Overview of benefits, limitations, by-products and applications of the main technologies of converting waste into energy and biofuel

Waste can be described as any type of material or object that has no other use and is to be thrown away. Perspective, the generation of waste materials is unavoidable in a consumption-based society, and it makes waste management a major challenge considering the huge amounts of waste produced globally. In fact, in 2014, about 2.6 billion tons of waste was generated in the European Union (EU), of which 41% was disposed of in landfills, 36% was recycled, 10% was used in excavation operations, 7% was treated in sewage treatment plants and the remaining of 6% was burned for creating energy or oxidation and stabilization of waste. Accordingly, in recent decades, humanity has changed its focus on traditional waste management from the concept of "collection and disposal" in favor of hierarchical management of waste to increase sustainability.Nevertheless, even when environmentally friendly practices such as recycling and reuse are carried out, many operations are "downcycling", meaning that the recycled product has less economic value than its original objective, and is not as valuable as the original product made from strong raw materials. In this way, the linear economy model based on the pyramidal hierarchy of waste materials, which is used today, also has limitations. Indeed, there are still opportunities to increase productivity in many industrial processes, but these gains are likely to be increasingly marginal and undifferentiated. Therefore, the future acceptance of the circular economy concept, as opposed to the current linear model, is a necessary paradigm shift. This new concept is increasingly considered a source of innovation in products, processes, and business models and opens up great opportunities that should be used by companies and organizations as competitive advantages in a dynamic market to be used globally. The processing of raw biomass to produce energy, fuel and chemicals through a combination of different applied technologies is considered a promising path to achieve sustainable waste management, with many environmental and economic benefits. The main processes related to energy recovery and biofuel production are considered under the concept of biorefineries. This waste biorefineries are facilities that integrate the necessary technologies to convert biomass feedstock and other waste into usable products, ensuring that the circular economy moves from theory to the real world. Existing waste streams can be converted to biofuels (waste-to-liquids, WtL) or energy (waste-to-energy, WtE) technologies, both of which are expected to be a key element in future waste management. In general, energy and biofuel production technologies from waste are classified into three main thermochemical, biological and chemical processes. Thermochemical technologies include processes of combustion/incineration, gasification, steam explosion, pyrolysis, hydrothermal liquefaction, and torrefaction; biological technologies include the processes of anaerobic digestion, fermentation, enzyme purification, and microbial electrolysis, and chemical technologies include hydrolysis, solvent extraction, transesterification, and supercritical conversion.

The present research is a descriptive-review study whose data was obtained through library studies and various sources were used to process the material. Considering the importance of biofuels as a source of renewable energy, we tried to use as much as possible the most relevant and up-to-date sources containing valuable points regarding the types of energy and biofuel production technologies from biomass. In this review article, the possibility of using the remaining post-processed waste as available and low-cost bio-renewable resources in waste bio-refineries has been investigated. Waste biorefineries are facilities that integrate the necessary technologies to convert biomass feedstock and other waste into usable products, ensuring that the circular economy moves from theory to the real world. Existing waste streams can be converted to biofuels (waste-to-liquids, WtL) or energy (waste-to-energy, WtE) technologies, both of which are expected to be a key element in future waste management. Accordingly, in this paper, we briefly study the current status of the main WtL and WtE technologies in order to use them as a tool for the management of residual post-processing wastes and by-products resulting from them, and finally about Future developments on the mentioned technical options are briefly discussed.

In this review research, the possibility of using the remaining waste after processing as abundant and low-cost bio-renewable resources in waste bio-refineries in the future was investigated. Existing waste streams have a complex and diverse composition according to their source, which require new logistics platforms of classification and valuation. With the exhaustion of the linear economy of "collection and disposal", new methods of waste management are inevitable in the long term. In this way, waste biorefineries that generate green energy and produce virtual products with high value and zero waste (no waste) in a "closed loop" and "up-cycling" approach are the "landfills" of the future. It is expected that they will be very important and vital in bringing sustainable waste management into the real world that will allow transformative economic growth under the concept of circular economy. However, from the technologies reviewed, it can be concluded that individual WtL and WtE processes are almost always limited in their scope and produce multiple unwanted products. In this regard, the gasification process is largely considered a technology with greater potential and scope in individual applications. However, even this process has drawbacks such as reactor design, feed system, and bitumen production that require costly post-treatment and/or further technical improvements. In contrast, the combination of several WtE and WtL processes in an integrated waste biorefinery allows reducing and eliminate the drawbacks of each process. For example, in gasification, some of the unwanted materials produced may be used and valued by further chemical processing, and even syngas can be upgraded. This new pyramid of waste valorization creates opportunities for specific technologies such as explosive depressurization and drying to become practical applications by reinforcing other well-established technologies in an integrated approach. Future research should primarily focus on establishing a hierarchy of processes to produce the highest value products, and then gradually progress to low-cost products and energy production. However, for this vision to become a reality, an increased effort by researchers is required with continued and sustained support from all potential stakeholders. More pilot or semi-pilot scale demonstration projects should be realized in the coming years, focusing on aspects such as energy balance and cost-benefit analysis that will ensure the viability of the proposed solutions