(367d) Performance of Ammonia/Natural Gas Co-Fired Gas Turbine with Two-Stage Combustor

Performance
of Ammonia/Natural Gas Co-fired Gas Turbine with Two-stage Combustor

Ammonia
has several advantages as renewable energy career [1-3]. Regarding the
manufacturing process, the Harbor-Bosch method is already established. For
transportation, ammonia can easily be liquefied at room temperature. Therefore,
both transportation and storage are much easier than in case of hydrogen. In
utilization, ammonia is combustible and used as a carbon-free fuel. However,
ammonia has different combustion characteristics from natural gas. For example,
the nitrogen atom contained in ammonia molecule causes high NOx emission
through fuel NOx mechanisms [3]. Laminar burning velocity of ammonia is much
lower; it might increase unburnt ammonia emission and cause unstable operation
of gas turbine. Much effort has been made to overcome these points and establish
stable low NOx combustion methods [4-10]. Among these, combustion methods for
pure ammonia developed by Hayakawa et al. showed characteristic effects on NOx
emission [9, 10]. These results showed that NOx emission became the smallest
around primary zone equivalence ratio of 1.2 in a two-stage premixed combustor.
However, this method has the risk of flashback in real gas turbine operation
near stoichiometric condition. Therefore, in this study the effect of primary
zone equivalence ratio is examined for a diffusion flame burner which co-fires
ammonia with natural gas. Secondary ammonia injection was adopted in the demonstration
tests as a means of low NOx combustion for ammonia / natural gas two-stage
combustor [11, 12]

For
the demonstration, IM270, a simple cycle gas turbine manufactured by IHI
Corporation [13], was used. Figure 1 and 2 show photos of gas
turbine engine test equipment and schematics of two-stage combustor with
diffusion flame burner, respectively. The test system consists of Selective
Catalytic Reduction (SCR) unit, natural-gas compressor and high pressure
ammonia supply unit. The high pressure ammonia supply unit first pressurizes ammonia
to 2 MPaG and then gasifies it in a steam vaporizer, before releasing it to the
combustor. In engine testing, the gas turbine is first started and then power
is increased up to 2 MWe power generation output firing natural gas, before
ammonia is supplied to the combustor. Ammonia supply to the engine is measured
in terms of the heat input ratio of ammonia to total fuel. This ratio is called
gammonia mixing ratioh in this study. Operation of the gas turbine engine
turned out to be stable in the whole range of ammonia mixing ratios from 0 to 25%
with diffusion flame burner. Primary zone equivalence ration of diffusion flame
burner was set to 0.85. This value is closer to stoichiometric condition than
that in lean premixed burner used in the past studies [12].

 Figure
3 shows NOx emissions in the demonstration tests. Results obtained with lean
premixed burner [12] are also plotted for comparison. All Ammonia is injected
into secondary zone in all test conditions. With the diffusion flame burner,
NOx emissions are 220 and 264 ppm@16%O2 at ammonia mixing ratios of 20 and 25%,
respectively. Comparison of NOx emissions at ammonia mixing ratio of 20% reveal
that with the diffusion flame burner lower NOx emissions can be realized than
with the lean premixed burner. It was also shown that the increase of NOx
emission with ammonia mixing ratio differs for the two combustors. In the case
of lean premixed burner, NOx emission rapidly increases, when ammonia mixing
ratio is increased from 0 to 5%, then it was nearly saturated for about 5%. In
the opposite, with the diffusion flame burner, NOx emission gradually
increases, when ammonia mixing ratio is increased from 0 to 25%. Though fuel
mixing methods are different for two burners, these differences are assumed to
be caused by difference in primary zone equivalence ratio.

 Results
of engine tests show that NOx emissions obtained with the diffusion flame
burner with primary equivalence ratio of 0.85 are lower than that of the lean
premixed burner. However, the primary zone equivalence ratio is still lean
conditions in these tests. To further investigate the effect of primary zone
equivalence ratio, NOx emissions for rich conditions in the primary zone should
be investigated.

 This
work was supported by Council for Science, Technology and Innovation (CSTI),
Cross-ministerial Strategic Innovation Promotion Program (SIP), gEnergy Carriersh (Funding agency: JST).

Fig. 1
Fig.
1 Photos of demonstration test equipment

(Left:
2MW class gas turbine engine, right: high pressure gasified ammonia supply unit)

Fig. 2
Schematic of combustor

(Left:
two-stage combustor, right: diffusion flame burner)

Fig. 3
Effect of ammonia mixing ratio on NOx emission in engine test

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[13]    IHI Corporation
home page, https://www.ihi.co.jp/powersystems/en/lineup/IM270/index.html