The recovery furnace uses as fuel the organic content in the black liquor coming from the pulp washers.  But at this point that liquor is only about 15% black liquor solids (BLS) and must be concentrated to at least 60% before it can be burned in the recovery boiler.

 Evaporator.  Most of that concentration happens in a multiple-effect evaporator,  a piece of equipment in which the water in the weak liquor is boiled in a sequence of vessels, each held at a lower pressure than the last.  Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires process steam from the boilers.  The black liquor leaves this evaporator at a concentration of about 55% solids.  The condensate from this process is probably the highest strength wastewater stream in the mill.

 The black liquor is further concentrated in a non-direct contact evaporator (NDCE) which is actually part of the recovery furnace itself.   The NDCE further concentrates the black liquor from 55% to 65% solids by indirect exposure to steam.   This is a great improvement over the older method of bringing the liquor into direct contact with the flue gases of the recovery furnace, a process that is largely responsible for a pulp mill’s characteristic odor, as the sodium sulfide in the black liquor reacts with the carbon dioxide in the furnace exhaust to produce hydrogen sulfide.   Old-timers in PT will tell you that the mill’s aroma used to be much stronger and pretty much continuous.

 Recovery furnace.  What is now termed “heavy black liquor” is burned in the recovery furnace, augmented by recycled fuel oil to aid in combustion.  The high pressure steam produced is enough to supply a major portion of the Mill’s steam load and electrical power needs.

 But the chemical recovery furnace is more than just a power boiler.  It is a complex device which also serves as a chemical reactor, a chemical recovery unit and an internal high efficiency SO2 scrubber.  In the process of burning the black liquor, sodium sulfate is converted to sodium sulfide (Na2SO4 + 2C  -->  Na2S + 2CO2).   This, and the sodium carbonate (Na2CO3) which remains unchanged in the burning process, are sent to the chemical recovery area of the mill in the form of the molten smelt.  There it is further transformed into the white liquor used in the digesters of the kraft pulp mill.


Emissions and Controls

 Because of the high sulfur content of the black liquor fuel, there is a lot of SO2 created in the burning of it.  The recovery furnace is designed to minimize the release of this SO2, since the sulfur is required in the chemical recovery cycle.  Under normal operating conditions, internal controls are more than 98% efficient in capturing SO2 from the flue gas to be sent to chemical recovery.  

In addition, three electrostatic precipitators (ESPs) work to capture much of the particulate (PM) emissions,  most of which are sodium salts which are also sent to the chemical recovery area for reclamation.   Two of these ESPs are Research Cottrell, rebuilt in 1993, and one is Environmental Elements​, installed in 1987.

Even so, the recovery furnace is the single biggest source of air emissions at the mill.  The main pollutants (in order by tonnage) are carbon monoxide (CO), sulfur dioxide (SO2), nitrous oxides (NOX),  small particulates (PM2.5) and various VOCs.  There is also a relatively small amount of formaldehyde released.



Trinity BART Report, 2007:

Trinity BART Report Addendum:

DOH Health Consultation, 2008:


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