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The global interest in moving away from carbon dioxide (CO2) and greenhouse gas (GHG) emitting fuels is primarily motivated, not only by global warming, but also the need to better utilize combustible waste products. In a 2010 study conducted at the Technical University of Denmark, researchers investigated the pyrolytic behaviors of coal and four different waste components, as well as their mixtures through the use of a simultaneous thermal analyzer (STA)
The Efficiency of Waste Combustion
A considerable amount of municipal solid waste components is comprised of up to 80% CO2 neutral fuels; therefore, researchers are often interested in developing waste combustion techniques that are efficient and safe for the environment. One of the most common combustion techniques utilized on power plants around the world involves the co-combustion of these waste products with coal.
One of the most critical steps of this co-combustion process is pyrolysis, as this step has been shown to affect the volatile release and char reactivity of waste mixtures. Since most waste products vary significantly in their composition, it is often a challenge for researchers to predict their reactivity during combustion accurately. To address these concerns, the 2010 study investigated the interaction within different mixtures of coal and four major waste components during pyrolysis.
Methodology
The four major waste components investigated in this study included wood, paper, polypropylene (PE) and polyvinyl chloride (PVC) and Columbian bituminous coal. To obtain pyrolysis measurements of these individual samples, as well as their mixtures, a NETZSCH STA 490C with platinum/rhodium crucibles were used to measure the weight loss of the samples as a function of time.
The pyrolysis experiment involved heating samples within the weight range of 3 to 15 milligrams (mg) at a rate of 20 K/min with a flow of 100 ml/min N2. To begin, samples were originally heated to 120 °C and remained at this temperature for 30 minutes until the heating temperature ultimately reached 800 °C. Both thermogravimetric (TG) and derivative thermogravimetric (DTG) data were obtained for all samples and their mixtures.
Results
The pure coal sample was found to exhibit a slow decomposition reaction that occurred over a wide temperature interval. This reaction characteristic is expected due to the various types of bonds, many of which are resonance stabilized, that are present within most coal samples. The pure wood sample, which is comprised primarily of cellulose, hemicellulose, and lignin, was also found to exhibit a slow decomposition reaction over a wide temperature range.
Paper, which exhibits a similar chemical composition to that of wood, exhibited similar decomposition curves. However, a clear second decomposition reaction of paper was found to occur at 620 °C, which was attributed to the unknown chemicals that are typically added to paper during production processes. The decomposition of PE was found to exhibit a steep decomposition curve at approximately 485 °C, whereas the decomposition of PVC generally occurs in two different bond breaking steps.
Regarding the decomposition of the waste mixtures, the strongest interactions were found between wood and PVC, paper and PVC, as well as coal and PVC mixtures. Slightly strong interactions were also identified for coal and PE, paper and PE, wood and PE, as well as PE and PVC samples. No interactions were found between coal and paper, coal and wood or paper and wood.
Conclusion
Since the only waste component that was found to interact with coal consistently was PVC, the researchers of this study concluded that the interaction between coal and an actual waste mixture sample would demonstrate similar results.
Reference
- Munther, A., Wu, H., & Glarborg, P. (2010). Thermogravimetric analysis of combustible waste components. The Technical University of Denmark, Department of Chemical Engineering.
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