A process for selective catalytic reduction of nitrogen oxides contained in a gas stream and regeneration of a deactivated SCR catalyst, wherein said process comprises: providing a first processed flue gas stream, containing nitrogen oxides and sulfur compounds, and a second processed flue gas stream, containing nitrogen oxides and sulfur compounds; passing said first processed flue gas stream to a first SCR system having an upstream inlet and a downstream outlet that includes a first SCR catalyst and contacting said first processed flue gas stream with said first SCR catalyst in the presence of ammonia for a time sufficient to provide a deactivated first SCR catalyst deactivated by sulfur compounds and yielding from said first SCR system a first denitrified flue gas stream for discharge into a stack; passing said second processed flue gas stream to a second SCR system that includes a second SCR catalyst and yielding from said second SCR system a second denitrified flue gas stream for discharge; discontinuing said step of passing said first processed flue gas stream to said first SCR system by isolating said first SCR system to provide a closed system; introducing at an introduction rate a portion of said second denitrified flue gas stream into said closed system as a regeneration gas used to regenerate said first SCR catalyst and to yield a regeneration effluent gas containing SOx and ammonia; and circulating at a circulation rate all or part of said regeneration effluent gas from said downstream outlet to said upstream inlet; and removing at a removal rate a portion of said regeneration effluent gas from said closed system and introducing said portion of said regeneration effluent gas into a parallel flue gas treating system that includes a second acid gas removal unit, a second particulate removal unit and said second SCR system, wherein said parallel flue gas treating system provides for removing at least a portion of the SOx and ammonia contained in said regeneration effluent gas of said first SCR system. providing a first processed flue gas stream, containing nitrogen oxides and sulfur compounds, and a second processed flue gas stream, containing nitrogen oxides and sulfur compounds; passing said first processed flue gas stream to a first SCR system having an upstream inlet and a downstream outlet that includes a first SCR catalyst and contacting said first processed flue gas stream with said first SCR catalyst in the presence of ammonia for a time sufficient to provide a deactivated first SCR catalyst deactivated by sulfur compounds and yielding from said first SCR system a first denitrified flue gas stream for discharge into a stack; passing said second processed flue gas stream to a second SCR system that includes a second SCR catalyst and yielding from said second SCR system a second denitrified flue gas stream for discharge; discontinuing said step of passing said first processed flue gas stream to said first SCR system by isolating said first SCR system to provide a closed system; introducing at an introduction rate a portion of said second denitrified flue gas stream into said closed system as a regeneration gas used to regenerate said first SCR catalyst and to yield a regeneration effluent gas containing SOx and ammonia; and circulating at a circulation rate all or part of said regeneration effluent gas from said downstream outlet to said upstream inlet; and removing at a removal rate a portion of said regeneration effluent gas from said closed system and introducing said portion of said regeneration effluent gas into a parallel flue gas treating system that includes a second acid gas removal unit, a second particulate removal unit and said second SCR system, wherein said parallel flue gas treating system provides for removing at least a portion of the SOx and ammonia contained in said regeneration effluent gas of said first SCR system.