Gas Turbine Handbook : Principles and Practices

240 Gas Turbine Handbook: Principles and Practices
The exact figures depend on specific operating conditions. Specific
heating value of the syngas can vary, therefore the entire delivery
system was sized to accommodate the resulting volumetric flow
changes. Piping and valve materials were upgraded to nickel based
super alloys and higher grade stainless steels for their improved high
temperature properties and corrosion resistance. The low Btu content
and elevated temperature of the syngas imply that large volumes of
gas are required. Flow from the gasifier is accommodated in a 16-
inch [41cm] diameter pipe with a 12-inch [30cm] diameter manifold
supplying syngas to the six combustors via 6-inch [15cm] diameter
flexible connections. By contrast, the corresponding figures for natural
gas are 4 [10cm], 3 [7.6cm] and 1.25 inches [3.1cm] respectively.
With combustor case bleed, syngas, natural gas, steam injection,
compressor discharge purge bleed and inlet bleed heat manifolds
all positioned closely on the unit, careful attention must be paid to
manifold arrangement. (Figure 14-3, View of MS6001FA Manifold Arrangement)
Syngas at 1000°F [538°C] burns spontaneously when in contact
with air. The fuel system is designed to eliminate syngas-to-air interfaces.
Both steam and nitrogen purging and blocking systems are being
evaluated. General Electric has used nitrogen as a purge medium
in previous IGCC applications. While operating on syngas, nitrogen
would be supplied at a pressure higher than the syngas and would
use a block and bleed strategy to prevent contact with the purge air
supply. Operating on natural gas, the syngas would be blocked by
an additional stop valve and a stop ratio valve. The nitrogen purge
valves open to flush syngas from the entire system downstream of the
stop valve, and then close. Compressor discharge air is then used to
purge the line from the control valve to the combustor while nitrogen
provides a block and bleed function between the control valves. Correct
sequencing of the valves maintains separation between syngas
and air. A steam based system would be functionally similar. (Figure
14-4.)
Combustion System
The MS6001FA uses a reverse flow can-annular combustion arrangement
common to all GE heavy duty gas turbines. There are six
combustors, equally spaced and angled forward from the compressor
discharge case. Compressor discharge air flows over and through