MAXIMIZING DIESEL REPLACEMENT AT LOW LOADS THROUGH HYDROGEN-ENHANCED AMMONIA/DIESEL DUAL-FUEL COMBUSTION

Abstract

Dual-fuel ammonia (NH3) combustion has been investigated as a promising way to meet the International Maritime Organization’s (IMO) CO2 reduction mandate for internal combustion engines in the maritime sector. High unburned NH3 emissions limit the replacement of the direct-injected diesel at low loads. One possible avenue to tackle this problem is to reform NH3 to hydrogen (H2) on-board using a decomposition catalyst. The NH3/H2 blends resulting from the decomposition process have been evaluated on a single-cylinder version of a Cummins B6.7 250 engine variant using gas bottles for simplicity. The objective of the study was to quantify the improvement in diesel replacements limits via potential combustion enhancement provided by H2. A diesel/NH3 baseline was established at 1200 RPM and 4.2 bar (IMEPn) with self-imposed constraint of 30,000 ppm of unburned NH3 in the exhaust and a COVIMEP of <4%, which resulted in a maximum total premixed energy fraction (TPEF) of 77%. Hydrogen was then introduced by replacing a fraction of the premixed NH3 at pre-determined hydrogen premixed energy fractions (HPEF) under constant IMEPn and combustion phasing (CA50) conditions. The IMEPn and CA50 were maintained by adjusting the commanded diesel duration and SOI timing, respectively. The results indicated considerable increase in TPEF via hydrogen enhancement; 77% TPEFmax at 0% HPEF to 93% TPEFmax at 20% HPEF. Despite an improvement in the unburned NH3 emissions at higher HPEFs, the engine-out NH3/NOx ratio was unacceptably large at the tested condition from an aftertreatment perspective. A statistically significant improvement in thermal efficiency (ηt) as compared to the baseline was not evident until a HPEF of 20%, whereas the CO2, eq emissions trended lower with only 5% HPEF in the mixture. A substantial room for improvement exists relative to a conventional diesel combustion (CDC) case.