(328e) A Kinetic Investigation of Coal Conversion to Metallurgical Coke: Influence of Particle Size, Density and Porosity

A series of experiments were performed using a thermogravimetric analyzer (TGA) to extract the kinetic parameters associated with converting sub-bituminous coal to metallurgical coke.  The TGA was close-coupled to an online micro-gas chromatograph in order to continuously measure the vapor products evolved during reaction with a Helium flow of 100 ml min^-1.  The particle diameter of the coal sample tested ranged from 5mm to 1000mm.  Heating rates of 3 and 10 ^oC min^-1 were used to observe the evolution of volatile matter and the production of tar and tar surrogates.   Additionally two reactor confinement configurations were tested, a 14.2875 mm wide by 1.5875 mm deep (shallow pan) and a 6.35 mm wide by 1.1125 mm deep (cylindrical crucible), to elucidate the effect of bed depth on vapor product evolution.  These configurations provide insight into the relative ratio between condensation, cracking and volatile species reactions.  Finally, coal bed density and porosity variations were investigated.  The bed density ranged between 0.8267 and 1.653 g cm^-3and the porosity varied accordingly to the particle size and bed density.  Influence of these experimental conditions on reaction kinetic parameters will be presented. Understanding the influence of particle size, bed density, and porosity on the kinetics of this process is a concern to the preprocessing of coal.   

Coke is used as a reducing agent in an iron blast furnace for steel production. Coke is produced from bituminous coal. At high temperatures, solid coal undergoes partial melting and resolidification resulting in hard porous carbon deposits referred to as coke. The interest of this work is to better understand the coal-to-coke, carbon deposition, volatile matter and tar evolution process. Solid granular or powdered samples are thermally processed at constant heating rate in inert environment similar to pyrolysis atmospheres. The main focus for this research is the reaction mechanism for slow to intermediate heating rates which produce gases and solids. Analyzing the coal in different configurations provides insight into the relative ratio between condensation, cracking and volatile species reactions. Additional parameters studied were bed density and porosity. In general, this work explores the influence of particle size, bulk density, macroporosity, heating rate, and crucible confinement on the kinetics of converting coal into metallurgical coke.

Clean bituminous coal from a single source was used as the reference for scoping and testing purposes. Various bio-carbon materials are to be discussed and pursued. Experiments using a Netzsch Luxx 409 STA (TGA) will be used to determine kinetic rate parameters for reaction rate calculation. The TGA was coupled to online gas chromatograph to measure combustion products and supplement the determination of the reaction mechanism. This type of analysis has been successfully done before for coal and biomass gasification experiment.  Importantly the TGA heating rate can be adjusted widely to ensure proper heat transfer to samples minimizing transport impacts. The heating rate can be accurately controlled from 0.5 ^oC min^-1 and the maximum furnace temperature is 1250 ^oC.  The low heating rate tracking has been demonstrated to be very precise and repeatable. The instrument has differential scanning calorimetry (DSC) capability used for the detection of phase transition during heating. The effluent of the TGA is measured in real-time, for pre-selected gases, such as CO₂, O₂, CH₄, C_xH_y, CO, H₂, etc.  There is no limitation on the gas chosen for real-time analysis, only that it be selected prior to beginning a test. This system has been extensively used to determine reaction mechanisms for various solid material under different atmospheres.  A condenser can be used a tar trap which allows characterization of the tars following each test, provided there is enough generated. The condensed tars could then be characterized separately. However, this method lead to composite analysis of all tar generated during the particular test. It is possible to analyze the tar directly using GC/MS or FT-IR in real-time evolution.

The key to experimental designs are to ensure kinetically controlled regimes. The sweep gas flow rate governs the residence time of volatiles as well as convective heat and mass transport. The sweep gas flow rate remained constant in these experiments. One of the confinements tested was a pan-like crucible (14.2875 mm wide, 1.5875 mm deep) filled with a sample of uniform particle sizes and sonicated for 10 minutes. The other confinement was is in a more traditional cup-like crucible (6.35 mm wide, 1.1125 mm deep) filled with the same mass as the pan-like crucible and sonicated in the same manner. The confinements between the pan and the cup will exemplify different chemistry. In the cup crucible, we attempt simulate the confinement found in the vertical slot oven. Volatile material will enter the gas phase and have to diffuse throughout the material. Secondary reaction will occur as the gas diffuses out. These secondary reactions could be condensation or deposition. In the pan crucible, we attempt simulate the confinement found in a horizontal slot oven. The volatile material will enter the gas phase and will be carried off by the sweeping gas. The gaseous products were measured by a coupled micro gas chromatograph. For the packed confinements, the samples of uniform particle size will be compressed to constant bulk densities. The varying particle size in turn varies porosity in the sample. The porosity governs the diffusion of gases out of the particle. The smaller particles give a more kinetically controlled regime. Between the sonicated and compressed experiments we can simulate grinded and compressed feedstock. Porosity as function of conversion can be investigated in future work. Multiple heating rates were studied in each configuration. The higher heating rate is used to drive off a greater amount of volatile material. The upper limit was chosen to be 10 ^oC min^-1. The lower heating rate was chose for the slow heating rate used in the coal to coke processes and was based upon ASTM standard test.