Investigation of reactions occurring in waste combustion ash using thermal analysis coupled with gas analysis and characterization.

Waste-to-energy (WtE) ash was investigated for thermal reactions that generate gas components such as hydrogen and carbon dioxide. An evolved gas detection method coupled with thermal gravimetric analysis and differential scanning calorimetry provided insight into the possible reactions occurring in WtE ash at temperatures ranging from 90°C to 600°C in an inert environment. The combined analysis shows that H2 is produced from WtE ash at temperatures ~298°C and is detected until ~480°C. CO2 appears in the evolved gas starting at 290°C and continues to increase as the temperature is increased. The results reveal that the processes releasing H2 and the CO2 are independent of each other, and the CO2 generation depends on the constant input of energy. These results enable the identification of the possible processes occurring in WtE ash decomposition of Friedel's salt at 280°C and dehydration of Ca(OH)2 at 410°C, both of which release H2O that reacts with the aluminium present to release H2. At temperatures higher than 480°C, an alumina layer is formed preventing further production of H2. X-ray diffraction analysis done on the WtE ash verifies the presence of chemical phases that support the proposed reactions. The outcome of this study enables identifying the possible reactions in WtE ash that can be causing the energy changes seen during disposal, storage and transportation of ash. These results can give direction for detailed understanding and development of the kinetics and the mechanisms of the reactions occurring in WtE ash which is important for optimization of reuse and disposal of ash.

Quantitative analysis of residential plastic recycling in New York City.

More than 420,000 tonnes of plastic waste is produced every year in New York City (NYC). This plastic represents 15% of municipal solid waste in NYC and is in line with New York State and United States averages. This material is managed by NYC's dual-stream recycling system and industry-leading material recovery facilities. However, not all plastic collected for recycling (diverted) is ultimately sold to be remanufactured into new products (recovered). This study utilizes publicly available data to quantify and compare the diversion and recovery rates of residential plastics in NYC to provide quantitative context of such a process in a large metropolitan area. In 2018, 35.2% of plastics suitable for recycling were diverted, indicating a potential to improve collection. Of these, only 53.4% of plastics diverted for recycling were ultimately recovered through sale into the markets. This is aligned with the theoretical maximum recycling potential described in other scholarly work. The 53.4% recovery rate of diverted plastics indicates that an increase in diversion would not yield an equivalent increase in recovery. Additionally, barriers to the recovery of plastic waste impact the actual recycling rate. The literature and this study recognize that contamination, technology limitations, and the availability of markets all influence the sorting and selling of plastics. Furthermore, plastic recycling has recently received significant attention due to the implementation of China's National Sword policy. This study demonstrates that from 2017 to 2018, while the sales of plastics #3-7 decreased, the overall recovery rate of plastics in NYC was not impacted by China's National Sword policy.

Approaching a zero-waste strategy by reuse in New York City: Challenges and potential.

In New York City (NYC), the aspiring target of zero waste to landfills is robustly engaging the often poorly understood solid waste management technique of reuse. The reuse activities occurring in NYC are reported, accounting for the quantities of reuse of various products, such as furniture, appliances and automobile accessories, amongst others. The quantities of products are translated to reuse mass and net CO2-eq emissions saved as a consequence of reuse. This quantitative assessment employs the Reuse Impact Calculator (RIC), based on the Waste Reduction Model (WARM). The RIC is a novel calculator used to quantitatively assess the environmental impact of material reuse. It uses the information about the material to be reused from the WARM database and estimates the emissions and energy savings based on the product's final destination, that is, reuse, landfill, recycle or composting. A close monitoring of reuse activities in NYC shows 45 × 106 kg of reuse occurring for different products that would otherwise be directed to landfills. The net emissions reduced annually by reuse is approximately 122 × 106 kg of CO2-eq. This article compares the NYC reuse activities with that occurring in some select cities of the world. It is shown that the maximum recycle potential is saturated at 66%, and only auxiliary strategies like reuse can achieve the zero waste to landfill ambitions. Furthermore, this work discusses the role of reuse in the circular economy, wherein the resource utilization is maximized by increasing the shelf life of the product, and thereby enabling a maximum reuse potential.

