PADDLE MIXER-EXTRUSION REACTOR FOR TORREFACTION AND PYROLYSIS

This work is focused on the fundamental understanding and the development of paddle mixer reactors (or modified screw augers). This work will contribute to the effort of the thermal conversion of biomass and wastes. We developed and studied two paddle systems (i) 25-mm lab-scale (up to 1 kg/hr) and (ii) 101-mm pilot-scale (up to 100 kg/hr). Thermal behavior of the two systems was studied and it was estimated that the lab-scale system has a high heating rate of up to 530 °C/s. Residence times were thoroughly measured and were determined as a function of rotation frequency and volume fraction. We also determined the specific process energy requirements and the specific heat of the material. Extensive pyrolysis experiments were carried out with many types of biomass. It was found that solid/liquid yields were comparable to those measured in circulating fluidized bed at NREL. Modification of the pilot-scale system is required to enhance the mass flow rates and the heating rate. Fiber and plastic waste blends were thoroughly investigated in a mixture of 40% plastic and 60% fiber. Extensive torrefaction experiments were carried out and thermal and mechanical properties of the torrefied material were measured and correlated with mass loss. Degradation reaction of waste blends was modeled using a first-order reaction. Excellent fit between the experimental and modeling results was obtained. Activation energy and pre-exponential factors were determined. One major finding was that the paddle mixer significantly increased the homogeneity of the waste blend and it is further increased as the size of the material reduces. Density was measured and found that at a density of ~1200 kg/m3, the water intake was 0.7% after 30 days of immersion in water. Extensive grinding study was carried out with these torrefied waste blends and the grinding energy behavior was found similar to that of PRB coal. Heat content was measured, and it was shown that the initial heat content is ~30 MJ/kg and as the torrefaction process proceeds the value increases to ~35 MJ/kg at ~51% mass loss. Combustion experiments were carried out and showed that with the reduction of volatile matter (due to thermal degradation) the combustion time has increased

Chlorine removal from U.S. solid waste blends through torrefaction

The amount of solid waste generated annually is increasing around the world. Although the waste has a high calorific value, one major obstacle that may prevent it from becoming a feedstock for power applications is the existence of polyvinyl chloride (PVC), which causes corrosion and emission issues after combustion due to its high chlorine content. Torrefaction is known to release hydrochloric acid; thus, it has been applied in this study for the reduction of chlorine from potential waste feedstocks. Fiber-plastic (60-40%) waste blends, with different chlorine content levels, as well as PVC were used in the current study. Torrefaction was conducted at 400 °C. Chlorine and heat content were measured. Experimental results showed that organically bonded chlorine was reduced during torrefaction as a function of mass loss. The chlorine removal efficiency was only dependent on temperature and residence time, not chlorine level. The heat content of the sample increased with mass loss up to a maximum of ~34 MJ/kg at ~45% mass loss. It was also observed that at ~30% mass loss, the organic chlorine content per unit heat content reduced by ~90%, while the heat content was ~32 MJ/kg, and ~90% energy was retained

Properties of torrefied U.S. waste blends

Power generation facilities in the U.S. are looking for a potential renewable fuel that is sustainable, low-cost, complies with environmental regulation standards and is a drop-in fuel in the existing infrastructure. Although torrefied woody biomass, meets most of these requirements, its high cost, due to the use of woody biomass, prevented its commercialization. Industrial waste blends, which are also mostly renewable, are suitable feedstock for torrefaction, and can be an economically viable solution, thus may prolong the life of some of the existing coal power plants in the U.S. This paper focuses on the torrefaction dynamics of paper fiber-plastic waste blend of 60% fiber and 40% plastic and the characterization of its torrefied product as a function of extent of reaction (denoted by mass loss). Two forms of the blend are used, one is un-densified and the other is in the form of pellets with three times the density of the un-densified material. Torrefaction of these blends was conducted at 300°C in the mass loss range of 0-51%. The torrefied product was characterized by moisture content, grindability, particle size distribution, energy content, molecular functional structure, and chlorine content. It was shown that although torrefaction dynamics is of the two forms differs significantly from each other, their properties and composition depend on the mass loss. Fiber content was shown to decrease relative to plastic upon the extent of torrefaction. Further, the torrefied product demonstrates a similar grinding behavior to Powder River Basin (PRB) coal. Upon grinding the fiber was concentrated in the smaller size fractions, while the plastic was concentrated in the larger size fractions

Torrefied plastic-fiber fuel pellets as a replacement for fossil fuels — a case study life cycle assessment for Green Bay, Wisconsin, USA

