethylene (PET), polyurethane (PU)); (v) hygiene waste (diapers, leno); and (vi) biodegradable waste (chicken meat, potato peel). Waste materials were chosen to represent the most abundant waste that can be found in the municipal solid waste stream. Based on TG results, kinetic parameters according to the Coats–Redfern method were determined. The pyrolysis activation energy was the highest for cotton, 134.5 kJ × (mol∙K)−1, and the lowest for leather, 25.2 kJ × (mol∙K)−1. The DSC analysis showed that a number of transformations occurred during pyrolysis for each material. For each transformation, the normalized energy required for transformation, or released during transformation, was determined, and then summarized to present the energy balance. The study found that the energy balance was negative for only three waste materials—PET (−220.1 J × g−1), leather (−66.8 J × g−1), and chicken meat (−130.3 J × g−1)—whereas the highest positive balance value was found for potato peelings (367.8 J × g−1). The obtained results may be applied for the modelling of energy and mass balance of municipal solid waste pyrolysis.

Scenario Analysis for Selecting Sewage Sludge-to-Energy/Matter Recovery Processes

The sewage sludges are the byproducts of the wastewater treatment. The new perspective of the wastewater value chain points to a sustainable circular economy approach, where the residual solid material produced by sewage sludge treatments is a resource rather than a waste. A sewage sludge treatment system consists of five main phases; each of them can be performed by different alternative processes. Each process is characterized by its capability to recover energy and/or matter. In this paper, a state of the art of the sludge-to-energy and sludge-to-matter treatments is provided. Then, a scenario analysis is developed to identify suitable sewage sludge treatments plants that best fit the quality and flowrate of sewage sludge to be processed while meeting technological and economic constraints. Based on the scientific literature findings and experts’ opinions, the authors identify a set of reference initial scenarios and the corresponding best treatments’ selection for configuring sewage sludge treatment plants. The scenario analysis reveals a useful reference technical framework when circular economy goals are pursued. The results achieved in all scenarios ensure the potential recovery of matter and/or energy from sewage sludges processes.