A pulp mill toolkit: new equipment and operating methods can help mills tackle common pulp mill problems.
The basic design principle for pulp mills requires the proper material selection for each position and application, said Heikki Jaakkola, recovery research and development manager for Andritz Oy, Finland. "There are no 'one size fits all' solutions today," he noted. "For example, modern evaporation plants operating at over 75% dry solids concentration with high effective alkali concentrations and temperatures over 140[degrees]C require duplex steels with better corrosion resistance. The basic material for evaporators is AISI 304. AISI 316 is not suitable for black liquor evaporators."
Metallurgy selection in the bleach plant depends on process conditions in each bleaching stage, said Olavi Pikka, fiberline research and development manager for Andritz Oy. Processes that contain chlorine chemicals require materials with higher corrosion resistance. Typical materials in these positions are titanium, SMO[TM] stainless steel, and fiber reinforced polymer (FRP) composite.
"One area often overlooked is the optimum selection and use of bleaching chemicals," said Pikka. "If the chemical consumption is optimized, there is a decreased risk of overdosing and corrosion."
Olavi Tervo, vice president, special projects for Andritz AG, points out that recovery boilers operating with higher black liquor dry solids (greater than 75%) typically have no S[O.sub.2] emissions. This minimizes corrosion in the boiler "back end" (convection surfaces).
According to Kari Saviharju, recovery boiler research and development manager, Andritz Oy, understanding the underlying phenomenon behind corrosion is vital. "Key areas are lower furnace and superheaters in the recovery boiler," he said. "The combination of primary air port design plus the right composite materials play an important role." He noted that systems for high dry solids, including the liquor piping, require specific attention. For example, 316 steel is not safe. "The trend toward higher steam temperatures (above 500[degrees]C) and pressures creates a more challenging environment for material corrosion resistance," he stated.
MINIMIZING AIR EMISSIONS
In a modern pulp mill, all exhaust gases undergo collection and treatment to avoid emissions of harmful substances. New washers have atmospherically closed designs to avoid generating exhaust gases like those of older "open" washers--vacuum washers--according to Janne Vehmaa, fiberline research and direction manager, Andritz Oy.
Today, vent gases collected from all tanks go to the noncondensible gas (NCG) system of a mill, according to Jaakkola. "Evaporators with secondary condensate segregation capabilities ensure emission-free recovery and reuse of condensates within the mill," he stated. "An example of an odor-free pulp mill is Rosenthal, Germany, where all gases are collected and treated."
Current requirements for total reduced sulfur (TRS) and S[O.sub.2] emissions are attainable if the make-up lime is clean and calcium-containing chemicals are continuously regenerated, noted Jouni Jantti, research and development engineer, Andritz Oy. "Nitrogen oxide (NOx) emissions are minimized by optimal control of combustion in the lime kiln," he said. "Dust emissions are controlled using proper dimensioning of electrostatic precipitators. The tendency now to burn high concentration gases in the lime kiln collected from the NCG system within the pulp mill makes control of emissions more difficult. Vent gases from the recausticizing tanks can be collected if the tanks are well sealed. The collection of diluted gases could be improved in old mills within the recausticizing area."
In the recovery arena, Tervo identified two major contributors to reduced emissions in the recovery boiler. These are high black liquor solids and the new Vertical Air[TM] distribution system--a new retrofit product from Andritz. The system significantly minimizes S[O.sub.2], carbon monoxide (CO), and NOx emissions in a boiler.
Saviharju stated that a recovery boiler can burn gases in a manner that does not increase emissions. "Using high dry solids firing and modern combustion technology, S[O.sub.2] emissions are today practically zero," he said. "Also, NOx emissions are an important issue in many European countries. Conversion factors from nitrogen in black liquor to NOx can be less than 15% using modern combustion technology. This compares with conversion factors exceeding 30%-35% in conventional recovery boilers."
Saviharju added that TRS emissions are very low in modern boilers and that CO emissions can be adjusted to below 10 ppm if low NOx combustion is not necessary. Low excess oxygen content is preferable for low NOx emissions. This may increase the CO level to 100-150 ppm.
MAXIMIZING ENERGY EFFICIENCY
Technology development within Andritz now leads to fiberlines having fewer unit operations and less process equipment compared with earlier installations, said Vehmaa. "This means less power consumption overall as well as lower capital and operating costs. Also, the efficiency and capacity of process equipment has increased dramatically as production lines become bigger. This leads to lower power consumption per ton of pulp."
Vehmaa added that the design of individual equipment and process flow-sheets--closed systems and closed process circulations--translate to lower heat losses requiring less energy to run the process.
In a modern lime kiln, good heat economy is possible assuming the kiln has good thermal insulation and is fed with lime mud of a consistent quality and high dry solids content according to Mika Kottila, research and development manager, Andritz Oy. "In the recausticizing process, speed-controlled pumps save electrical power," he said. "Future improvements can be made in utilizing heat generated during the lime slaking process. Today, the heat is used only for the generation of hot water."
In the recovery area, boiler energy efficiency will improve by increasing the steam temperature and pressure, according to Saviharju. "Key elements are high dry solids liquor firing and low excess air combustion using the Vertical Air[TM] system that make feedwater preheating possible. Other preheats also must be effective to increase the power-to-heat ratio in energy production. In modern mills, dry solids concentration is approximately 80%, and the highest figures exceed 85% in kraft pulping."
Jaakkola noted that a modern 6- or 7-effect evaporator plant and high firing liquor dry solids (>80%) increase electrical power production of the recovery boiler. In a modern system, the foul condensate stripper is integrated within the evaporator plant so that practically no extra energy is necessary for condensate cleaning. Tervo added that high dry solids black liquor from the evaporators and the addition of a Vertical Air[TM] distribution system in the recovery boiler produce less flue gas losses, a cleaner boiler, and more complete combustion in the boiler.
"The result is increased production of electrical power for the same 'fuel' input," said Tervo. "In addition, a new higher pressure and temperature (1500-1700 psig and 920[degrees]F) recovery boiler will also maximize energy production. A biofuel gasifier retrofit for a lime kiln and the existing power boiler can replace fossil fuel in the power boiler with biogas."
About the author: Alan Rooks, editorial director of Solutions! magazine, edited this article using contributions from experts from Andritz Oy and Andritz AG.
Edited by Alan Rooks, Editorial Director
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|Title Annotation:||Practical Solutions|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Oct 1, 2003|
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