There are two ways of reducing NOx, and I think ADBlue or any post burn approach is the wrong way around as your curing the result not the cause.
Carbon Monoxide and NOx are created when you burn a fuel fast and at high temp. In the past on Diesel engine emission controls were basically just EGR valves, these were used to feed exhaust gas back into the inlet to reduce the burn temp during combustion.
EGR valves have their own issues, first that's un filtered corrosive exhaust gasses your feeding back into your engine. Second this type of system can only work at idle or overrun on a turbo diesel, any other time the inlet manifold is at higher pressure than the exhaust. So when your pulling hard and the turbo is spun up to pressure there is no emissive controls, but this is when you need them most.
For me I believe the best approach is water methanol, this has been done for years on petrol, diesel, and jet engines. This works in two ways, both reducing NOx, and CO. First the Methanol improves the burn efficiency and this also has a result of giving extra power. Second the water cools the burn therefore slowing it down and reduces the CO and NOx, it also cools the intake air giving higher compression, again aiding efficient burn and producing more power.
With Water Meth, you do not need an EGR valve that has been proven to reduce engine life, you do not need to add chemicals to the exhaust, and you get more power for less emissions. I can't work out why this approach is not adopted main stream?