On the combustion and pyrolysis of fossil and alternative fuels to minimize their environmental impacts

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Climate change and generation of solid wastes are among the most pressing challenges of the 21st century. Concerns about the environmental impact of greenhouse gas emissions from the combustion of fossil fuels have promoted the use of alternative sources of energy. These include renewable biomass and organic wastes (both agricultural and post-consumer). Options include firing these alternative fuels in dedicated boilers or co-firing them with coal in existing coal-fired boilers, as such infrastructure is already available, and it only requires limited modifications. In both cases, knowledge on the details of the pyrolysis and combustion behaviors of these fuel types is needed. Thus, in this work investigations have been conducted where: (i) Torrefaction, pyrolysis and combustion characteristics of pulverized biomass (woody, herbaceous and crop wastes) were assessed. Pyrolysis and combustion of raw and torrefied biomass particles took place under high heating rates and temperatures, similar to those prevailing in utility furnaces. Using optical and scanning electron microscopy it was determined that biomass particles undergo fusion and melting, and thereby their typical initial needle shapes transform to spheroidal or elliptical shapes. (ii) Combustion details of biomass were investigated by burning individual biomass particles, both raw and torrefied, of a priori-know mass, size and shape, under experimental conditions that are suitable for utility boilers. Based on a combination of these experimental techniques and appropriate numerical modeling it was determined that torrefied biomass chars that are formed under high heating rates and high temperature conditions burned as thin-wall cenospheres. (iii) A methodology was established for finding the torrefied biomass grind size that is suitable for firing or co-firing with coal in existing pulverized fuel boilers. Based on pyrometric and cinematographic observations it was determined that the burnout times of biomass particles of nominal sizes smaller than 300 μm (Mesh 200) matched those of coal particles of 75-90 μm (Mesh 50), which is a size selected based on industry standards. Hence, grinding torrefied biomass to smaller particle sizes may not be necessary as it results in higher grinding costs (iv) A methodology was developed for lowering the emissions of greenhouse gases from co-firing of high-sulfur coals with low-sulfur coals of different ranks. It was found that high alkali content, highly fragmenting lignite coals can serve as effective sulfur sorbents for high sulfur bituminous coals. (v) A previously proposed method for "up-cycling" post-consumer wastes into the value-added products of carbon nanotubes (CNTs) and gaseous fuels was improved to increase both their yields and their quality. Based on explored combinations of methods for pre-treating different types of stainless steel catalysts that increase the yield of CNTs from common waste plastics, it was found that the catalyst type, composition and pre-treatment method, as well as the type of feedstock, are all influential on the yields and physical characteristics of the synthesized CNTs.