A system for reducing the amount of nitrogen oxides in exhaust gas of an internal combustion engine, comprising - a selective catalytic reactor system comprising: • a selective catalytic reactor arranged to catalytically convert the exhaust gas to a purified exhaust gas by reducing the amount of nitrogen oxides in the exhaust gas using a reducing agent; • an exhaust gas inlet to lead exhaust gas from the internal combustion engine into the selective catalytic reactor; • an exhaust gas outlet to lead the purified exhaust gas out of the selective catalytic reactor system; and; • a nitrogen oxides analyser arranged in the exhaust gas outlet for measuring the amount of nitrogen oxides present in the purified exhaust gas; - a reducing agent injection system comprising a container for storing reducing agent, an injector for injecting the reducing agent into the selective catalytic reactor system or the exhaust gas inlet of the selective catalytic reactor system thereby contacting the exhaust gas with the reducing agent, and a metering device for controlled injection of the reducing agent into the selective catalytic reactor system or the exhaust gas inlet of the selective catalytic reactor system; - a machinery system arranged to determine a significant engine load signal; and; - a process controller receiving information from the machinery system and the nitrogen oxides analyser and sending information to the reducing agent injection system; CHARACTERIZED IN THAT the process is configured to a) modify the incoming significant engine load signal based on the measured amount of nitrogen oxides present in the purified exhaust gas thereby generating a modified engine load signal, b) map the modified engine load signal in function of a reference amount of injected reducing agent for a specified nitrogen oxides reduction efficiency; and c) regulate the amount of reducing agent to be injected based on the results of step (b). - a selective catalytic reactor system comprising: • a selective catalytic reactor arranged to catalytically convert the exhaust gas to a purified exhaust gas by reducing the amount of nitrogen oxides in the exhaust gas using a reducing agent; • an exhaust gas inlet to lead exhaust gas from the internal combustion engine into the selective catalytic reactor; • an exhaust gas outlet to lead the purified exhaust gas out of the selective catalytic reactor system; and; • a nitrogen oxides analyser arranged in the exhaust gas outlet for measuring the amount of nitrogen oxides present in the purified exhaust gas; • a selective catalytic reactor arranged to catalytically convert the exhaust gas to a purified exhaust gas by reducing the amount of nitrogen oxides in the exhaust gas using a reducing agent; • an exhaust gas inlet to lead exhaust gas from the internal combustion engine into the selective catalytic reactor; • an exhaust gas outlet to lead the purified exhaust gas out of the selective catalytic reactor system; and; • a nitrogen oxides analyser arranged in the exhaust gas outlet for measuring the amount of nitrogen oxides present in the purified exhaust gas; - a reducing agent injection system comprising a container for storing reducing agent, an injector for injecting the reducing agent into the selective catalytic reactor system or the exhaust gas inlet of the selective catalytic reactor system thereby contacting the exhaust gas with the reducing agent, and a metering device for controlled injection of the reducing agent into the selective catalytic reactor system or the exhaust gas inlet of the selective catalytic reactor system; - a machinery system arranged to determine a significant engine load signal; and; - a process controller receiving information from the machinery system and the nitrogen oxides analyser and sending information to the reducing agent injection system; a) modify the incoming significant engine load signal based on the measured amount of nitrogen oxides present in the purified exhaust gas thereby generating a modified engine load signal, b) map the modified engine load signal in function of a reference amount of injected reducing agent for a specified nitrogen oxides reduction efficiency; and c) regulate the amount of reducing agent to be injected based on the results of step (b).