Engine Testing of Dissolved Sodium Borohydride for Diesel Combustion CO ₂ Scrubbing

Improvements in the efficiency of internal combustion engines and the development of renewable liquid fuels have both been deployed to reduce exhaust emissions of CO₂. An additional approach is to scrub CO₂ from the combustion gases, and one potential means by which this might be achieved is the reaction of combustions gases with sodium borohydride to form sodium carbonate. This paper presents experimental studies carried out on a modern direct injection diesel engine supplied with a solution of dissolved sodium borohydride so as to investigate the effects of sodium borohydride on combustion and emissions. Sodium borohydride was dissolved in the ether diglyme at concentrations of 0.1 and 2 % (wt/wt), and tested alongside pure diglyme and a reference fossil diesel. The sodium borohydride solutions and pure diglyme were supplied to the fuel injector under an inert atmosphere and tested at a constant injection timing and constant engine indicated mean effective pressure (IMEP).

The 0.1 % sodium borohydride diglyme solution and pure diglyme exhibited durations of ignition delay shorter than that of the reference fossil diesel, while testing of the 2 % sodium borohydride solution resulted in failure of the fuel injector. Addition of the dissolved sodium borohydride was found to increase the duration of fuel injection required to maintain a constant engine IMEP by as much as 50 %, with a concurrent increase in the duration of combustion also observed. Levels of exhaust unburnt hydrocarbons did not increase with the addition of sodium borohydride suggesting a detrimental effect of the sodium borohydride on the injector efficiency. Measurement of exhaust emissions of particulate mass and scanning electron microscopy (SEM) of exhaust soot samples both indicated greatly increased soot emissions with the addition of sodium borohydride. Chemical analysis of the exhaust soot samples found no evidence of sodium carbonate or bicarbonate compounds, possibly suggesting deposition of sodium compounds inside the engine combustion chamber.