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Liquid Lignin as Fuel
By Michael Lake

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: [email protected].


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