Fuel Flexibility and Petroleum Coke Combustion at Provence 250 MW CFB

Abstract

The Provence 250 MWe CFB boiler was originally designed in 1992 for a local coal (Provence lignite) with a high sulfur and high ash content. This large CFB, features a pant leg bottom furnace, 4 cyclones and 4 fluid bed heat exchangers to provide the active temperature control of the furnace and reheated steam final temperature. After start up in 1995 with this local coal, several other fuel types were burnt. Mainly 3 fuels were tested over the last 5 years, on a long term basis, with various degrees of combination with the Gardanne coal up to full firing alone: • a lean coal (Gard, France), low volatile bituminous type, not far from semi anthracite type coals, • an imported coal, representative of low ash low sulfur content imported coal, • a petroleum coke, with a high sulfur content. This last test demonstrates the widest flexibility with regards to fuel reactivity range of a CFB plant with this architecture. The fuel, limestone and ash handling/injection systems were able to cope with this fuel diversity without equipment modifications. Since these tests were conclusive both on the pollutant emissions and on the operating concern, a permit to burn petroleum coke in commercial operation in a 250 MWe CFB boiler has been obtained in December 2001. This paper presents the main features and the results of the petroleum coke tests performed and compare them with the feedback on operating conditions of the boiler and emissions performances for Provence lignite and imported coal. These positive results demonstrate the wide fuel capability of large CFB boilers with this boiler architecture. First, they emphasize the critical role of advanced cyclones to accept fuels with very different reactivities and minimize limestone consumption. Second, these results show the role of the Fluid Bed Heat Exchangers system to control actively the furnace temperature, while controlling the reheated steam temperature without using spray.