POWER February 2021 - 30

OPERATIONS & MAINTENANCE
How to Avoid Full Refractory System
Failure in Wood Combustion
Wood-burning facilities have seen a spike in refractory problems due to operating
units at higher temperatures and at increased capacities than in the past. Refractory
professionals offer three recommendations to reduce slagging, spalling, and corrosion
in today's wood-combustion systems.
Dan Szynal, et al.
he combustion of wood, wood
waste, and wood by-products is
not new technology. Refractory
materials and engineering designs have
successfully been used in wood-burning
industries for more than 100 years. Yet, if
wood has been used in boilers and other
combustion applications for more than
a century, then why are so many power
generation systems experiencing so
many refractory challenges? It could be
that wood waste is no longer considered
a disposable waste, but rather, is being
used as a primary or secondary source
of fuel for power generation systems.
Incinerators and boilers used in power
generation experience more refractory
challenges as combustion temperatures
and/or fuel volumes increase. Some of
the newer system designs call for high
operating temperatures, and many times
companies are retrofitting and redesigning
their older systems to increase capacity
and/or efficiency.
There are also different slagging problems
associated with different types of
wood and wood waste from various locations
around the country. On occasion, a
system that worked well with one type of
wood waste encounters problems when
a different type of waste is burned. The
long-servicing, super-duty refractory used
to protect incinerators and boilers that had
T
2. This chart shows the heat release of wood per cubic foot (ft3) of combustion volume as
related to the fuel's moisture content. Source: Plibrico
worked well for the past 20 years, now
may suddenly fail in three to six months.
Therefore, the new, retrofitted, and redesigned
power generation systems of today
require a higher-quality refractory.
1. Typical wood analysis. Source: Plibrico
30
Wood Product Combustion
The main refractory challenge associated
with wood-product combustion is
slagging and the subsequent spalling,
corrosion, buildup, and/or dissolution of
the refractory. This is due to a temperature
dependent reaction between the
combustion ash and refractory. Moisture
in the wood fuel has a direct effect on
furnace temperature, and subsequently,
on the refractory challenges.
Wood-scrap combustion encompasses
wood, sawdust, sawmill scrap,
finishing scrap (trim), bark, or any combination
of these items. A typical wood
analysis is shown in Figure 1. While 1%
of wood products are typically ash content,
ash can range from 0.3% to 4%,
depending on the type of wood product
used. The moisture content of the
wood fuel is also important because it
can play a direct role in determining the
furnace operating temperature. Table
1 lists the moisture content of various
types of wood products.
The moisture content of wood fuel
has a direct effect on its heat output and
furnace operating temperature as shown
in Figure 2. Perfectly dry wood of almost
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any type has an approximate Btu output
of 9,000 Btu/lb, with the exceptions of
resinous pine, plywood, and composition
board scrap where resins are used as a
binder. These exception materials may
produce a heat output of up to 12,000
Btu/lb. Scrap moisture content decreases
Btu output as shown by the net heat
values in Figure 2.
Furnace temperature will increase
as drier wood is burned. Therefore, a
change in wood fuel moisture and/or
volume burned can have a drastic effect
on furnace temperatures and refractory
degradation.
Effect of Moisture on Wood Product
Combustion
There are dramatic effects that a change
in moisture content can have in the heat
release in a furnace (Figure 3). Typically,
furnaces are designed to operate at a
heat release rate of 25,000 to 60,000
Btu/ft3
of combustion volume per hour.
For example, burning wood scrap with a
Wood Type
Green woods
Seasoned woods
Saw mill refuse
Kiln dried woods
Moisture content
25-40%
15-20%
20-55%
5-10%
Table 1. Moisture content of wood.
Source: Plibrico
POWER | February 2021
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POWER February 2021

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