The main reason for the NOx emissions is flame temperature, the higher the flame temperature, the highest the amount of NOx produced, since the work by Harry Ricardo in the 20's, it's known that a 1% increase in Exhaust Gas Recirculation reduces the flame temperature in 45ºF. How the addition LPG affects this? I'm not sure, but I guess, from the use of mixed Gas-Diesel Fuel in City Buses engines, that adding LPG, that have a lower intrinsic energy i.e. when burnt, it release less calories than Gasoline or Diesel fuel does, thus, LPG may have favorable effects on NOx emissions.

as EGR method is used to reduce NOx emission in diesel engine by mixing the exhaust gases in combustile mixture, the flame temperature is going to be reduced. NOx emission is directly proportional to combustion temperature because NOx formation is due to oxidation of N at high temperatures. So as in your case, LPG is added to diesel fuel, works in similar way as EGR, results in lowering NOx emission.

As I understand the question the LPG is not added to the diesel fuel but instead supplied with the inlet air (this is the normal meaning of dual fuel operation). Then there will be a lean, nearly homogeneous, mixture of LPG and air which is ignited by the diesel pilot injection. Lean combustion of a premixed charge takes place at a lower temperature than the mostly stoichiometric diffusion combustion with direct injection of diesel fuel. This is most likely the reason for the reduction in NOx.

Hi!: I guess you may find interesting the SAE paper 865033, by S. Veinovic et al: 'Economy increase for Otto engines by introducing exhausts into carburettor', it's part of a SAE volume, published in 2005, ISBN 0-7680-1498-0: '100 years of engine developments', edited by Daniel J Holt, with the Preface title 'Engine development - over 100 years and still going strong' Hope you like it. Salut †