Olympian in size, recovery boilers are getting faster, higher and stronger, but North America accounts for only about half of the newly installed units
Coming soon to a recovery boiler near you - even higher operating pressure? Still higher temperature? More exotic (and expensive) tube metallurgy? How about a leap to gasification technology instead? Or perhaps you need to know what to do with the recovery boiler you're operating now.
With the biggest, newest black liquor recovery boilers now tagged at approximately $150 million, the focus on chemical recovery operations continues to ratchet up in importance, whether your point of concentration is design, operations or business.
"Faster, higher, stronger" are the Olympian characteristics of leading-edge kraft mills, now built at 3,000 tonnes/day and still increasing, according to Esa Vakkilainen (Poyry Forest Industry Oy, Finland) at the recent International Chemical Recovery Conference in Quebec, QC, May 29-June 1. The triennial gathering provided a comprehensive platform of information from around the world to some 200 delegates via four days of presentation sessions and posters.
Vakkilainen offered a broad perspective in which to place today's kraft recovery systems, whether new or "experienced." The largest recovery boilers can now process 5,000 tonnes/day (dry solids basis) of black liquor, more than 25% higher than just five years ago and double that of a decade earlier. Superheated steam temperature continues to rise in order to maximize power generation.
During the past 10 years, two -thirds of the installations have been at existing mills. But over this period, North America has accounted for only about half the newly installed recovery capacity in (pick any one) Europe, South America, or Asia. The average recovery boiler in North America or Europe is about 30 years old. With a life expectancy probably not exceeding 40 years, time is running out rapidly.
Besides huge size, what other attributes can you expect in the most modern recovery boilers?
* Steam pressures now range up to 1,800 psig, temperatures to 500°C (but not necessarily both in the same boiler for the same reason)
* As-fired black liquor solids content of 80-85%
* Superior materials of construction (especially tubing) for corrosion resistance in what's essentially an industrial volcano
* Potassium and chloride removal systems
Other parts of chemical recovery plants are receiving attention too, such as high-rate, high-capacity black liquor evaporation systems (rated at 1,000 1 H2O/h) with seven or eight effects to reach percent liquor solids in the 80s. Without state-of-the-art evaporation, it is difficult to maximize the potential of a high-efficiency recovery boiler. These evaporation sets use duplex stainless steel for surfaces exposed to black liquor above 70% solids, plus heat treatment to reduce liquor viscosity.
A Phase Change?
So where does gasification fit into this picture of kraft recovery boilers? For now, it is only as a booster for incremental processing capacity. With the continuing technological development of conventional recovery boilers, their size, capacity and operational characteristics expand to meet the ever more gargantuan needs of modern kraft mill designs. Ongoing improvements in materials, sensing techniques, process control, and operational reliability provide very tough competition for any new technology to crack.
Gasification could be a competitor - perhaps, some day, somewhere - but it is a long shot. Ten years into development on an industrial scale at Weyerhaeuser's New Bern, NC, mill, the Chemrec high-temperature/atmospheric pressure form of this technology offers incremental capacity equivalent to -350 tonnes/day of pulp. It struggled with operational problems early on, and the refractory lining continues to pose a challenge (it lasts about two years). Chemrec also has a pressurized pilot gasifier under development in Sweden. But few people are imagining gasifiers that could compete head-to-head with modern conventional recovery boilers. Perhaps gasifiers will succeed on biomass earlier than on black liquor.
