Time Series Analysis and the Propagation of Pressure Pulses in a Solids Circulation System

In advanced energy process technologies, like the advanced pressurized circulating fluidized bed (CFB) process conceptualized by Foster Wheeler Corp., there exists a need to transport and control the flow of solids from a high pressure vessel to a low pressure vessel. Solids transport has long been done in the refinery industry and in the CFB combustor industry, but always from a low pressure vessel to a high pressure with the use of a pressure building standpipe. The key issue in the transport of solids from a high pressure vessel to a low pressure vessel is in the control of this process. At low temperatures, a valve can be used. However, at high temperatures, the use of a valve is not possible. A non-mechanical solution is required: the N-Valve. The N-Valve is a non-mechanical valve developed to control the transfer solids from a high pressure vessel to a low pressure vessel. Unlike conventional non-mechanical valves (L-Valve, J-Valve and Loop seal) pressure does not have to be built within a standpipe to promote the transfer of the solids. The differential pressure between the two vessels is more than enough to cause the solids to flow from the high pressure vessel to the low pressure vessel. Thus, the N-Valve was configured with a vertical lift which dissipated the pressure difference between the two vessels. The work presented in this paper outlines the successful operation of a cold flow scaled model of the N-Valve conceptualized for the advanced pressurized CFB. Specifically, the paper reports on the stability of the solids flow rate entering and exiting the N-Valve, the ability to control/modulate solids flow with move air, and the effect of the particulate filter back pulse on the system static pressure and on the N-Valve pressure drop. Introduction The N-Valve is a non-mechanical valve developed to control the transfer solids from a high pressure vessel to a low pressure vessel. Unlike conventional non-mechanical valves (L-Valve, J-Valve and Loop seal) pressure does not have to be built within a standpipe to promote the transfer of the solids. The differential pressure between the two vessels is more than enough to cause the solids to flow from the high pressure vessel to the low pressure vessel and given that you could have a mechanical valve operate in the hot-erosive hostile environment solids transport could be done quite easily and effectively. The problem is that mechanical valves will not operate in the hot erosive environment. Thus, the N-Valve was configured with a vertical lift which dissipated the pressure difference between the two vessels. N-Valve Concept In an Advanced Pressurized Fluid Bed (APFBC) application [1] an N-valve has been proposed to transfer a char fuel from a high-pressure, spouted fluid bed carbonizer to a lower pressure, circulating fluid bed combustor. An N-Valve is a non-mechanical device that is used to provide a pressure seal and to control the solids flow rate between a high pressure vessel and a low pressure vessel. Foster Wheeler and PSRI had designed a completely integrated APFBC system using this non-mechanical valve in the Power System Development Facility (PSDF) in Wilsonville, Alabama [1,2]. In that pilot plant the N-valve was designed to carry hot char from a fluidized bed carbonizer to a circulating fluidized bed combustor. A simplified sketch of an N-Valve system is shown in Figure 1. In this figure, solids are shown being transported from the high pressure fluidized bed to a lower pressure riser of a circulating fluidized bed reactor. Solids spill from the pressurized fluidized bed at A and descend to point B where they are picked up in a vertical transport section. The solids are transported vertically up in a step that dissipates the pressure difference between the high pressure vessel and the low pressure vessel (location C). The solids then fall into the riser (location D). The “N” in the N-Valve is more apparent when looking at the pressure plot in Figure 1. The pressure plot for the N-Valve looks like the letter “N”. The two solids descent lines are represented by the vertical lines on the pressure plot and the lift line consumes the excess pressure between the two vessels and is represented by the line with the negative slope connecting the two vertical lines. Figure 1 N-Valve Concept Sketch C C D Lo w Pr es .R is er High Pres. Fluid Bed A