New York City's Reuse Impact Calculator: Quantifying the zero waste impact of materials reuse.

In 2015, the city of New York (NYC) introduced a plan to reduce the volume of collected solid waste by 90% by 2030 and envisioned the expansion of reuse opportunities as one of its main drivers. The assessment of the contributions from reuse initiatives to the advancement of waste prevention and waste reduction goals requires a quantitative understanding of the scope of reuse activities. The high population density in NYC and well-organised collection efforts by The City of New York Department of Sanitation (DSNY) (DSNY) have resulted in a structure that enables the informal sector to readily contribute and access the reuse market. Importantly, the scale of the operations in NYC enable the results found to be a model for other municipalities of similar size. This article presents the Reuse Impact Calculator, developed from the need to automate and quantify the environmental impact of product reuse by nonprofit enterprises in NYC. Specifically, we will explain the development process, show the novel characteristics of this calculator, describe the software in terms of data input, auto mapping functionality and calculations and present a case study to demonstrate the implementation of the Reuse Impact Calculator. This calculator is a dynamic and easily modifiable tool that converts diverse datasets to comparable conditions and allows the assessment of the impact of reuse organisations to waste prevention in NYC.

Investigation on electrical surface modification of waste to energy ash for possible use as an electrode material in microbial fuel cells.

With the world population expected to reach 8.5 billion by 2030, demand for access to electricity and clean water will grow at unprecedented rates. Municipal solid waste combusted at waste to energy (WtE) facilities decreases waste volume and recovers energy, but yields ash as a byproduct, the beneficial uses of which are actively being investigated. Ash is intrinsically hydrophobic, highly oxidized, and exhibits high melting points and low conductivities. The research presented here explores the potential of ash to be used as an electrode material for a microbial fuel cell (MFC). This application requires increased conductivity and hydrophilicity, and a lowered melting point. Three ash samples were investigated. By applying an electric potential in the range 50-125 V across the ash in the presence of water, several key property changes were observed: lower melting point, a color change within the ash, evidence of changes in surface morphologies of ash particles, and completely wetting water-ash contact angles. We analyzed this system using a variety of analytical techniques including sector field inductively coupled plasma mass spectrometry, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, and tensiometry. Ability to make such surface modifications and significant property changes could allow ash to become useful in an application such as an electrode material for a MFC.

Heat Generation and Accumulation in Municipal Solid Waste Landfills.

There have been reports of North American landfills that are experiencing temperatures in excess of 80-100 °C. However, the processes causing elevated temperatures are not well understood. The objectives of this study were to develop a model to describe the generation, consumption and release of heat from landfills, to predict landfill temperatures, and to understand the relative importance of factors that contribute to heat generation and accumulation. Modeled heat sources include energy from aerobic and anaerobic biodegradation, anaerobic metal corrosion, ash hydration and carbonation, and acid-base neutralization. Heat removal processes include landfill gas convection, infiltration, leachate collection, and evaporation. The landfill was treated as a perfectly mixed batch reactor. Model predictions indicate that both anaerobic metal corrosion and ash hydration/carbonation contribute to landfill temperatures above those estimated from biological reactions alone. Exothermic pyrolysis of refuse, which is hypothesized to be initiated due to a local accumulation of heat, was modeled empirically to illustrate its potential impact on heat generation.

Mechanistic Insights into Catalytic Ethanol Steam Reforming Using Isotope-Labeled Reactants.

The low-temperature ethanol steam reforming (ESR) reaction mechanism over a supported Rh/Pt catalyst has been investigated using isotope-labeled EtOH and H2 O. Through strategic isotope labeling, all nonhydrogen atoms were distinct from one another, and allowed an unprecedented level of understanding of the dominant reaction pathways. All combinations of isotope- and non-isotope-labeled atoms were detected in the products, thus there are multiple pathways involved in H2 , CO, CO2 , CH4 , C2 H4 , and C2 H6 product formation. Both the recombination of C species on the surface of the catalyst and preservation of the C-C bond within ethanol are responsible for C2 product formation. Ethylene is not detected until conversion drops below 100 % at t=1.25 h. Also, quantitatively, 57 % of the observed ethylene is formed directly through ethanol dehydration. Finally there is clear evidence to show that oxygen in the SiO2 -ZrO2 support constitutes 10 % of the CO formed during the reaction.

