10-04-2017, 09:08 PM
abstract
This work presents the development of a simple analytical model of performance for heavy duty gas turbine
combustors and its use for the analysis of main emissions for a set of syngas fuels. This set of syngas fuels has
been selected as a wide representation of different compositions of syngas fuels, from fossil or vegetal
origins. Their combustion processes have been modelled as a set of chemical reactors in serial and a detailed
kinetic model, simulating a conventional diffusion flame combustor. In each slice, the thermodynamics and
the kinetics have been modelled using perfect stirred reactor models. The combustor model has been
validated with the GE MS7001F gas turbine experimental data. From this validation the model applicability
range has been established for combustor outlet temperatures above 1200 K. Finally the combustor model
has been applied to the comparison of different syngas fuels emissions in three new generation gas turbines.
1. Introduction
The traditional fuel prices oscillations and their geographic
distribution, is making that the use of fuels with alternative origins,
as those generated from renewable sources or from more abundant
fossil sources, is increasingly considered [1]. Hydrogen rich fuels being
available the second step of the process is related to gas turbines
engineering since this is the equipment where these fuels are to be
burnt. Among all its components, fuel injectors and combustor are
those that need to be modified more due to the higher mass flows
required to achieve similar rated power [2 4], with a lot of work to be
done at the turbine redesign as well to adapt it to the higher mass
flows[3]. Gas turbines manufacturers are committed in the development
of engines capable of burning hydrogen rich fuel as efficiently
and reliably as possible and most of them are running specific
programmes for it: Siemens [5,6], General Electric [7,8], Mitsubishi
Heavy Industries [9] and Alstom [10], adapting their gas turbines to
fuels from variable origins [1], and with pilot plants in different
localizations [11] (Table 1).
Under the denomination of synthetic gas (syngas) fuels, mixtures of
gases with different fractions of hydrogen, methane, carbon monoxide,
carbon dioxide and hydrocarbons are considered. Main differences
compared with the gas natural as fuel are associated to their composition.
The combustible and inert gases mixture, hydrogen and
methane, have quite different LHV (Low Heat Value) and transport
properties. Also the high percentage of inert gases, like CO2, diminishes
the LHV of the mixture and constrains the kinetic of the combustion,
mainly because of the heat capacity increase of the syngas and therefore
the decrease of the maximum attainable combustion temperature.
Through thismechanismthe reaction rate and combustion products are
also affected. For this reason they are usually classified as Low Caloric
Value Fuels (LCV) [1,4].
Gas turbines are primarily designed to be fuelledwith natural gas, the
use of syngases as fuel will present two main redesign challenges [12]:
different combustion characteristics.
higher volumetric and mass flow rates of fuel gases through the
gas turbine combustor and turbine.
Recent designs of gas turbine combustors use lean-premix combustion
technology to avoid hot spots in the combustor; however
syngas fuels cannot be directly used in natural gas lean-premixed
combustors designs due to the combined effect of the shorter autoignition
delay and faster flame speed of hydrogen. This combination
produces an unacceptable risk of the combustion flame propagating
upstream, or flashing back, into the lean-premix zone [12], under
these considerations, premixing becomes a very questionable practice
[3]. So, in a first step of the design, the gas turbines adapted for syngas
fuel would use single-stage diffusion combustors featured in older gas
turbine designs until lean-premix combustors for syngas fuels are
developed.
This work is focused on the simulation of the reaction mechanisms
with reactants from syngas fuels to predict the main gaseous emissions
in heavy duty gas turbines. It presents the development of an analytical
model of performance for heavy duty gas turbine combustors
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