NOVELTY - Selecting and preparing boiler ash residue from a wood waste boiler of pulp and paper mills for mercury adsorption such that the boiler ash residue has a carbon content of about 70% or more, and a Brunauer-Emmett-Teller (BET) surface area of about 300 m2/g or more or a butane adsorption of about 130 mg/g or more; involves when the boiler ash residue has a BET surface area of less than about 300 m2/g or a butane adsorption of less than about 130 mg/g, subjecting boiler ash residue to activation with steam or carbon dioxide, to increase the BET surface area to about 300 m2/g or more.    USE - The methods are useful for selecting and preparing boiler ash residue from wood waste boiler of pulp and paper mills for mercury adsorption; for preparing a brominated sorbent; and for reducing mercury emissions from exhaust system which comprises at least combustion gas stream and particulate collection device (all claimed).    ADVANTAGE - Low-cost activated carbon is provided from boiler ash residue from wood waste boilers of pulp and paper mills. Advantageously, the material that forms the boiler ash residue from waste wood boilers of pulp and paper mills has undergone a heat treatment process in the boiler to decrease waste volume at the pulp or paper mill, and to produce energy for the pulp or paper mill. As a result of the heat treatment process, the boiler ash residue contains considerable amounts of carbon with a porous structure. In addition, boiler ash residue from waste wood boilers of pulp and paper mills has a low volatile content.    DETAILED DESCRIPTION - Selecting and preparing boiler ash residue from a wood waste boiler of pulp and paper mills for mercury adsorption such that the boiler ash residue has a carbon content of about 70% or more, and a Brunauer-Emmett-Teller (BET) surface area of about 300 m2/g or more or a butane adsorption of about 130 mg/g or more; involves when the boiler ash residue has a BET surface area of less than about 300 m2/g or a butane adsorption of less than about 130 mg/g, subjecting boiler ash residue to activation with steam or carbon dioxide, to increase the BET surface area to about 300 m2/g or more or the butane adsorption to about 130 mg/g or more. INDEPENDENT CLAIMS are included for:    (1) a boiler ash residue selected and prepared by the method;    (2) a method of preparing a brominated sorbent, involving contacting boiler ash residue prepared above and a bromine source, which bromine source is: (i) elemental bromine and/or hydrogen bromide or (ii) a solid bromine source, to form a brominated sorbent;    (3) a brominated sorbent prepared by the method; and    (4) a method for reducing mercury emissions from an exhaust system which comprises at least a combustion gas stream and a particulate collection device, involving: (i) introducing the brominated sorbent into the combustion gas stream at one or more points upstream of a particulate collection device; and (ii) collecting the brominated sorbent from the combustion gas stream, with the proviso that the brominated sorbent is not exposed to temperatures above the ignition temperature of the brominated sorbent; or a method for reducing mercury emissions from an exhaust system which comprises at least a combustion gas stream and a particulate collection device, involving: (i) introducing a brominated sorbent into the combustion gas stream at one or more points upstream of a particulate collection device; and (ii) collecting the brominated sorbent from the combustion gas stream, where the brominated sorbent is formed from boiler ash residue from a wood waste boiler of pulp and paper mills and a bromine source which is (i) elemental bromine and/or hydrogen bromide or (ii) a solid bromine source, with the proviso that the brominated sorbent is not exposed to temperatures above the ignition temperature of the brominated sorbent.