Conoco is successfully operating a prototype 50 MMBTU/hr., solid fuel-fired, Multi-Solids Fluid Bed Combustor (MS-FBC) to generate 80 quality 2450 psig steam for tar sands recovery from the San Miguel-4 formation in Maverick County in South Texas. The facility is designed to burn high ash coal or high sulfur petroleum coke with environmentally acceptable emission levels of SO, NO, CO and particulates. More than 8,000 hours of on line operation have been logged since startup of the fluid bed combustor in January 1982. This paper describes the selection process and criteria used by Conoco in selecting this technology, the key process design considerations and comparative economics with conventional oilfield steam generators. It also discusses the overall system performance and plant modifications which have contributed to improvements in the operability and reliability of the fluid bed combustor steam generator.