PSDF Gasification Process Description

The PSDF gasification process, shown in the figure below, features high pressure solids feed systems; a KBR (formerly Kellogg Brown & Root) Transport Gasifier; a high pressure filter vessel, the Siemens Westinghouse particulate control device (PCD); continuous ash depressurization systems developed at the PSDF for ash removal; a slipstream syngas cleanup unit to test various pollutant control technologies; and a novel piloted syngas burner.

Coal and sorbent (when required for sulfur capture) are separately fed into the Transport Gasifier through lock hopper feed systems. Coal is ground to a nominal particle diameter between 250 and 400 microns. Sorbent, either limestone or dolomite, is ground to a nominal particle diameter of 10 to 100 microns. For start-up purposes, a direct propane-fired burner is operated to heat the gasifier.

The Transport Gasifier, a pressurized, advanced circulating fluidized bed reactor, consists of a mixing zone, riser, solids separation unit, seal leg, standpipe, and J-leg. The gasifier is equally capable of using air or oxygen as the gasification oxidant. Steam and either air or oxygen are mixed together and introduced in the lower mixing zone, while the coal, sorbent, and additional air and steam (if needed) are added in the upper mixing zone. These feed streams are mixed together in the upper mixing zone. The gas and solids move up the riser before entering the solids separation unit, which removes larger particles by gravity separation. Between the first and second stages of the solids separation device is the seal leg, which prevents backflow of solids. The solids collected by the solids separation unit are recycled back to the gasifier mixing zone through the standpipe and J-leg. The standpipe level can be reduced by removing solids through the continuous course ash depressurization system. The nominal gasifier operating temperature is 1,800^oF, and the gasifier system is designed to have a maximum operating pressure of 294 psig with a thermal capacity of about 41 MBtu/hr.

The gas exits the Transport Gasifier, passes through the primary gas cooler where the gas temperature is reduced to about 750^oF, and enters the PCD for final particulate cleanup. The metal or ceramic filter elements used in the PCD remove essentially all the dust from the gas stream. The PCD utilizes a tube sheet holding up to 91 filter elements, which are attached to one of two plenums. Process gas flows into the PCD through a tangential entrance, around a shroud, and through the filter elements into the plenums. Failsafe devices are located downstream of the filter elements to stop solids leakage by plugging in the event of element failures. High pressure nitrogen back-pulsing, typically lasting 0.2 seconds, is used to clean the filters periodically to remove the accumulated solids and control the pressure drop across the tube sheet. The solids fall to the bottom of the PCD and are removed through the continuous fine ash depressurization system.

After exiting the PCD, a portion of the syngas can be directed to the piloted syngas burner (PSB), a gas turbine combustor designed to burn coal-derived syngas with a lower heating value below 100 Btu/SCF. After combusting in the burner, the gas passes through a turbine before exiting the turbine stack. An associated generator supplies power from the turbine to the electric transmission grid.

A small portion of the syngas, up to 100 lb/hr, can also flow to an advanced gas cleanup system downstream of the PCD. The gas cleanup system is a specialized, flexible unit, capable of operating at a range of temperatures, pressures, and flow rates, and provides a means to test various pollutant control technologies, including removal of sulfur, nitrogen, and chlorine compounds. The syngas cleanup system also includes a CO₂ removal unit.

The main stream of syngas is then cooled in a secondary gas cooler, which reduces the temperature to about 450^oF. Some of this cooled is compressed and sent to the gasifier for aeration to aid in solids circulation. The remaining syngas is reduced to near atmospheric pressure through a pressure control valve. The gas is then sent to the atmospheric syngas combustor which oxidizes carbon monoxide, reduced sulfur compounds (H₂S, COS, and CS₂), and reduced nitrogen compounds (NH₃ and HCN). The gas from the atmospheric syngas combustor goes to a heat recovery boiler, through a baghouse, and then is discharged out a stack.