Real Measurements Lead to Real Understandings and Better Operating Control of the Recovery Liquor Cycle, 2023 PEERS Conference

Black liquor is a complex fuel of water, lignin fragments, hemicellulose, inorganic salts, sugars, starches, organic sulfur compounds and tall oil soaps. In contrast to the other fuels of the world, the properties of black liquor are poorly understood. It changes from species to species, from mill to mill, and from day to day. It is consistently inconsistent. Because of the changing properties of this fuel, the recovery boiler experiences operating problems associated with fouling, loss of thermal efficiency, environmental discharges, and corrosion. Many of the earliest measurements of black liquor were inferential. These included temperature and density. The first attempts at providing online data occurred approximately 50 years ago and employed the use of auto titration which took advantage of the fact that mills were accustomed to tests based on titration. In 1996 Paprican (Pulp and Paper Research Institute of Canada) now FPInnovations, introduced the use of the FT-NIR (Fourier Transform Near Infrared) spectrometer to determine actual liquor parameters, online, at a Northern Ontario kraft mill. This approach did not use chemicals like the auto titration process. In molecular spectroscopy, the scan of the spectral frequency range allows for the individual components to be determined from their unique features. In 50 seconds, 128 scans are co-added to yield a spectrum of the liquor sample. In as little as a few minutes ,including sampling time ,information on numerous online black liquor parameters can be provided .

Many of these parameters such as lignin, inorganic/organic solids and soap determination are not presently available from other measurement approaches. Additional samples to determine other parameters can be handled at the same sampling station. The spectrometer, in turn, can receive information from several sampling stations using fiberoptic cables. In 2023 this technology can no longer be considered new. It is, however, an evolving system, as new applications, developed in collaboration with partners offer new approaches of control to the kraft liquor cycle by measuring the actual (real) components of black liquor. This paper will summarize the benefits of spectroscopy-based measurement systems and demonstrate the potential use of this new information to improve the understanding and control of the chemical recovery and energy process of the Kraft mill.

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