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Thermodynamics, Transport, and Fluid Mechanics
Analysis of solids flow rate through non-mechanical L-valve in an industrial scale Circulating Fluidized Bed using Group B particles Esmail R. Monazam, Ronald W Breault, Lawrence J. Shadle, and Justin M. Weber Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01051 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018
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Industrial & Engineering Chemistry Research
Analysis of solids flow rate through non-mechanical L-valve in an industrial scale Circulating Fluidized Bed using Group B particles Esmail R. Monazam+, Ronald W. Breault*, Lawrence J. Shadle, Justin M. Weber
National Energy Technology Laboratory U. S. Department of Energy 3610 Collins Ferry Rd. Morgantown, West Virginia 26507-0880 + REM Engineering Services, PLLC 3537 Collins Ferry Rd. Morgantown, West Virginia 26505
ABSTRACT A series of statistically designed experiments were conducted using two different Geldart Group B particles in a 0.3 m diameter circulating fluidized bed cold model to evaluate the non-mechanical L-valve for controlling the solids flow rate. The objective of this study was to investigate the effects of standpipe aeration, L-valve aeration, solids inventory and superficial gas velocity through the riser on the solids flowrate. A stochastic correlation was developed for calculating the solid flow rate as a function of these variables. It was found that the solid flow rate increased directly proportional to each of these variables. Dimensional analysis was used to generate an expression for the solids turnover ratio. The model developed in the present study is intended to simulate flow in the present CFB loop. It is not intended for scale-up and will probably not apply for alternative CFB loops with different geometries / pressure balance conditions.
Keywords: Circulating Fluidized Bed, L-valve, non-mechanical valve, solids flow, Group B particles INTRODUCTION Circulating fluidized bed (CFB) has become the leading new combustion technology for power generation because of its favorable heat rates, environmental performance, and fuel flexibility.1 A typical CFB is made of main components including: a riser, a gas–solid cyclone, a standpipe and a solid recycle valve. In general, the solid circulation rate in CFBs is controlled by adjusting a valve (mechanical or non-mechanical), installed at the bottom end of the standpipe. Mechanical valves are difficult to operate in harsh environments under elevated temperature and pressure conditions because of sealing and mechanical problems, and the problems are greatly exacerbated by the presence of granular materials. Non-mechanical valves (L-valve, V-valve, and J-valve) are commonly employed for such multiphase *
Corresponding author: Tel. 304-285-4486; fax – 304-285-4403; email –
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applications.2-7 Non-mechanical valve employ external gas injection to control the solid circulation rate in CFB’s. In the design of CFB’s, the solid circulation rate is one of the important parameters, especially when is used as a catalytic reactor to enhance the reaction rates. Many researchers have conducted fundamental experiments to estimate the solid circulation rate through non-mechanical valves. Non-mechanical L-valves have been employed widely to control the solid circulation rate in CFB’s due to its simplicity and inexpensive.8 Knowlton and Hirsan4,7 were the first to investigate the relationship of solids flow rate and the volumetric flow rate of air in an L-valve. They found that smaller diameter L-valves have lower throughput than larger diameter L-valves. Knowlton and Hirsan7 reported that the solid flow rate in the L-valve is controlled by the total amount of gas passing through the L-valve. This includes both external aeration and the gas flow coming down the standpipe with the solids. Yang and Knowlton9 correlate the solid flow rate as a function of pressure drop between the exit of the horizontal section and the aeration tap in L-valves. They demonstrated that there is a linear relationship between the aeration rate in the L-valve, and the effective open area in the horizontal section of the L-valve. Yang and Knowlton9 concluded that the minimum aeration rate required to flow solids depends upon the geometry of the horizontal leg of the L-valve. Geldart and Jones2 studied the effect of aeration rate on solid circulation rate. At low aeration rate, most of the particle are in stagnant state, while only the solid in the upper section flow. At high aeration rate, dune-like structures appear in the bottom of horizontal the section and causing fluctuations in pressure and solid flow rate. Most of the L-valve studies have been carried out in an open configuration with a simple hopper, Lvalve, and reservoir. 2,8,10-15 Knowlton and Hirsan ran tests in a configuration using valves in the standpipe and riser which allowed both open and closed loop operations.4,7 Alternatively, a slide valve is often incorporated under a fluidized bed standpipe creating a decoupled-closed configuration.9 These practices help to decouple the standpipe from the riser. It should be recognized that in these systems the decoupling is not absolute and, unlike valves with sonic gas flow two-way communication of gas flow through the valve is possible. In an open system the solid flow rate depends strongly on the pressure drop across the standpipe of the L-valve or vice versa. Therefore, most correlations are developed to predict the solid flow rate as a function of pressure drop or vice versa (pressure drop as a function of solid flow rate). In this study, a cold flow circulating fluidized bed was operated with a non-mechanical L-valve to control the solids flow rate of Geldart B particles at ambient temperature. This represents a coupled closed loop configuration. The objective of this study was to determine the influence of standpipe aeration, L-valve aeration, solids inventory, and riser superficial gas velocity on the solids flow rate. A stochastic correlation that predicts the solids flow rate across the L-valve as a function of these variables is developed and discussed. EXPERIMENTAL SECTION The experiments were conducted in a circulating fluidized-bed cold flow unit, which is shown in Fig. 1. The CFB was a closed loop system with the unique capability of accurately measuring solids flows and adjusting solids inventory. The main components in the loop consisted of a riser, a two-stage cyclone, a standpipe, and a non-mechanical L-valve. The riser was 30.48-cm diameter and 15.45 m high which consisted of metal and acrylic spool pieces such that solid movement in the riser could be observed through the transparent acrylic sections. The main fluidizing air was fed through a perforated plate having
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Industrial & Engineering Chemistry Research
1,444 holes of 0.3175 cm diameter to provide 40% open flow area through the distributor. The distributor is located between the flanges located 0.43 m below the centerline of the L-valve. Solids exit from the top of the riser to the primary cyclone through a blinded-tee arrangement, which create a stagnant volume of solids extending approximately 90 cm above the exit point. Solids collected by the primary cyclone fall into a 25.4-cm diameter standpipe of 12.2 m high. A secondary cyclone and a baghouse remove fine particles not captured by the primary cyclone. The materials collected in the secondary cyclone represents abraded fines of very small particle size (
