Combustion Strategies for Syngas and High-Hydrogen Turbines

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3.2
Combustion Strategies for
Syngas and High-Hydrogen
Fuel
3.2-1 Introduction
The technical challenges surrounding syngas and hydrogen fuel
combustion have been outlined in section 3.1. Given the issues presented
there, various options can be considered for combustor design and operation.
First, it is critical to defi ne the type of combustion system that will be used.
There are two broad categories: diffusion fl ame combustors, and premixed
combustors. These are described below, but before discussing the combustion
strategies, it is useful to review how NOx pollutants are formed.
3.2-2 NOx Formation
George Richards
National Energy Technology Laboratory
3610 Collins Ferry Rd.
P.O. Box 880
email: george.richards@netl.doe.gov
phone: (304) 285-4458
Nate Weiland
National Energy Technology Laboratory
P. O. Box 10940
Pittsburgh, PA 15236
email: nathan.weiland@netl.doe.gov
phone: (412)386-4649
Pete Strakey
Energy Systems Dynamics Division
National Energy Technology Laboratory
3610 Collins Ferry Rd.
P.O. Box 880
Morgantown, WV 26507-0880
phone: (304) 285-4476
email: peter.strakey@netl.doe.gov
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There are several routes to form NOx pollutants and these may be
broadly catalogued as thermally-generated, fl ame-generated, or fuel-bound
NOx. Different authors use different names to catalogue these mechanisms and
there is still continuing research to understand the most prominent mechanisms
at ultra-low NOx conditions. For example, in hydrogen fueled systems, the
prominence of H radicals may contribute to NOx in a manner that is different
than in systems fueled by natural gas.1
Thermal NOx is formed by oxidation of nitrogen in air and requires
suffi cient temperature and time to produce NOx. A rule of thumb is that below
approximately 1700K, the residence time in typical gas turbine combustors
is not long enough to produce signifi cant thermal NOx. Where temperatures
higher than 1700K cannot be avoided, it is necessary to limit residence time to
control NOx formation, which favors very short combustor designs. Thermal
NOx production also increases with the square root of operating pressure,
making it more diffi cult to reduce in higher-pressure aeroderivative gas
turbines.
As the name implies, fl ame-generated NOx occurs in the fl ame front,
created on the short time scale associated with primary combustion reactions.
There are a variety of chemical mechanisms involved, all linked to intermediate
combustion species that exist only in the reaction zone of the fl ame. It is
important to understand that in practical combustors, the reaction zone is just
a small portion of the total combustor volume –most of the combustor volume
is sized to complete the relatively slow approach to equilibrium products
(notably CO to CO2 oxidation). Thus, residence time in the whole combustor
does not affect the fl ame-generated NOx produced – a signifi cantly different
behavior compared to thermal NOx. A convincing demonstration of this
point was presented by Leonard and Stegmaier2 who studied multiple fl ame
holders, operating conditions, and residence times from 2 to 100 milliseconds,
demonstrating that the fl ame temperature alone (not residence time) determined
the NOx production for emissions under 10 ppmv. Fig. 1, is useful to estimate
the fl ame NOx produced at a given fl ame temperature, accounting for ideal,
and “poor” premixing (not carefully defi ned in note 2). Note that the effect of
poor premixing raises the NOx levels by as much as a factor of three. These
data were recorded in turbulent fl ames, where combustion products are mixed
with the fresh reactants right at the fl ame. It has been suggested that other
combustion confi gurations, without signifi cant stirring between the fl ame front
and products, may reduce the fl ame generated NOx.3 This may be the basis for
NOx reductions reported in the Low-Swirl Combustion section.
Finally, fuel-bound NOx is produced by nitrogen species in the fuel
reacting with air during combustion. For coal syngas, the most prominent
fuel nitrogen species is ammonia, generated during gasifi cation from nitrogen
compounds in coal. The ammonia should ideally be removed from the
fuel before entering the combustor, or it will be converted to NOx by most
combustion strategies. Where this is not possible, rich-lean strategies have
the most potential to reduce NOx pollutants. In this approach, combustion
is fi rst carried out under fuel-rich conditions, followed by completing
combustion under fuel lean conditions. In fuel rich conditions, with suffi cient
residence times, the ammonia can be reduced to nitrogen and water, rather than