Although molecular H2 is the most obvious choice, there are still challenges relating mainly to its distribution and on-board storage (Zuttel et al., 2010). By comparison, NH3 is surprisingly overlooked, and yet the technology and facilities for its production, storage, handling and distribution are well established and extensive. By comparison, in recent years there has been a great deal more research into the use of H2, particularly for dual-fuelling of CI engines (Szwaja and Grab-Rogalinski, 2009, Abu-Jrai et al., 2007, Saravanan and Nagarajan, 2010). Although NH3 can be used as a source of dilute H2, the volumetric energy density of liquid NH3 is 45% higher than that of liquid H2 (Kroch, 1945), arising from strong H2 bonding between NH3 molecules in the condensed phase (Gray et al., 1966, Jorgensen and Ibrahim, 1980). One of the major concerns about NH3 is its high apparent toxicity, which is a combined measure of its volatility and intrinsic toxicity. This has been addressed in a detailed hazard assessment of each step in the distribution and storage of liquid NH3, carried out by the Risø National Laboratory in Denmark (Duijm et al., 2005), which has defined the required control measures for its safe public use as a vehicle fuel. The study emphasises the similarity in physical properties between liquid NH and liquefied petroleum gas (LPG), and
recommends that the failsafe fuel-tank design (lightweight porous carbon-fibre shell with polymer lining) developed for LPG is used for ammonia-fuelled vehicles.
NH3 is neither a greenhouse gas nor a direct emitter of CO2 but the route by which it is currently manufactured generates CO2 as a by-product. The first step in the route (steam reforming or auto-thermal reforming of hydrocarbon to produce syngas) is common to the manufacture of H2 but requires a subsequent synthesis step (the Haber process) in which the H2 combines with N2. Although the synthesis step is exothermic, it nevertheless incurs additional (though relatively small) energy and carbon penalties. However, as pointed out by Klerke et al. (2008), NH3 production lends itself to permanent carbon-capture because CO2
has to be removed from the feed-gas in the synthesis step to avoid poisoning the catalyst.
Furthermore, when the energy requirement for liquefaction is factored in, the efficiency of liquid H2 storage is only 54%, while the overall efficiency for NH3 synthesis followed by liquefaction and storage of the liquid is 85%, assuming the same starting point of high pressure H2 (Klerke et al., 2008). In the future, either dissociation or hydrolysis of catabolically-formed urea could provide the most sustainable route for the large-scale manufacture of NH3 (Rollinson et al., 2011).
2.5.1.1 On-board Storage and Supply
Urea has been widely evaluated as an on-board source of NH3 for catalytic SCR aftertreatment systems, where the role of NH3 is to reduce NOx to nitrogen. Some of the advantages of urea include its low volatility and toxicity in its solid form, and the fact that it can be injected as an aqueous solution (Lambert et al., 2004, Rollinson et al., 2011). On the other hand, the disadvantages of aqueous urea include a high freezing point and the possible formation of isocyanic acid as a by-product during reaction with exhaust gases (Koebel et al.,
Chapter 2: Literature Review
7.95wt.% for aqueous urea, using neat NH3 is a much more weight-efficient option, especially for large scale application as a supplementary fuel (Rollinson et al., 2011). However, in the context of the progressive decarbonisation of IC engines, the most compelling disadvantage of urea is that it forms CO2 during the hydrolysis reaction that releases NH3, as shown below:
Reaction 31: ) *+ → ++
Overall, liquid NH3 is seen as a better option for improving light-duty combustion and for enabling advanced aftertreatment technologies (e.g. HC-SCR) which require promotion by H2 (Abu-Jrai and Tsolakis, 2007, Houel et al., 2007b).
2.5.1.2 Effects of Ammonia on IC Engines
The feasibility of using NH3 as an alternative to hydrocarbon fuels was demonstrated on fleet scale in Europe during World War II, when it was used to fuel buses in Belgium during a shortage of conventional fossil fuels (Kroch, 1945). Later, in the 1960s, the U.S Army carried out more systematic studies of NH3 as a substitute for hydrocarbons in SI and CI engines (Gray et al., 1966, Pearsall and Garabedian, 1967). These studies highlighted the effectiveness of dual fuelling CI engines with diesel and NH3. On its own, NH3 has a high ignition resistance, which can be overcome by injecting enough diesel fuel directly into the cylinder to initiate combustion (Pearsall and Garabedian, 1967). Another perceived disadvantage of NH3
is its low heating value but this is largely compensated for by the low A/F ratio at stoichiometry, which means that the energy content per unit mass of a stoichiometric mixture is only 7% lower than that of a comparable diesel/air mixture.
Despite its long history as an alternative or supplementary fuel, our understanding of the effects of NH3 on thermal combustion efficiency and emission quality is still in its infancy. In the most detailed study of NH3 addition reported to date, Reiter and Kong (2008, 2011) have
investigated the combustion and emissions characteristics of a dual-fuelled turbocharged diesel engine. They have shown that CO2 and soot emissions can be reduced without sacrificing engine power when 40-60% of the fuel energy delivered to the engine is provided by NH3. The major issue, however, relates to the emissions directly resulting from the NH3. Although the predominant reaction of NH3 in the engine is its clean combustion to produce only N2 and H2O (Reaction 32), there is a persistent slip of unconverted NH3 that results in a concentration of 1000-3000 ppm in the exhaust. Additionally, when most of the fuel energy is provided by NH3, some of the NH3 can undergo over-oxidation (Reaction 33), resulting in very high rates of NOX emissions. However, Reiter and Kong (2008, 2011) have also shown that when < 40% of the fuel energy is provided by NH3, NOX emissions are lower than for a diesel engine running entirely on hydrocarbon fuel because substitution by NH3 lowers the combustion temperature, and hence reduces the thermally-induced formation of NOX
(Reaction 34).
Reaction 32: +++%$+.6#& → #. 8 +6+-
Reaction 33: % ++ 8→ , + %
Reaction 34: + →
Further, in forming H2 from NH3, the minimum ignition energy is lowered from 8 mJ to 0.018 mJ, while the laminar burning velocity is increased from 0.015 ms-1 to 3.51 ms-1 (Saika et al., 2006). At the same time, although the proportion of unburnt NH3 may remain the same, the substantially lower concentration being fed to the engine should result in much lower slip of NH3 through to the exhaust.
In a practical on-board H2 production system, it is believed that liquid NH3 will be released from a storage tank into an evaporator where it will undergo a phase change to form
Chapter 2: Literature Review
gas phase NH3, which can then be catalytically dissociated to H2 and N2. The dissociation of NH3 (Reaction 35) is limited by a chemical equilibrium between the forward and reverse reactions, which results in a small percent of NH3 in the product stream at temperatures below 700°C (at 1 atm) (Faleschini et al., 2000). However, a temperature of 400°C or higher will promote stable dissociation (Saika et al., 2006). It can be assumed that a dissociated gas stream containing 1-2% unconverted NH3 can be produced in a catalytic reactor, where the endothermic heat of dissociation can be provided by the combustion of some of the NH3 over the catalyst and through the recovery of waste heat from the engine exhaust. In a further adaptation of the same concept, a diffusion membrane can be implemented to separate the H2
from the N2 and unreacted NH3 in the exit stream from the catalytic reactor. In this scenario, the function of the NH3 is as a H2 carrier and as a heat recovery medium.
Reaction 35: + → + + ∆ 6- = +%,;<=3#