Power Engineering - February 2018

around 9ppm. But we also know that these requirements are typically enacted gradually. Switching to operating at 9 ppm NOx now for regulations that might not come into effect for 5 or 10 years means incurring a lot of unnecessary operating costs in the meantime. At the same time, many old burners need to be replaced -- and if you’re going to replace one, shouldn’t you replace it with an ultra-low NOx model so that you’re not facing another replacement in a few years when regulations change?

The way out of this conundrum is the latest breed of low-NOx burners that offer configurable NOx emissions concentrations.

These now exist in both FGR and lean premix forms. In the case of the FGR configurable burners, the amount of flue gas that gets recirculated is programmable over a wide range, and down to the lowest theoretically possible levels. In the case of the lean premix configurable burners, that fuel-to-air ratio is configurable, from super lean to very rich. These configurable systems are the best option for areas that don’t yet face the strictest NOx standards. By switching to one of these burners, NOx emissions can be set to exactly meet current NOx regulations, while maximizing efficiency. When and if these standards change in the future,

the burner can be simply reconfigured to meet the new requirements.

RETROFITTING FOR
BIOFUEL: THE NEW
FRONTIER OF LIQUID
WOOD
Along with a trend towards lower NOx
emissions, there is a trend toward lower
carbon footprints and the use of renewable
biofuels. Until recently, the only viable op-
tion for boilers was solid-mass biofuels -- in
particular, wood pellets or chips.

The wood for these fuels can be sustain-
ably sourced from tree farms, and replanting
the trees used to produce the fuel can mostly
offset the carbon emitted from burning
them. What has limited their widespread
adoption, however, are the significant costs
of switching to such a system. An entirely
new boiler is generally required to enable
burning solid chunks of fuel as opposed to
burning liquid or gas. The solid biomass
fuel will need to be burned over a grate or
in a fluidized bed boiler, which is signifi-
cantly different from a standard firetube or
watertube boiler. Additionally, burning solid
biomass produces much more ash, and the
burner has to be equipped so that this can
be regularly removed from the boiler. All
of this means that switching to solid
biomass fuels can easily exceed $20M in
upfront investment. That type of investment
is generally only made sense for institutions
with very strict carbon or renewable com-
mitments (for example, universities which
adopted strict carbon commitments under
the American College & University Presi-
dents’ Climate Commitment).

However, it is now possible to retrofit for biofuel because liquid wood, ( renewable fuel oil or bio-oil) has become viable for use in existing gas or oil boilers for the first time. Liquid wood is produced by heating wood in the absence of oxygen, in excess of 500º Celsius. Because there is no oxygen, the wood does not combust: it first becomes charcoal, and then further decomposes into gas and liquid. When heated above 700º C, pyrolysis occurs very quickly and the yield will be around 60% bio-oil, 20% char, and 20% “syngas.”