Abstract
The interest in ammonia as a high-density hydrogen carrier for long-term electricity storage is growing. A clean and efficient Combined Heat and Power (CHP) system is envisioned for power production from stored ammonia, to which Homogeneous-Charge Compression-Ignition (HCCI) engines are promising. Although recent studies showed a high equivalence ratio potential for ammonia HCCI engines, its resistance to auto-ignition forces the use of high intake temperatures, which limits the engine power density. Moreover, the feasibility of clean and highly efficient ammonia combustion has not been demonstrated. Therefore an HCCI test bench has been modified to reach an effective compression ratio of 22:1, which allowed operation from neat hydrogen up to 95%vol. ammonia. A full cartography of the ammonia-hydrogen load range, related efficiencies and emissions is obtained and the impact of the equivalence ratio (from 0.1 to 0.6), the intake temperature (from 50 ◦C to 240 ◦C) and Exhaust Gas Recirculation is assessed. Thanks to a reduced combustion intensity, ammonia allows a 50% IMEP increase compared to neat hydrogen, while maintaining equivalent combustion efficiencies. Using the ammonia-hydrogen blending ratio as a control parameter, the combustion timing and load control is broadened. The EGR impact was mixed: a positive reduction of NO emissions but a negative impact on NO2 and unburned emissions. Finally, the trade-off for the ideal ammonia-hydrogen blending ratio is discussed. To optimise the concept, boosted conditions with higher stroke-to- bore ratios must be aimed at.
Highlights
With the increasing penetration of renewables, excess and shortage of electricity will reflect the mismatch between demand and generation at different timescales: from minutes to seasons
Hydrogen must still be operable without intensive ringing, and the ideal compression ratio (IMEP wise) for hydrogen has been demonstrated to be around 17:1-18:1 (Caton and Pruitt, 2009)
The increased resistance starts to appear at around 15%vol of ammonia and increases non-linearly, yet according to Rapid Compression Machine (RCM) exmeriments (Pochet et al, 2019a) the rise in ignition delay should be tapering off reaching neat ammonia
Summary
With the increasing penetration of renewables, excess and shortage of electricity will reflect the mismatch between demand and generation at different timescales: from minutes to seasons. A technology with a low investment on the storage capacity is needed to effectively store the massive energy quantities needed to shift seasons. Power-to-Fuel (P2F) shows a very high potential with investment per unit energy stored three orders of magnitude lower than battery-based solutions and one to two orders lower than Pumped Hydro Storage (PHS) or Compressed Air Energy Storage (CAES) (Hedegaard and Meibom, 2012; Jülch, 2016). Hydrogen is not convenient for massive storage given its very low energetic density of about 10 MJ/m3 under normal conditions. Hydrogen compression or liquefaction cannot achieve energetic densities higher than 8 GJ/m3. It is a very expensive storage solution given the high pressure or low temperature to produce and that the tank must withstand. The hydrogen storage form is only preferred for mid-term (∼weeks) and limited capacity electricity storage
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