The Kraft paper process was invented in 1879 and produces a stronger finished product that other paper manufacturing methods. One of the waste streams is known as black liquor and is a mixture of solids and water. It contains lignin, hemicelluloses and chemicals used in the pulping process. The original process had no use for this harmful waste stream and it was dumped into nearby waterways, to their detriment!!! Mr. G.H. Tomlinson invented the recovery boiler in the early 1930’s. This development made the Kraft process the manufacturing method of choice, as explained below.
As you may have guessed, the recovery boiler is a piece of equipment that uses black liquor, thereby preventing its discharge into lakes and rivers. The black liquor that comes from the process has a very high water content, it won’t burn. However, if the water is evaporated off such that the solids content is higher (65% – 80%), the material will burn. The balancing act that paper manufacturers have to pull off is to concentrate the liquor enough so it will burn, but not too much or else it won’t flow through the nozzles of the boiler. When they are successful, they burn the black liquor in the furnace section of the boiler. So, the first thing a recovery boiler recovers is heat.
The heat from the burning black liquor is used to turn water into saturated steam. That steam is used elsewhere in the process for heating. For example, the cooking and digestion steps all occur at elevated temperatures. Steam is also used in the black liquor evaporation process. Later design developments would allow the production of superheated steam to first turn a turbine and generate electricity before being used elsewhere for heating in the process. So, the second thing a recovery boiler recovers is power.
The paper manufacturing process requires steam for heating and power for pumping and other electrical uses. If the recovery boiler weren’t providing them, the manufacturer would have to buy fuel for another boiler and buy electricity to power their pumps. So, the third thing a recovery boiler recovers is dollars! This is the advantage of the Kraft process, from a business perspective, that elevates it above other paper processes.
The inorganic chemicals (primarily sodium, sulfur, and calcium containing compounds) are also transformed via the combustion process. The solid “smelt” discharged from the bottom of the recovery boiler is sent back and treated to become the white liquor used in the pulping process. In this way, the inorganic compounds are trapped in a closed loop within the process. So, the fourth thing the recovery boiler recovers is the pulping compounds.
As environmental regulations have required lower emissions from manufacturing facilities, design improvements to recovery boilers have lowered the amount of sulfur and NOx that leave the units. This has primarily been accomplished by altering the way air is introduced into the furnace section. So, in a way, the last thing a recovery boiler recovers is a better environment than if it wasn’t being used.
Jennifer Morningstar is President and Senior Consulting Engineer of The Warren Group. A licensed professional engineer in several states and a NAFI Certified Fire and Explosion Investigator, she holds a Bachelor of Science Degree in Chemical Engineering from Virginia Polytechnic Institute and State University, as well as a Master of Business Administration from the University of South Carolina. Over her 20-year engineering career, Jennifer has conducted forensic investigations involving chemical release/exposure, OSHA process safety management, industrial accident investigation, equipment failures, fires & explosions, and scope of damage/cost to repair. Jennifer is a member of the National Association of Fire Investigators (NAFI), the South Carolina chapter of the International Association of Arson Investigators (SCIAAI), and the American Institute of Chemical Engineers (AIChE).