Article pubs.acs.org/EF
Reactivity Analysis of Pakistani Thar Lignite Reserves in Oxidizing Thermogravimetric Analysis Atmospheres Asim H. Rizvi,† S. Sheraz Daood,*,‡ M. Tayyeb Javed,§ Shahid Munir,† Mohamed Pourkashanian,‡ and William Nimmo‡ †
Institute of Chemical Engineering and Technology, University of the Punjab, Lahore 54590, Pakistan Energy Engineering Group, Energy 2050, Department of Mechanical Engineering, University of Sheffield, First Floor, The Arts Tower, Western Bank, Sheffield S10 2TN, United Kingdom § Department of Chemical Engineering, Pakistan Institute of Engineering and Applied Sciences, Nilore, Islamabad, Pakistan ‡
ABSTRACT: In this study, three lignite coal samples from the largest lignite-based coalfield in Sindh, Pakistan, were sourced together with four bituminous coal samples from the Khushab District in Punjab, Pakistan. A non-isothermal thermogravimetric analysis of these coal samples was performed at two heating rates of 10 and 40 °C/min in a Shimadzu TGA-50 thermogravimetric analyzer under reaction environments comprising 21−30% O2/70−79% N2 and 21−30% O2/70−79% CO2 to investigate their specific reaction kinetics under these four different combustion atmospheres. The inherent activation energies and orders of reaction under the aforementioned reaction atmospheres were calculated using the Friedman method (a differential isoconversional approach) and the Ghetti method. In this work, the primary dominating factor on the rate of mass loss from devolatilization was the higher heating rate (40 °C/min), and the second factor was the higher oxygen concentration of 30%. Overlapping of devolatilization and char oxidation zones was also observed at higher oxygen concentrations of 30%. Increased reactivity of the Pakistani lignite was observed at a heating rate of 40 °C/min and high oxygen concentrations of 30%.
1. INTRODUCTION The demand for electrical power in this day and age continues to increase with the increase in population and technological and economic advancements. Coal combustion is imperative in energy production worldwide, especially considering that there are 892 billion tonnes of proven global coal reserves.1 Oxy-fuel combustion technology has been suggested as one of the most promising technologies for CO2 capture from power plants. In this concept, oxygen mixed with recycled flue gas (RFG) replaces the normal combustion air, and this results in a flue gas that consists primarily of CO2 (concentrations greater than 95%), instead of nitrogen, also controlling the flame temperature. Several numerical and experimental studies were conducted to understand the suitability of this concept.2−5 The non-isothermal thermogravimetric analysis (TGA) method is employed extensively by many researchers who wish to evaluate the combustion characteristics of solid fuels because of its simple, accurate, one-step experimental run under oxy-fuel combustion environments.6−8 TGA is an economical and simple conventional method compared to a pilot-scale combustor. It provides a prompt and definitive quantitative method for thermochemical processes. TGA studies aiming to investigate the most important reactivity profile parameters for lignite (peak temperatures and apparent energy of activation) lack extensive research, especially for the newly explored resources.8 In recent works, researchers have underpinned the importance of poor quality brown coal in the overall energy mix.9,10 Pakistan has the world’s largest lignite coal reserve comprising 175 billion tons spread over 9600 km2 and having an electrical potential equivalent to 100 000 MW or 50 billion tonnes of oil equivalent.11 This particular site was discovered by © XXXX American Chemical Society
the Geological Survey of Pakistan in 1992; however, because of the lack of intensive investigation, the role of coal in the energy mix never gained significant importance in Pakistan.12 The technological option of either directly using lignite as a fuel or its products as a substitute to oil or natural gas necessitates studies under various reaction atmospheres.8−10,13 The important combustion parameters have been evaluated for the various reaction atmospheres (air, oxygen-enriched air, oxyfuel, and oxygen-enriched oxy-fuel), especially for lignite and lignite−biomass blends.13−17 The combustion performance has been reported to significantly increase, especially for an O2 concentration of
