Method and system for controlling an ammonia coverage degree profile

A system arranged for controlling at least one sectional ammonia coverage degree profile NH3_profile for at least one selective Catalytic Reduction (SCR) catalyst included in an exhaust gas treatment system arranged for treating an exhaust stream output from a combustion engine;

• the system comprising:
• a determination unit, arranged for determining at least one sectional ammonia coverage degree profile NH3_profile_det for said at least one SCR catalyst including at least two sectional ammonia coverage degrees NH3_det_1, NH3_det_2, . . . , NH3_det_n for at least two sections of said at least one SCR catalyst, based on a flow F, a temperature T and a composition C of said exhaust stream upstream of said at least one SCR catalyst, said at least one sectional ammonia coverage degree profile NH3_profile_det indicating ammonia stored along said at least one SCR catalyst;
• a comparison unit, arranged for comparing said at least one sectional ammonia coverage degree profile NH3_profile_det with at least one sectional reference profile for an ammonia coverage degree NH3_profile_ref for said at least one SCR catalyst, said at least one sectional reference profile for an ammonia coverage degree NH3_profile_ref being related to an available capacity for reduction of nitrogen oxides NOx for said at least one selective Catalytic Reduction (SCR) catalyst, and including at least two sectional ammonia coverage reference degrees NH3_ref_1, NH3_ref_2, . . . , NH3_ref_n, for said at least two sections of said at least one SCR catalyst, wherein said reference profile is determined by solving mass balances over said at least one SCR catalyst for an equilibrium ammonia coverage profile that corresponds to a desired concentration of an exhaust component downstream of said at least one SCR catalyst, said comparing including comparing said at least two sectional ammonia coverage degrees NH3_det_1, NH3_det_2, . . . , NH3_det_n with said at least two sectional ammonia coverage reference degrees NH3_ref_1, NH3_ref_2, . . . , NH3_ref_n;
• and
• a control unit, arranged for controlling, based on said comparing, at least one of a concentration of nitrogen oxides CNOx in said exhaust stream to be output from said combustion engine and a dosage of a reductant including ammonia NH3 to be injected into said exhaust stream upstream of said at least one SCR catalyst, wherein if said comparing indicates that there is a difference Diffprofiles between said at least one ammonia coverage degree profile NH3_profile_det and said at least one reference profile for said ammonia coverage degree NH3_profile_ref, said controlling includes controlling at least one of said concentration of nitrogen oxides CNOx and said dosage of said reductant such that said difference Diffprofiles is reduced and wherein said difference Diffprofiles is calculated as a weighted summation of at least two sectional differences Diff1, Diff2, . . . , Diffn between said at least two sectional ammonia coverage degrees NH3_det_1, NH3_det_2, . . . , NH3_det_n and said at least two sectional ammonia coverage reference degrees NH3_ref_1, NH3_ref_2, . . . , NH3_ref_n, for said at least one SCR catalyst, wherein each one of said at least two sectional differences Diff1, Diff2, . . . , Diffn is weighted by a sectional weighting coefficient W1, W2, . . . , Wn, and wherein said weight coefficients are determined to convert each sectional difference to a required addition or subtraction of a concentration of ammonia in said section in order to yield a required reductant dosing, said weight coefficients being determined to convert each sectional difference to a required addition or subtraction of the concentration of NOx in said section in order to yield a required NOx concentration to output from said combustion engine.