The measurement of the rate of burning of different coal chars in an electrically heated fluidised bed of sand

• Published in: Chemical engineering science Vol. 62; no. 1; pp. 608 - 618
• Main Authors: Fennell, P.S., Kadchha, S., Lee, H.-Y., Dennis, J.S., Hayhurst, A.N.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2007
• Subjects: Combustion of coal chars; Fluidised beds; Sampling; Sampling; Combustion of coal chars; Fluidised beds
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2006.09.024

This work measured the rates of burning of three coal chars. This was done by adding small batches ( ∼ 3 mg ; particle size 106– 150 μ m ) of a char to a hot bed of silica sand (diam. 90– 126 μ m ) fluidised by different mixtures of O 2 + N 2 , varying from 0 to 100 vol% O 2 . The bed was electrically heated and maintained at 700, 800, 900 or 950 ∘ C . The concentrations of CO 2 and CO in the gases leaving the bed were continuously measured after adding a batch of char and used to derive the rate of oxidation of carbon. It was found that [ CO ] ⪡ [ CO 2 ] in the off-gas. The indications are that C s + 1 2 O 2 → CO is a major reaction of a coal char, with the CO being oxidised in a fluidised bed to CO 2 either in bubbles or on top of the hot sand. The initial rate of burning of these tiny char particles was measured to check if it relates to the initial properties (B.E.T. surface areas, internal diameters of the pores, etc.) of each char. These physical properties were measured here. The char particles were sufficiently small ( mean diam. = 126 ± 20 μ m ) to ensure that external mass transfer did not control the rate of reaction. Likewise, the mass of char added in a batch ( ∼ 3 mg ) was small enough for inter-phase mass transfer to be unimportant. Furthermore, heat transfer from one of these fluidised beds to a burning char particle was so fast that a particle was no more than 2.5 K hotter than the fluidised bed. Finally, the size of the char particles, coupled with their fairly wide pores made in effect all their internal surfaces available for reaction with O 2 . In these experiments, the small size of the sand ensured that, even with U / U mf ∼ 10.7 , little elutriation of the char occurred. One difficulty encountered here is that char of the lignite can react so quickly that [ CO 2 ] measured in the off-gas rises to a maximum and falls to zero in times sometimes as short as ∼ 30 s . That the measured [ CO 2 ] was changing quickly, coupled with the fact that there was mixing of gases inside and above the bed, made it difficult to measure the initial rate of reaction. A procedure for deconvoluting the measured [ CO 2 ] is arrived at, but is seen to be problematic. Char of an anthracite coal oxidises slower than a lignitic char; it does so with kinetics whose order in O 2 is ∼ 0.9 at 700 ∘ C , but ∼ 0.35 at 950 ∘ C , rendering it impossible to measure an activation energy. The order for a bituminous char is ∼ 1 2 , but is lower at higher temperatures. Measurements for the lignitic char are uncertain. The indications are that its burning rate is zeroth order in oxygen; the associated activation energy appears to be close to zero. It is also concluded that a char's reactivity is not entirely determined by the surface area it has available for reaction.