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Energy & Fuels 2006, 20, 2572-2579
Simulation of Blast-Furnace Raceway Conditions in a Wire-Mesh Reactor: Interference by the Reactions of Molybdenum Mesh and Initial Results Long Wu, N. Paterson,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK ReceiVed June 22, 2006. ReVised Manuscript ReceiVed July 24, 2006
Direct coal injection into the blast-furnace raceway is being used as a means of reducing the metallurgical coke requirement. However, at high rates of injection, operating difficulties are found, which are caused by incomplete coal conversion. A novel trapped air injection system has been built for a wire-mesh reactor to enable tests with short exposure times to air that are intended to simulate typical residence times in blastfurnace raceways. Initial tests have shown that the molybdenum wire-mesh sample-holder reacts with O2 under conditions intended for this work. By varying the proportions of solid MoO2 (weight gain), vapor phase oxides (weight loss) may form, depending on reaction conditions. Oxide formation pathways thus become relevant to coal weight loss determinations during experiments. If, in addition to solid MoO2 formation, significant formation of vapor phase oxides occurs, then the weight change is more complicated to understand and the impact on the O2 concentration cannot be unravelled. Furthermore, it turns out that O2-scavenging by the mesh affects the amount of O2 that is available to react with the coal sample. It was concluded that it is only possible to conduct reliable tests under conditions which the favor the formation of solid MoO2 only, as this leads to a quantifiable weight gain. Its impact can then be accounted for in the evaluation of the experimental weight change. In the case of MoO2 formation, the impact of the mesh oxidation on the amount of O2 available to react with the sample can also be estimated. It has been found that the wire-mesh reactor, equipped with the trapped air injection system, can be used to obtain valid data at up to 1600 °C and 0.5 MPa. This pressure is similar to that of the blast-furnace raceway, but the temperature is several hundred degrees lower. However, preliminary tests have shown that useful kinetic data on the extents of reaction can be obtained with the equipment, provided it is operated under conditions that minimize the formation of vapor phase Mo oxides.
Introduction The cost of the metallurgical coke used in blast-furnaces is high in both economic and environmental terms. This has led to the use of coal to replace a portion of the coke feed, as this reduces the impact of coke manufacture on the production of the iron. The coal is added through a lance into the preheated air (or enriched air) blast as it passes, at high velocity, from the tuyere to the raceway at the base of the furnace shaft.1 A schematic diagram of the blast-furnace injection system is shown in Figure 1. The raceway is a high temperature reaction zone that forms at the point where the air blast meets the base of the coke bed. It has been found that operational problems are encountered when a high coal injection rate is used (i.e., when it exceeds approximately 200 kg of coal/ton of pig iron2). These include unacceptable increases in the level of fines production, which are blown out from the furnace top, and poor drainage of molten slag and iron downward through the bed. These problems are related to the extent of reaction that occurs in the raceway region of the furnace. Several previous studies3,4 have attempted to examine the factors that influence the extents of reaction occurring in this region. However, a more detailed * Corresponding author:
[email protected]. (1) Biswas, A. K. Principles of Blast Furnace Iron Making: Theory and Practice; Cootha: Brisbane, 1981. (2) Yamakuchi, K.; Ueno, H.; Tamura, K. ISIJ Int. 1992, 36 (6), 716. (3) Maki, A.; Sakai, A.; Takagaki, N.; Mori, K.; Ariyama, T.; Sato, M.; Murai, R. ISIJ Int. 1996, 36 (6), 650. (4) Chung, J. K.; Hur, N. S. ISIJ Int. 1997, 37 (2), 119.
Figure 1. Schematic diagram of the blast-furnace injection system.7
understanding of the complex physical and chemical processes governing the extents of coal reactions in this region is needed. In an earlier study in this laboratory,5 a wire-mesh reactor was modified to enable the simulation of conditions existing in the tuyere and raceway region of a blast-furnace. This type of reactor enables precise and variable control of the reaction conditions (time, temperature, heating rate, pressure, and gas composition), using a monolayer of fuel particles, held between (5) Pipatmanomai, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2003, 17 (2), 489.
10.1021/ef060289r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/07/2006
Simulation of Blast-Furnace Raceway Conditions
two layers of electrically conducting wire mesh.6 A continuous stream of sweep gas can provide a reactive environment and minimizes interactions between pyrolyzing char and evolved volatiles. The preliminary development of the wire-mesh reactor for pulsed air tests simulating coal behavior in blast-furnace tuyere and raceways has already been described.5 Briefly, a pair of solenoid valves was used, to enable short pulses (5-500 ms) of air or O2-enriched air to be injected through the sample holder, once the particles had reached the peak experimental temperature. The short pulse injection system enabled injectant coal reactions in the raceway to be studied under more representative conditions than had hitherto been achieved. However, the air injection rate was limited, and initially, the system could only operate slightly above atmospheric pressure and temperatures up to 1500 °C. The pressure limitation was due to the maximum allowable inlet pressure (and the large pressure drops) of the solenoid valves, while the temperature was limited by the properties of the thermocouple wires (PtPt/Rh). This apparatus enabled the measurement of the extents of successive pyrolysis, char combustion, and CO2-gasification reactions which occurred under conditions that approached the raceway conditions. The release of volatiles was found to be complete within the heatup period (∼300 ms, at a heating rate of 5000 °C s-1). For 20 ms air pulse times, the complete consumption of the inlet O2 occurred (with concentrations in the range 21-75%) and the extent of combustion was limited by the amount of O2 supplied (i.e., it was diffusion limited). Extents of CO2-gasification were measured in the temperature range 800-1500 °C. The results showed that reaction times of tens of seconds were needed to achieve measurable extents of gasification. Overall, the results indicated that the extents of combustion and gasification of the char in the raceway (residence times
