Mechanical Engineering Journal (Sep 2019)
Emission characteristics of a lean-premixed ammonia/natural-gas gas-turbine combustor and effect of secondary ammonia injection
Abstract
To develop an NH3/natural gas-fired gas-turbine combustor with minimal modifications to a natural gas-fired combustor, the effect of co-firing NH3 with natural gas on emission characteristics and combustion efficiency is investigated experimentally under atmospheric pressure. In this experiment, a lean premixed combustor that has basically the same structure and same size as those of an actual 2 MW-class gas-turbine combustor is used and the effects of equivalence ratio, NH3 mixing ratio, and NH3 injection method were evaluated. The results of NH3/natural gas/air premixed combustion show that NO emission shows a local maximum at a certain NH3 mixing ratio, when equivalence ratio is fixed. The NH3 mixing ratio where this maximal NO emission occurs shifts to higher ratios, when equivalence ratio is increased towards stoichiometric condition. At the same time, the maximal NO emission increases. The variation of NOx concentration can be explained by the change in the selective non-catalytic reduction (SNCR) effect, which depends on temperature and NH3 concentration in the flame. In particular, the influence of NH3 concentration on the SNCR effect appears to be stronger than that of temperature. On the other hand, concentrations of N2O and unburnt NH3 increase with increasing NH3 mixing ratio in the very lean case, when decreasing flame temperature approaches the lean flame blow-off limit. When NH3 is directly injected into the lean premixed flame of natural gas, NO and N2O concentrations are simultaneously reduced; this is attributed to the local increase of NH3 concentration and flame temperature. Finally, it is proposed to consider NOx as unburnt component in the calculation of combustion efficiency. When NH3 is fired, the enthalpy content of NOx is very high, and its influence on combustion efficiency cannot be ignored. It is also found that NH3 direct injection is effective in increasing combustion efficiency, achieving more than 99.6% of the combustion efficiency even if the enthalpy of NOx is considered.
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