Purpose: The commercial-scale production of torrefied plastic-fiber fuel pellets from waste plastics and waste fibers may offer a viable alternative to fossil fuel–based energy. In this study, the environmental impact of fuel pellets produced and consumed in Green Bay, Wisconsin, USA is evaluated and compared to the status quo of grid energy production from fossil fuels (i.e., coal or natural gas). Methods: A cradle-to-grave life cycle assessment was conducted using a functional unit of 1 kWh of energy produced using torrefied plastic-fiber fuel pellets versus production of energy from coal or natural gas. Regional data along with relevant manufacturing data was used to inform the inventory of the production of the torrefied fuel pellets, which are manufactured using waste fibers and waste plastics sourced from within 5 km of the torrefaction facility and consumed within 50 km of the facility. Since fuel pellets are produced from waste inputs and contain biogenic carbon sources, impacts were assessed with/without credit for biogenic carbon and with/without the burden of the torrefaction inputs. Results and discussion: The production of 1 kWh of energy using torrefied plastic-fiber fuel pellets was determined to produce between 0.303 and 0.757 kg CO2 eq emissions due to combustion and between 0.062 and 1.105 kg CO2 eq additional emissions as a result of the manufacturing process, with the ranges dependent upon the allocation method selected. Under a burden-free allocation due to waste materials used as inputs, along with a credit for biogenic carbon emissions, the system produces 0.365 kg CO2 eq per 1 kWh of energy; however, under a full-burden allocation with no credit for biogenic carbon emissions, 1.862 kg CO2 eq per 1 kWh of energy is produced. This highlights the differences between allocation scenarios and role of credits for biogenic carbon emissions when evaluating systems. Conclusions: The usage of torrefied plastic-fiber fuel pellets produced using waste plastics and fibers is a reasonable alternative to the status quo of waste disposal coupled with the production of grid energy from fossil fuels. In addition to the reduction in GHG emissions, the use of the process would also help to alleviate the environmental burden of waste plastics

Integration of Thermal Treatment and Extrusion by Compounding for Processing Various Wastes for Energy Applications

Waste generation is increasing, and a significant portion of the wastes is being landfilled. Torrefaction of such wastes to produce clean fuels is one of the potential solutions. This paper studied torrefaction of mixed fiber-plastic wastes at 300 °C in an integrated torrefaction-extrusion screw reactor with a throughput of up to 70 kg/h. The study experimentally measured the thermomechanical properties of the torrefaction-extrusion process and the pellets produced. The study presents the results for thermal dynamics, the effect of shaft configuration on residence time, specific mechanical energy (SME), heat transfer coefficient (U), specific heat (C) of mixed wastes, and mechanical and rheological properties of pellets. First, the thermal dynamics of the system were studied along the corresponding response of heaters with and without the flow of materials measured. The residence time measurement showed 20% and 40% cut flighting had about 2.3 and 3.7 times more residence time compared to a regular screw. The specific heat of the heterogeneous mix blend was measured at 1.58 kJ/(kg °C). The average overall heat transfer coefficient was measured experimentally for the reactor at 52.5 W/(m2 °C). The correlation between specific mechanical energy and mass flow showed more than 3 times decrease in specific energy consumed when the feed rate was increased from ∼10 to 50 kg/h. Thermomechanical analysis, flexural testing, and rheological testing were performed on the produced pellets to measure pellet properties

Properties of pellets of torrefied U.S. waste blends.

With the continued growing U.S. population, solid waste generation will increase, which will lead to undesired and significant growth in landfilling. Thermal treatment can turn these high calorific value wastes into fuels that can be used in small-to-large power plants. This article focuses on using blends with 40% plastic and 60% fiber wastes and converting them into densified solid fuel by torrefaction and extrusion. The material was torrefied at 300 °C to obtain torrefied samples with different mass losses, ranging from 0% to a maximum of 51%. The torrefaction results showed a clear synergy between plastics and fibers. The torrefied material was then extruded into 9 mm diameter rods and the products were characterized by molecular functional group analysis, thermomechanical analysis, dynamic mechanical analysis, dynamic rheological measurement, density measurement, flexural testing, water absorption test, size distribution measurement, heat content test, and combustion test. The fiber content in the material decreased as mass loss increased, and the process reduced significantly the variability of the material. The heat content increased as the mass loss increased. The plastic in the feedstock acted as a process enabler as it imparted properties like bindability, water resistance, high heat content, and increased degradation reaction rate

Comprehensive kinetic study of thermal degradation of polyvinylchloride (PVC)

