Key Combustion Issues Associated with Syngas

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3.1
Key Combustion Issues Associated with Syngas
3.1-1 Key Combustion Issues Associated with Syngas and High-Hydrogen Fuels
and High-Hydrogen
Fuels
Combustion of syngas and high-hydrogen fuels requires attention to key
combustion issues, especially if low emissions are to be achieved using these fuels.
Current combustion systems operated on natural gas have evolved to the point where
low single digit NOx emissions are possible with lean premixed strategies. However,
the price of this evolution has been a signifi cant increase in sensitivity to various
perturbations such as changes in ambient conditions and variation in pipeline natural
gas composition. In light of these observed sensitivities, strategies over an above lean
premixed are continuing to be evaluated as discussed in “Combustion Strategies for Syn-
gas and High Hydrogen Fuel”. To reduce risk and development time, it is desirable to
apply the experience of developing low emissions combustion systems for natural gas to
syngas and high-hydrogen fuels. However, the range of compositions found in syngas
and high-hydrogen fuels varies more substantially than similar properties of pipeline
natural gas. By way of example, consider the ranges of composition shown in Table
1. Table 2 summarizes the range and average values of the fuel constituents shown.
As a result of the wider range of composition found in syngas and high
hydrogen fuels, strategies well suited for low emissions performance on natural gas
may not necessarily work best for syngas and hydrogen containing fuels. That said,
it is important to note that the variation indicated in Table 1 and Table 2 is somewhat
misleading. Specifi cally, if a given feedstock and gasifi cations process is considered, the
variation found will be much less. By way of example, if the processes are limited to PSI
Wabash, Tampa, El Dorado, and Motiva, a representation of variation found in coal/pet
coke fed, oxygen blown gasifi cation systems can be established as illustrated in Table 3.
The other point to be made by way of introduction is that hydrogen poses the
most signifi cant challenge in terms of the combustion system. As a fuel, hydrogen
behaves differently than a hydrocarbon in many ways including specifi c heat
(hydrogen has a much higher specifi c heat than other gases), diffusivity (hydrogen
has a much higher diffusivity than other gases), fl ammability limits (hydrogen has a
wide range of volume concentrations over which it is fl ammable), and fl ame speed
(hydrogen has a much higher laminar fl ame speed than do other gases). As a result,
the presence of hydrogen creates issues for combustion that require a different
perspective than would a hydrocarbon fuel. Further, mixtures of gases often exhibit
non-linear behavior and little data are available on the types of mixtures found in
syngas.
With this in mind, the key issues that are associated with combustion of syngas and
hydrogen containing fuels can be broadly classifi ed into two major areas: reaction location
and stability. These two areas are discussed in detail in “Static and Dynamic Stability”.
Reaction location is an issue for all strategies and is related to the chemistry
and time scales associated with the system. In strategies which involve premixing the
fuel and oxidant (especially for lean strategies), the possibility of reaction evolving into
the premixing region is a major concern. Given the high fl ame speeds of hydrogen, this
concern must been examined carefully. Another issue related to the reaction location is
ignition delay. With 1-5 msecs of premixing time available, at typical gas turbine inlet
temperatures and pressures, ignition delay does not appear to be a major concern for
the fuels of interest—however, the predictions of ignition delay have been developed
largely in the absence of data at these conditions. As a result, better understanding of
whether autoignition might be a factor is really needed to confi rm this expectation.
Stability can broadly be divided between static and dynamic regimes. It
is often reasoned that the wide fl ammability limits of hydrogen can allow stable
operation at leaner (and therefore cooler) reaction temperatures. This extension of
the lean blow off or static stability limit is an inherent benefi t that should be realized
from the presence of hydrogen. However, the extent to which this limit can be
achieved in practice and, furthermore, its sensitivity to variation in composition is
a key issue. On the other hand, dynamic stability, which is less predictable than
static stability, can arise for combustion systems due to various reasons. How fuel
composition impacts the propensity of a system to exhibit dynamic stability issues is
another concern that must be addressed.
Vincent G. McDonell
Advanced Power and Energy Program
University of California
Irvine, CA 92697-3550
949-824-7302 ext 121
vgm@apep.uci.edu
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