CFD analysis of municipal solid waste combustion using detailed chemical kinetic modelling.

Nitrogen oxides (NO x ) emissions from the combustion of municipal solid waste (MSW) in waste-to-energy (WtE) facilities are receiving renewed attention to reduce their output further. While NO x emissions are currently 60% below allowed limits, further reductions will decrease the air pollution control (APC) system burden and reduce consumption of NH3. This work combines the incorporation of the GRI 3.0 mechanism as a detailed chemical kinetic model (DCKM) into a custom three-dimensional (3D) computational fluid dynamics (CFD) model fully to understand the NO x chemistry in the above-bed burnout zones. Specifically, thermal, prompt and fuel NO formation mechanisms were evaluated for the system and a parametric study was utilized to determine the effect of varying fuel nitrogen conversion intermediates between HCN, NH3 and NO directly. Simulation results indicate that the fuel nitrogen mechanism accounts for 92% of the total NO produced in the system with thermal and prompt mechanisms accounting for the remaining 8%. Results also show a 5% variation in final NO concentration between HCN and NH3 inlet conditions, demonstrating that the fuel nitrogen intermediate assumed is not significant. Furthermore, the conversion ratio of fuel nitrogen to NO was 0.33, revealing that the majority of fuel nitrogen forms N2.

Technical assessment of the CLEERGAS moving grate-based process for energy generation from municipal solid waste.

A technical analysis has been completed for a commercial-scale two-stage gasification-combustion system. The CLEERGAS (Covanta Low Emissions Energy Recovery GASification) process consists of partial combustion and gasification of as-received municipal solid waste (MSW) on a moving grate producing syngas followed by full combustion of the generated syngas in an adjoining chamber and boiler. This process has been in operation since 2009 on a modified 330-tonne day(-1) waste-to-energy (WTE) line in Tulsa, Oklahoma. Material balances determined that the syngas composition is 12.8% H2 and 11.4% CO, the heating value of the gas in the gasifier section is 4098 kJ Nm(-3), and an aggregate molecular formula for the waste is C6H14.5O5. The analysis of gas measurements sampled from the Tulsa unit showed that the gasification-combustion mode fully processed the MSW at an excess air input of only 20% as compared to the 80-100% typically found in conventional WTE moving grate plants. Other important attributes of the CLEERGAS gasification-combustion process are that it has operated on a commercial scale for a period of over two years with 93% availability and utilizes a moving grate technology that is currently used in hundreds of WTE plants around the world.

Landfill gas collection efficiency: Categorization of data from existing in-situ measurements.

Landfill methane emissions are commonly estimated using cover-type dependent default collection efficiency values, with a first-order decay model or measured gas collection. Current default collection efficiencies used in the United States were predominately derived from 4 studies conducted during or prior to 2007 that relied on flux chambers. Flux chambers are limited by small sample sizes, placement restrictions, and the inability to measure emissions from gas or leachate collection systems. Since 2007, over 14 new studies have been completed using more advanced technologies that allow for direct measurement of methane plumes from most or all of a landfill's surface. On average, these measurements are 2-3 times greater than emissions predicted by current models and collection efficiency defaults. In lieu of measuring emissions from all landfills, updating collection efficiency defaults can bring modeled emissions into better alignment with measurements. To this end, collection efficiency estimates derived from measured data were categorized into cover types and then adjusted to account for cases where whole plume measurement was an amalgamation of multiple cover types. The resultant adjusted default values were 41% for daily cover, 69% for intermediate cover, and 71% for final cover. Direct measurement of landfill methane emissions is preferrable to account for the full range of variables driving landfill emissions, including collection system design and operation. However, applying these updated defaults back into the landfill emission models eliminates underprediction of landfill emissions for the dataset reviewed, and would provide a more accurate estimate of landfill gas emissions where measurements are unavailable.

Effect of carbon dioxide on the thermal degradation of lignocellulosic biomass.

