September 26, 2012 | ||||||||
Proper head fit maintains roll integrity and paper quality | ||||||||
· www.tappi.org · Subscribe to Ahead of the Curve · Newsletters · Ahead of the Curve archived issues · Contact the Editor |
Liquid Lignin as Fuel The industry is considering
lignin recovery as a low-capital option for expanding pulp production
of their mills when they are limited by their recovery boiler capacity.
The typical mill may want to remove as much as 50,000 ton/yr of lignin
from their black liquor to debottleneck their system, yet large-scale,
high-value markets for lignin products take time to develop.
A new process from Liquid Lignin Company called "Liquid Lignin as Fuel
(LLF)," provides an interim solution and takes a significant first step
toward producing lignin for value-added markets.
LLF generates a lignin-rich fuel which is fired in a biomass boiler
as shown below:
Black liquor is fed into the top of a column operating near the black
liquor process temperature and under a slight pressure sufficient to
keep the system from boiling. Carbon dioxide is sparged into the bottom
of the column. As the falling black liquor reacts with the rising carbon
dioxide, its pH is reduced from about 14 to about 9, and the lignin
precipitates as a true liquid phase. In the settler, which is an integral
part of the bottom of the carbonation column, dense liquid-lignin phase
separates easily from the lignin-depleted carbonated black liquor which
is returned to the mill's black liquor system.
The liquid-lignin phase exits the bottom of the settler and is then
depressurized. A portion of the water in the phase vaporizes, and solidified
liquid-lignin (SLL) results. The moisture content of SLL is about 30
percent and the ash content is less than 20 percent. The ash in the
SLL is primarily sodium bicarbonate.
A stand-alone, dedicated biomass boiler is a good option for burning
the SLL. The SLL should not be blended with normal hog-fuel in an existing
biomass boiler since the high level of sodium will react with the silica
in the hog-fuel, resulting in significant sodium silicate glass formation.
Also SLL is not recommended as a fuel for a lime kiln, since high sodium
levels lead to ring formation and other deleterious operation of the
kiln. In the biomass boiler, the sodium bicarbonate of the SLL is converted
to sodium carbonate, which can be collected as concentrate from the
boiler's ash-recovery system and incorporated into the host mill's green
liquor system. All of the sodium in the original black liquor is recovered.
Another unique feature of the LLF is the ease of scrubbing of the vent
gases. The vent gas from the carbonation column consists mainly of residual
CO2 and H2S, but no air. This concentrated gas can be readily absorbed
in a white-liquor scrubber. Bubbles of the concentrated CO2 and H2S
react rapidly with NaOH in the scrubbing fluid. In laboratory tests,
bubbles of this concentrated vent gas collapse quantitatively in as
little as two-feet of the scrubbing fluid. Valuable NaSH is recovered
as H2S reacts with NaOH. This efficient vent-gas scrubbing system is
facilitated because of the absence of air which normally impedes scrubbing
operations.
Commercial installation of the LLF system provides a platform for future
lignin manufacture. As value-added markets for lignin develop, a slipstream
of the liquid-lignin phase could be withdrawn to provide the feed for
LLC's Sequential Liquid-Lignin Recovery and Purification (SLRP™) or
any of the alternative batch process. The capital costs of the future
lignin system will be much lower since the first carbonation step is
provided by the LLF system.
The LLF system can be designed to fit both the need to remove lignin
to unload a Recovery boiler and to provide future lignin manufacture
and sale. Turn-down capability of LLF's carbonation column is straight-forward,
since the throughput of the LLF is determined by the diameter of the
column. The height of the column is fixed and is independent of the
throughput of the system.
Liquid Lignin Company can assess the feasibility of their LLF system
using a batch lab-scale carbonation which they provide free-of-charge.
They will present the LLF system in more detail at a poster presentation
at the upcoming TAPPI PEERS & IBBC conference meeting in Savannah, GA,
October 17-19, 2012. Register
for the conference online.
Dr. Michael Lake can be contacted at: michael@liquidlignin.com.
Now that you
are Ahead of the Curve, stay there by joining TAPPI. |
|||||||