The plastic waste accumulation has been increasing and a solution other than landfilling is required. Due to the high cost of recycling, thermal treatment could be an option. However, the existence of polyvinyl chloride (PVC) would release hydrochloric acid which would cause emission problems as well as damage to the reactor systems. The thermal degradation of PVC has been studied over the years. However, the mechanism of the PVC thermal degradation is not fully developed. Specifically, the mechanism of the PVC thermal degradation at medium temperatures, which is more practical for industries, is still lacking. A degradation temperature of 300 °C was used to study the dehydrochlorination behavior of PVC. A rather comprehensive mechanism with four consecutive reactions has been developed based on the micro-pyrolysis experiments and has been validated and proved by predicting the mass loss, chlorine content, heat content and elemental composition with high precision experimental data in different reactors with/without heat transfer coupling

Accurate Characterization of Mixed Plastic Waste Using Machine Learning and Fast Infrared Spectroscopy

We present a combination of convolutional neural network (CNN) framework and fast MIR (mid-infrared spectroscopy) for classifying different types of dark plastic materials that are commonly found in mixed plastic waste (MPW) streams. Dark plastic materials present challenges in fast identification because of the low signal-to-noise ratio. The proposed CNN architecture (which we call PlasticNet) can reach an overall classification accuracy of 100% and can identify the constituent materials in a multiplastic blend with 100% accuracy. The fast MIR system can collect spectral data at a rate up to 400 Hz, and the CNN model can reach prediction speeds of 8200 Hz. Therefore, this method provides an avenue to be able to characterize MPW in a real-time high-throughput manner

Integrated torrefaction-extrusion system for solid fuel pellet production from mixed fiber-plastic wastes: Techno-economic analysis and life cycle assessment

The world is witnessing an unprecedented generation and accumulation of fiber-plastic wastes resulting in various challenges due to inconsistency, waste-stream heterogeneity, conveying issues, self-heating, and difficulty in pelletization. This study presents a novel pilot-scale system that integrates torrefaction and extrusion to convert mix fiber-plastic waste into fuel pellets. The produced pellets have low cost, high heating value, better uniformity, and low environmental impact. They can be used as solid fuels or as feedstock for pyrolysis and gasification. To evaluate the pellet cost and its environmental impact, we performed Techno-Economic Analysis (TEA) and Life Cycle Assessment (LCA). The TEA integrates research findings from the torrefaction-extrusion project with the techno-economic models and estimates the costs, energy consumption, and mass balances for pelletizing and torrefaction. The analysis indicates that the baseline cost of producing uniform pellets is about $55.28/dry tonne (2020$). LCA results indicate that the torrefied product has cradle-to-gate embodied greenhouse gas emissions that are net negative, although they are higher than a comparable forest-derived woodchip product. Fossil energy demand for the torrefied product is lower than the forest-derived chip, indicating the torrefied product has strong potential for use as an environmentally beneficial feedstock for future processing

Kinetic Study of Paper Waste Thermal Degradation

Paper waste generation has been rising in the past decades, with a large amount being landfilled. These paper wastes can be great energy sources after thermal treatment since they are considered carbon neutral. These wastes contain mainly cellulose, hemicellulose, lignin, and some minerals. The thermal decomposition of cellulose, hemicellulose, and lignin have been extensively studied, however, the knowledge of thermal degradation of paper wastes at lower temperatures, which are more practical for industrial applications are still lacking. In this study, paper wastes have been characterized and thermogravimetric analyses were performed from 200°C to 400°C and the char produced were analyzed by nuclear magnetic resonance (NMR) spectroscopy and Fourier-transform infrared (FTIR) spectroscopy. Two kinetic approaches were taken while developing the kinetic model of paper waste thermal degradation: (i) reconstructing the TGA results of paper waste thermal degradation by an additive law of the degradation of cellulose, hemicellulose and lignin; (ii) considering paper waste as one material and develop a multi-step consecutive reaction mechanism that focuses on solid products at different temperatures. It was observed that there are potential interactions between cellulose, hemicellulose and lignin during paper waste degradation. Therefore, the second approach was concluded to be more plausible, and one set of kinetic parameters were determined according to the experimental results at different temperatures. These results provided insights into the degradation kinetic mechanism and solid product distribution of the paper waste. It was found that the first reaction was due to dehydration of cellulose and the 6th and 7th reaction can be attributed to the thermal degradation of lignin. The NMR and FTIR results also validated that the cellulose started degrading at lower temperatures, and lignin degradation became more pronounced at higher temperatures