The emission characteristics of Cl were investigated on the basis of the field experiments at three 410 t/h circulating fluidized bed boilers co-firing petroleum coke and coal. All of the boilers were equipped with advanced air pollutant control devices (APCDs) to meet the ultralow emission requirement, such as selective non-catalytic reduction, electrostatic precipitator (ESP) or fabric filters (FFs), and wet flue gas desulfurization (WFGD). Gaseous Cl was sampled simultaneously at both the inlet and outlet of the ESP or FFs and at the outlet of the WFGDs based on U.S. Environmental Protection Agency Method 26A. Feed fuel, limestone, bottom ash, ESP or FF ash, limestone slurry, flush water, wastewater, and gypsum were also sampled to determine Cl in each stream. Results showed that more than 86.1% of total chlorine was emitted into the flue gas in the gaseous form (e.g., HCl and Cl-2). HCl was the dominant species in the flue gas at the outlet of boilers, accounting for 85.1-88.0%. The removal efficiencies of total chloride are 15.6% by ESP and 19.0-19.7% by FFs, respectively. The removal efficiencies of total chloride by WFGDs are in the range of 91.2-96.1%. The majority of total chlorine went into wastewater and gypsum, accounting for 76.16-80.19 and 11.35-12.76%, respectively. About 1.4-4.3% of total Cl was finally emitted into the atmosphere.