Using biomass as a renewable energy source via currently available thermochemical processes (i.e., pyrolysis and gasification) is environmentally advantageous owing to its intrinsic carbon neutrality. Developing methodologies to enhance the thermal efficiency of these proven technologies is therefore imperative. This study aimed to investigate the use of CO2 as a reaction medium to increase not only thermal efficiency but also environmental benefit. The influence of CO2 on thermochemical processes at a fundamental level was experimentally validated with the main constituents of biomass (i.e., cellulose and xylan) to avoid complexities arising from the heterogeneous matrix of biomass. For instance, gaseous products including H2, CH4, and CO were substantially enhanced in the presence of CO2 because CO2 expedited thermal cracking behavior (i.e., 200-1000%). This behavior was then universally observed in our case study with real biomass (i.e., corn stover) during pyrolysis and steam gasification. However, further study is urgently needed to optimize these experimental findings.

Processing renewable and waste-based feedstocks with fluid catalytic cracking: Impact on catalytic performance and considerations for improved catalyst design.

Refiners around the globe are either considering or are actively replacing a portion of their crude oil inputs originating from fossil sources with alternative sources, including recycled materials (plastics, urban waste, mixed solid waste) and renewable materials (bio-mass waste, vegetable oils). In this paper, we explore such replacement, specifically focusing on the fluid catalytic cracking (FCC) operation. Five pyrolysis oils, obtained from municipal solid waste (MSW) and biogenic material (olive stones/pits), were fully characterized and tested at 10% loading against a standard fluid catalytic cracking (FCC) vacuum gasoil (VGO) feed in a bench scale reactor using an industrially available fluid catalytic cracking catalyst based on ultrastable Y zeolite to simulate fluid catalytic cracking co-processing. Despite having unique feed properties, including high Conradson carbon (e.g., up to 19.41 wt%), water (e.g., up to 5.7 wt%), and contaminants (e.g., up to 227 ppm Cl) in some cases, the five pyrolysis oils gave similar yield patterns as vacuum gasoil. Gasoline was slightly (ca. 1 wt%) higher in all cases and LPG slightly (ca. 1 wt%) lower. Olefinicity in the LPG streams were unchanged, bottoms and light cycle oil (LCO) showed no significant changes, while dry gas was slightly (up to -0.2 wt%) lower. Coke selectivity was also unchanged (maximum -7.7 wt%, relatively), suggesting minimal to no heat balance concerns when co-processing in an industrial fluid catalytic cracking unit. The results demonstrate the applicability of municipal solid waste and biogenic originating pyrolysis oils into a refinery. A catalyst design concept is explored, based on higher rare Earth oxide exchange and/or utilization of ZSM-5 zeolite, that would further minimize the impacts of replacing fossil oils with pyrolysis oils, namely one that shifts the 1% higher gasoline into LPG.

Mechanistic understanding of polycyclic aromatic hydrocarbons (PAHs) from the thermal degradation of tires under various oxygen concentration atmospheres.

The thermal degradation of tires under various oxygen concentrations (7-30%/Bal. N(2)) was investigated thermo-gravimetrically at 10 °C min(-1) heating rate over a temperature range from ambient to 1000 °C. Significant mass loss (~55%) was observed at the temperature of 300-500 °C, where the thermal degradation rate was almost identical and independent of oxygen concentrations due to simultaneous volatilization and oxidation. A series of gas chromatography/mass spectroscopy (GC/MS) measurements taken from the effluent of a thermo-gravimetric analysis (TGA) unit at temperature of 300-5000 °C leads to the overall thermal degradation mechanisms of waste tires and some insights for understanding evolution steps of air pollutants including volatile organic carbons (VOCs) and polycyclic aromatic hydrocarbons (PAHs). In order to describe the fundamental mechanistic behavior on tire combustion, the main constituents of tires, styrene butadiene rubber (SBR) and polyisoprene (IR), has been investigated in the same experimental conditions. The thermal degradation of SBR and IR suggests the reaction mechanisms including bond scissions followed by hydrogenation, gas phase addition reaction, and/or partial oxidation.

Utilizing carbon dioxide as a reaction medium to mitigate production of polycyclic aromatic hydrocarbons from the thermal decomposition of styrene butadiene rubber.

