Potential Applications of High Temperature Air Combustion Technology to Other Systems - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 354

6.6 COMPACT BOILERS

The basic structure of a boiler consists of a radiant component and a convective
component. The convective section of the boiler is much larger than the radiative
section. Most of the heat transfer occurs in the radiative section of the boiler.
Therefore, any attempt to reduce or eliminate the convective section of the boiler
will result in a compact size boiler. The compact boilers have small or no convective
section and take advantage of the features associated with the high temperature air
combustion processes (very high heat flux plus feedback of the exhaust thermal
energy into the combustor). The radiative heat flux with the high temperature air
combustion is much higher (in excess of 500%) than the conventional boiler. The
use of compact boilers in urban areas is particularly attractive from the point of view
of space and low emission levels, including NO x .

6.7 GAS TURBINE COMBUSTION, MICRO GAS

TURBINES, AND INDEPENDENT POWER
PRODUCTION

The characteristic performance goals for a gas turbine include time/range, complex-
ity/weight and size, cost, reliability/durability, practicality, and emissions/signature.
Combustion under lean premixed conditions is preferred for achieving low NO x emis-
sion and high efficiency. However, under very fuel-lean conditions it is not possible
to stabilize a flame. Flame stabilization becomes an issue when the combustor size is
small. Flame quenching becomes an issue for smaller size because of the large ratio
of surface area to volume of the combustor. This issue becomes even more important
when using gases of very low heating value. Application of high temperature combus-
tion air easily provides stable flame with even low calorific value gas (or liquid) fuel
provided the temperature of the combustion air is high (above the autoignition tem-
perature of fuel). 3 Since the autoignition temperature for most hydrocarbon fuels is
much below 1000˚C the fuel injected into the gas turbine combustion chamber will
ignite and burn quickly. It is to be noted that the pattern factor associated with high
temperature air combustion is very good because of the wider flame volume and low
to negligible temperature fluctuations in the flame. The ignition delay can be tailored
to satisfy the needs of the combustor. The high temperature air combustion technology,
therefore, has good potential for application to stationary gas turbines, in particular
micro gas turbines for independent power production (IPP). The emission of CO and
hydrocarbons is negligible with most hydrocarbon fuels under high temperature air
combustion conditions. Soot, particulates, and ignition delay can be an issue with high
carbon ratio fuels so that one needs to develop means of burning residual fuel oils in
stationary gas turbines. For gaseous and light liquid fuels soot and particulates are not
a major issue. Some data are available in the literature on the use of heavy fuel oils
with high temperature combustion air. 12 Emission of NO x is also low since the thermal
field uniformity in the combustion chamber is very high, which also results in a good
pattern factor. The thermal field uniformity under high temperature air combustion
conditions is far superior to any other known combustion method. An interesting gas