Residual Biomass Conversion to Bioenergy

1. Introduction

Greenhouse gases (GHG) concentration (CO₂, CH₄, and NOx mainly) in the Earth’s atmosphere has dramatically increased since 1960; in particular, the atmospheric CO₂ levels have increased from 320 ppm to 412 ppm from 1960 to 2019 [1], exacerbating global warming to a very dangerous threshold. The proof of this is the melting of glaciers and ice sheets that are contributing to rising sea levels. Even in temperate regions, the frequency of extreme weather conditions, be they wind, thunderstorms, hailstorms and flooding, or heat waves, has increased over the course of recent years.

Tackling such global issues is not an easy task, nor can it be done without concern for both economic and social aspects around the world. A carbon-free global economy is difficult to achieve, and far off. Nowadays, many different solutions can be used to mitigate global warming, but they require the transformation of the global energy sector including production, distribution, and consumption stages.

Biomasses represent a valuable renewable resource which can be conveniently used to cope with the energy problem and, at the same time, reduce greenhouse gas emissions. Residual biomass feedstock, which comprises a large amount of the whole biomass, is widely available in the world and could be effectively used to recover not only energy, but also biofuels and biomaterials without competition with the food sector. However, the different chemical and physical characteristics of biomasses require different and specific thermochemical, biochemical, or physical-chemical conversion pathway processes to directly obtain energy or extract biofuels and biomaterials.

The goal of this Special Issue was aimed at gathering researchers working on biomass topics from different fields to discuss scientific, technical, economic, and environmental issues arising from the use of residual biomass resources. The authors were invited to submit articles dealing with innovative technical issues, but also reviews, case studies, and analytical and assessment papers focused on the sustainable use of residual biomasses.

2. Short Review of the Contributions of This Issue

Experimental, numerical, and scenario investigations were considered and presented in the Special Issue. The papers can be broadly divided into two categories. Production technologies refer to investigations focused on energy conversion accomplished through experimental tests on different biomass types or presenting scenario investigations using a mixed approach, whereas the Processing Investigations papers are focused on the basic aspects of biomass use, such as combustion characteristics of different biomasses or the evaluation of PM emissions.

A brief review of the papers published in this Special Issue is presented below.

3. Production Technologies

One paper [2] discussed the experiments performed on a large-scale gasification installation to assess the system pressure and partial pressure of CO₂ on the efficiency of conversion and the quality of the produced gas using bark, lignin, and a blend of bark and wheat straw as feedstocks and softwood pellet as a reference fuel. The influence of gasification pressure was predominantly visible in higher yields of all hydrocarbons (including CH₄) and lower overall production of producer gas.

The authors in reference [3] investigated the lignocellulosic rice-processing residue pre-treatment in liquid hot water (LHW) at three different temperatures and two times to assess its effects on hydrolysates composition, matrix structural changes, and methane yield. The maximum methane yield (276 L kg⁻¹ vs) was obtained under pre-treatment conditions of 180 °C for 20 min. The results indicated significant potential for the use of LHW to improve methane production from rice-processing residue.

Charcoal production in Portugal is mostly based on the valorization of woody residues from cork oak and holm oak, the latter being considered a reference feedstock in the market. This has motivated the work by the authors of reference [4], which considered whether the carbonization process could be used to valorize alternative woody biomasses not currently used on a large scale. For this purpose, slow pyrolysis experiments were carried out with ten types of wood using a fixed-bed reactor. Emphasis was placed on the properties of the resulting charcoals considering its major market in Portugal.

The focus of reference [5] was to assess if the waste biomass residues in Tanzania could potentially produce renewable electrical energy for small-scale electricity generation using off-grid diesel generators coupled with anaerobic digestion (AD) and/or gasification. The biomass waste streams estimated are those arising from agriculture, forestry, livestock, and urban human waste. The results showed that, collectively, these residues could produce at least 1.2 times the electricity generated nationally in 2018 using AD and gasification coupled with a diesel gen-set engine.

The authors of reference [6] assessed an integrated technical and economic investigation scenario of the biological treatment plants, anaerobic digestion, and composting, dealing with the Organic Fraction of Municipal Solid Waste (OFMSW). The economic sustainability of such plants can be increased by integrating the whole system with lactic acid production, because of the high added value of the by-product and by substituting the composting process with the hydrothermal carbonization process to obtain Hydrochar. The authors showed that the integration processes could be a valid economic alternative and that the most interesting solution considers the utilization of the leachate produced during the anaerobic digestion process instead of fresh water for the hydrothermal carbonization process.

Reference [7] presented the results of the pelletization tests of post-harvest tobacco waste as a feedstock for fuel pellet production. The experiments were conducted on a prototype pelleting–briquetting device with a flat matrix. The obtained pellets were characterized by a high density above 1000 kg·m⁻³ and kinetic durability above 97%. The obtained pellets were combusted in a 25 kW fixed great boiler showing high amounts of CO, SOx, NO, and HCl, which suggests that the selection of a different technology for tobacco waste pellet combustion should be made.

4. Processing Investigations

The authors of reference [8] investigated the combustion characteristics of olive by-products by the TG-FTIR technique, considering twigs, leaves, and olive-mill waste from the two-phase decanting method, and wastewater from the three-phase system. Comprehensive combustion, ignition, burnout, and flammability indexes were also calculated. Interesting results were achieved by the authors by differentiating the best characteristics of the by-products. FTIR analyses presented differences in the exhaust gas composition for specific combustion temperature ranges.

A novel approach was developed in reference [9] by the authors to investigate the PM emissions, originating from residual biomass combustion, at different combustion conditions in a lab-scale grate-fired furnace, also including in situ PM measurements. Experimental results and computational fluid dynamics (CFD) analyses were utilized to develop a kinetic model for reduction of particulate matter emissions in biomass combustion. By combining the experimental measurements and information from the CFD analyses, a predictive kinetic model for PM10 reduction in biomass combustion was successfully developed.

A research study [10] was conducted on the thermal behavior of leaves of urban greenery and the products of their pyrolysis and extraction as assisted by microwaves, which was investigated using the FT-IR and UV spectroscopies and XRD techniques. The pyrolysis, at the first stage of combustion, displayed a decrease in the amount of dangerous compounds in the volatile products of pyrolysis, leading to a lower contribution of such compounds in combustion products. The main outcomes showed that urban greenery leaves could be subjected to combustion after extraction, and the obtained extracts could be used as a source of phytochemicals and chemical reagents.

5. Conclusions

A wealth of biomass resources has been taken into account in this Special Issue, ranging from agricultural residues to the organic fraction of municipal waste, leaves of urban greenery, and also tobacco waste. Different physical, as well as chemical compositions, require diverse processing systems. In any case, all of these biomasses share the opportunity to be fully exploited as energy sources to reduce the environmental impact of conversion energy systems.