Experimental and Theoretical Investigations on High-ash Coal-air Flames in High-speed Jets Stabilized Recirculating Flow

ABSTRACT

In this paper, experimental and theoretical investigations on the stability of coal-air flames in high-speed jets stabilized recirculating flow are presented. The current work reports the operational limits of high-ash coal-air flames stabilized by large velocity differentials. Experiments are performed to investigate the effects of particle size distribution, flow rate of primary and high-speed jets, and feed rate of coal on the stability of recirculating coal-air flames. Temperature measurements of the primary reaction zone are used to assess the stability of the reactor under various operating conditions. Experiments are carried out with two different particle size distributions, namely: (1) the fine ($<$355 $\mu$m), and (2) the coarse ($<700$ $\mu$m) distributions. Investigations are carried out with a range of primary jet velocities (4 m/s to 9 m/s), high-speed jets mass flow rates (1.24$\times {10}^{-3}$ to 4.94$\times {10}^{-3}$ kg/s), and feed rates (1.11$\times {10}^{-3}$ to 2.1$\times {10}^{-3}$ kg/s). A nondimensional ignition index ($\tau$) which is a ratio of the particle flow time ($t_f$) to particle ignition time ($t_{ig}$) is shown to indicate the stability of recirculating coal-air flames. Temperature measurements indicate that the operating conditions with τ>0.8$\tau >0.6$ are stable whereas those with $\tau <0.6$ lead to reactor quenching. Force balance on particles with numerically resolved velocity profiles is used to estimate the particle flow time. Particle ignition times are quantified by the extended unified ignition-devolatilization model. Results indicate a strong influence of primary jet velocity on the stability of the flame. Only a marginal increase in stability can be attributed to the high-speed jets. Particle size distribution, along with the feed rate plays a crucial role in determining the stability of recirculating coal-air flames. Temperatures with the fine distribution exceed ash-fusion temperature causing slag deposition in the reactor. Model-based inferences on the increase in stability of recirculating coal-air flames with preheated coal are presented.

KEYWORDS:

• Recirculating coal-air flames
• ash-fusion
• stability regimes
• high-ash coal
• zero preheating