The CO(2) cofeed impact on the pyrolysis of styrene butadiene rubber (SBR) was investigated using thermogravimetric analysis (TGA) coupled to online gas chromatography/mass spectroscopy (GC/MS). The direct comparison of the chemical species evolved from the thermal degradation of SBR in N(2) and CO(2) led to a preliminary mechanistic understanding of the formation and relationship of light hydrocarbons (C(1-4)), aromatic derivatives, and polycyclic aromatic hydrocarbons (PAHs), clarifying the role of CO(2) in the thermal degradation of SBR. The identification and quantification of over 50 major and minor chemical species from hydrogen and benzo[ghi]perylene were carried out experimentally in the temperature regime between 300 and 500 °C in N(2) and CO(2). The significant amounts of benzene derivatives from the direct bond dissociation of the backbone of SBR, induced by thermal degradation, provided favorable conditions for PAHs by the gas-phase addition reaction at a relatively low temperature compared to that with conventional fuels such as coal and petroleum-derived fuels. However, the formation of PAHs in a CO(2) atmosphere was decreased considerably (i.e., ∼50%) by the enhanced thermal cracking behavior, and the ultimate fates of these species were determined by different pathways in CO(2) and N(2) atmospheres. Consequently, this work has provided a new approach to mitigate PAHs by utilizing CO(2) as a reaction medium in thermochemical processes.

Abiotic decomposition of municipal solid waste under elevated temperature landfill conditions.

Abiotic decomposition of simulated Municipal Solid Waste (MSW) was investigated for thermal reactions that impact landfill gas components such as methane, carbon dioxide, and hydrogen. The gas composition and temperature were monitored as a function of heating rate and time. The tests were conducted at 483 kPa (70 psig), 55 wt% moisture, and 30 to 60 W controlled heat input in the presence of biological inhibitors. The gas composition trends show that for heat inputs higher than 46 W, the CH4/CO2 ratio diverges from the initial value of 1.0 to as low as 0.2, correlated to a decrease in CH4 concentration. Major findings of the study include that the primary gas composition ratio (CH4/CO2) starts to reduce from the baseline value of 1.0 as the heating rate is increased from 30 W to 51 W and further declines at significantly higher rates beyond 51 W. The hydrogen evolution was directly proportional to the amount of CH4 available in the system. Low levels of CH4 (<25%) correspond to decreased H2 levels in the system (<5%) whereas injection of CH4 gas in the system correspond with a renewed H2 generation The study provides insights into the operational conditions such as available heat and moisture leading to changes in landfill gas ratios.

The impact of pressure, moisture and temperature on pyrolysis of municipal solid waste under simulated landfill conditions and relevance to the field data from elevated temperature landfill.

Experiments were conducted with simulated Municipal Solid Waste (MSW) to understand the impact of pressure, moisture, and temperature on MSW decomposition under simulated landfill conditions. Three experimental phases were completed, where the first two phases provided baseline results and assisted in fine tuning parameters such as pressure, temperature, gas composition, and moisture content for phase three. The manuscript focuses on the results from third phase. In the third phase, the composition of the gases evolved from representative MSW samples was tested over time in two pressure conditions, 101 kilopascals (kPa) (atmospheric pressure) and 483 kPa, with varying moisture contents (38 to 55 wt%) and controlled temperatures (50 to 200 °C) in the presence of biological inhibitors. The headspace in the reactor in phase three was pressurized with gas mixture of 50/50 (vol%) of methane (CH4) and carbon dioxide (CO2) setting the initial CH4/CO2 gas composition ratio to 1.0 at time t = 0 days. The results established moisture ranges that affect hydrogen (H2) production and the CH4/CO2 ratio at different temperature and pressure conditions. Results show that at 85 °C, there was a change in the CH4/CO2 ratio from 1.0 to 0.3. Additionally, moisture contents from 47 to 43.5 wt% caused the CH4/CO2 ratio to increase from 1.0 to 1.2, yet from 43.5 to 38 wt%, the ratio reversed and declined to 0.3, returning to 1.0 for moisture levels below 38 wt%. Thus, moisture levels above 47 wt% and below 38 wt%, for the system tested, allow thermal reactions to proceed without a measured change in CH4/CO2 ratio. H2 generation rates follow a similar trend with moisture, yet definitively increase with increased pressure from 101 kPa to 483 kPa. The observed change in solid MSW and gas composition under controlled pressure, moisture, and temperature suggests the presence of thermal reactions in the absence of oxygen.

