In this paper, we present a thermochemical recuperation (TCR) reactor that was developed and experimentally evaluated with the objective to improve dual-fuel diesel-ammonia compression ignition engines. The novel system simultaneously decomposed ammonia into a hydrogen-containing mixture to allow high diesel fuel replacement ratios and oxidize unburned ammonia emissions in the exhaust; overcoming two key shortcomings of ammonia combustion in engines from previous literature. In the experimental work, a multi-cylinder compression ignition engine was operated in dual-fuel mode using intake-fumigated ammonia and hydrogen mixtures as the secondary fuel. A full-scale catalytic TCR reactor was constructed and generated the fuel used in the engine experiments. Results show that up to 55% of the total fuel energy was provided by ammonia on a lower heating value basis. Overall engine brake thermal efficiency increased for modes with high exhaust temperature where ammonia decomposition conversion in the TCR reactor was high but decreased for all other modes due to poor combustion efficiency. Hydrocarbon and soot emissions were shown to increase with replacement ratio for all modes due to lower combustion temperatures in-cylinder oxidation processes in the late part of heat release. Engine-out oxides of nitrogen (NOx) emissions decreased with increasing diesel replacement levels for all engine modes. High concentration of unburned ammonia was measured in the exhaust with increasing replacement ratio. This unburned ammonia predominantly oxidized to NOx species over the oxidation catalyst used in the TCR reactor. Ammonia substitution thus increased post-TCR reactor ammonia and NOx emissions in this work. Results show however that engine-out NH3 to NOx ratios were suitable for passive selective catalytic reduction, thus demonstrating that both ammonia and NOx from the engine could be readily converted to N2 if a different catalyst were used in the TCR reactor.