Role of plastics in decoupling municipal solid waste and economic growth in the U.S.

Analysis of data from the US Environmental Protection Agency (EPA) on municipal solid waste (MSW) generation rates correlated to personal consumption expenditure (PCE) uncovers a decoupling event occurring between 1997 and 2000. A comparison of waste generation rates for each material category found in MSW reveals that plastics increased by nearly 84 times from 1960 to 2013 while total MSW increased only 2.9 times. The increase in plastic waste generation coincides with a decrease in glass and metal found in the MSW stream. In addition, calculating the material substitution rates for glass, metal and other materials with plastics in packaging and containers demonstrates an overall reduction by weight and by volume in MSW generation of approximately 58% over the same time period. A quantitative calculation of a scenario where plastics were not used in packaging and containers to replace glass, metal, and other materials demonstrates that MSW generation rate rises equally with PCE. Therefore, this study has determined that the increase of plastic use is a contributing factor to the decoupling of MSW generation from PCE.

Perspectives on sustainable waste management.

Sustainable waste management is a goal that all societies must strive to maintain. Currently nearly 80% of global wastes are sent to landfill, with a significant amount lacking proper design or containment. The increased attention to environmental impacts of human activities and the increasing demand for energy and materials have resulted in a new perspective on waste streams. Use of waste streams for energy and materials recovery is becoming more prevalent, especially in developed regions of the world, such as Europe, the United States, and Japan. Although currently these efforts have a small impact on waste disposal, use of waste streams to extract value very likely will increase as society becomes more aware of the options available. This review presents an overview of waste management with a focus on following an expanded waste hierarchy to extract value specifically from municipal solid waste streams.

Urban energy mining from municipal solid waste (MSW) via the enhanced thermo-chemical process by carbon dioxide (CO2) as a reaction medium.

The enhanced gasification of municipal solid waste (MSW) using carbon dioxide (CO(2)) as the gasification medium was investigated to achieve environmentally benign and energy efficient ways for the disposal of MSW. Two main steps of thermal decomposition of MSW were observed. The first thermal degradation step occurs at temperature between 280 and 350°C and consists of the decomposition of the biomass component into light C(1-3)-hydrocarbons. The second thermal degradation step occurs between 380 and 450°C and is mainly attributed to polymer components, such as plastics and rubber, in MSW. To extend this understanding to a more practical level, MSW samples were tested in a drop tube reactor (DTR) at a temperature range from 500 to 1000°C under various atmospheres with CO(2) concentrations of 0-30%. The release of major chemical species from the DTR has been determined using a micro-GC. For example, CO (≈ 30%), H(2) (≈ 25%) and CH(4) (≈ 10%) were generated.

CO2 as a carbon neutral fuel source via enhanced biomass gasification.

The gas evolution, mass decay behavior and energy content of several woods, grasses, and agricultural residues were examined with steam and CO(2) gasification using thermogravimetric analysis and gas chromatography. CO(2) concentrations were varied between 0 and 100% with steam as a coreactant. Carbon conversion was complete with 25% CO(2)/75% steam compared to 90% conversion with pure steam in the temperature range of 800-1000 degrees C. The largest effect was from 0-5% CO(2) introduction where CO concentration increased by a factor of 10 and H(2) decreased by a factor of 3.3 at 900 degrees C. Increasing CO(2) from 5 to 50% resulted in continued CO increases and H(2) decrease by a factor of 3 at 900 degrees C. This yielded a H(2)/CO ratio that could be adjusted from 5.5 at a 0% CO(2) to 0.25 at a 50% CO(2) concentration. Selection of the gasification parameters, such as heating rate, also enabled greater control in the separation of cellulose from lignin via thermal treatment. 100% CO(2) concentration enabled near complete separation of cellulose from lignin at 380 degrees C using a 1 degrees C min(-1) heating rate. Similar trends were observed with coal and municipal solid waste (MSW) as feedstock. The likely mechanism is the ability for CO(2) to enhance the pore structure, particularly the micropores, of the residual carbon skeleton after drying and devolatilization providing access for CO(2) to efficiently gasify the solid.