State of the Art in Black Liquor Gasification Technology

Kevin Whitty Larry Baxter University of Utah Brigham Young University Salt Lake City, Utah, USA Provo, Utah, USA

ABSTRACT Black liquor, a biomass-based fuel formed in the paper making process, has traditionally been burned in recovery boilers to generate energy and recover the inorganic chemicals that comprise about a third of the fuel. Gasifying black liquor offers an attractive alternative to the recovery boiler that has the potential to double the power output, reduce operational costs, improve safety and increase the yield of paper in a mill. Over the years, more than a dozen companies have tried to develop a commercially viable process for gasifying black liquor. About half these technologies have centered on low temperature gasification in fluidized beds. The rest have focused on high temperature gasification, usually in entrained flow reactors. Many of the low temperature technologies encountered problems attaining good carbon conversion (improves with increasing temperature) while avoiding bed agglomeration (worsens with increasing temperature). Condensation of "tars" has also been problematic in low temperature gasification. The high temperature technologies have suffered from problems with liquor distribution and severe corrosion by the molten smelt. Nonetheless, two companies appear to be on the brink of commercializing black liquor gasification. MTCI's fluidized bed steam gasification process has been developed to the point where MTCI now has five commercial projects under way. Chemrec has sold an atmospheric version of their high temperature entrained flow gasifier, and development of the pressurized version is entering the final pre-demonstration phase. If progress in these technologies continues, gasifiers should be a proven alternative to the recovery boiler within the next decade.

INTRODUCTION Black liquor is a biomass-based fuel that is formed in the process of making paper. The chief chemical cycle for a typical pulping process is depicted in Figure 1. In the digester, wood chips are cooked in a caustic solution called white liquor (mostly sodium hydroxide) to liberate the fibers that form the pulp and eventually paper. Roughly half the mass of the wood, containing lignin and other organic material, dissolves into the solution to form black liquor. In order to make the black liquor combustible, it is concentrated from about 15% solids to about 70% solids. The composition of this "strong black liquor" is roughly 30% water, 35% inorganic chemicals and 35% organic material that was dissolved out of the wood. Black liquor is burned in a large recovery boiler, which is the single most expensive piece of equipment in the mill. The recovery boiler serves the dual purposes of producing steam and recovering the inorganic chemicals in the liquor for recycle to the process. As the liquor burns, the inorganic Black Liquor 10-15%

Flue Gas Black Liquor Black Concentration Liquor Chips 70-80% Recovery Boiler Digester

Smelt

Pulp

Causticizing White Green Water Dissolving Paper Liquor Liquor Tank

Figure 1. Pulping chemical cycle chemicals collect in the bottom as molten smelt, mostly sodium carbonate. The smelt runs out of the boiler into a tank where it is dissolved in water to form "green liquor." The green liquor is causticized to form white liquor, thus completing the chemical cycle. Because the energy content in black liquor is derived from wood, it is classified as a biomass- based, renewable energy source. Global black liquor production is roughly 180 million tons annually, corresponding to an energy input of 80,000 MW. An average mill produces about 1500 tds (tons dry solids) per day, and recovery boilers with capacities as large as 3850 tds/day (corresponding to a thermal input of 600 MW) are in operation. Gasification of black liquor offers an interesting alternative to the conventional recovery boiler- based process. The primary advantage of a black liquor gasification system is that it has the potential to significantly increase electrical power production and overall thermal efficiency in the mill. The exact magnitude of the energy benefits depends on the configuration of the gasification system and pulping process. Several studies have projected improvements in energy efficiency for model mills, and have found that electricity production for a black liquor gasification combined cycle (BLGCC) system can be more than double that of a recovery boiler system without any penalty to overall thermal efficiency [1,2]. In addition to increasing power production, gasification has the potential for better environmental performance and improved safety. A final incentive for pursuing black liquor gasification is that it opens up possibilities for advanced pulping methods that could increase the quantity of pulp produced per ton of wood. Inspired by these advantages, no less than a dozen serious attempts have been made to develop a commercial black liquor gasification system. The majority of these projects have been discontinued, highlighting the fact that black liquor gasification presents very serious technical challenges. Currently, there are only two companies actively pursuing commercialization of the technology. This paper summarizes the previous attempts at developing black liquor gasification and describes the two systems currently being actively developed. These two systems define the state of the art in black liquor gasification technology today.

2 HISTORY OF BLACK LIQUOR GASIFICATION Black liquor gasification technologies can be divided into two categories based on their operating temperature. Low temperature schemes operate below the melting point of the inorganic material in the black liquor (approx. 750°C) and all have used fluidized bed reactors. High temperature schemes operate above the melting point, and a variety of reactor types have been considered. The following two sections summarize past efforts to develop low and high temperature black liquor gasification solutions. The technologies in each section are presented in approximate chronological order. Low Temperature Gasification Technologies Several companies have tried to develop low temperature processes for black liquor gasification. All of these have used fluidized bed reactors, either bubbling or circulating, and the problems that were encountered were largely the same.

VTT Circulating Fluidized Bed Gasifier (1988–1992). The Technical Research Center of Finland (VTT) worked on a circulating fluidized bed gasification process in the late 1980's and early 1990's as part of the Finnish Fuel Conversion Research Program [3,4]. A 1.5 tds/day pilot unit designed to operate at pressures up to 20 bar was built in Äänekoski, Finland. The unit was operated both at atmospheric pressure and at high pressure, but encountered a number of difficulties. Agglomeration of the bed material was problematic. Changing the bed material from Na2CO3 to Al2O3 made a slight improvement. Poor conversion of the liquor was also a problem. Efforts to address this by staging the gasification were made, but were largely unsuccessful. Circulating the bed was also difficult due to the low density of the black liquor char. The program was finally abandoned in 1992 due to a lack of funds.

ABB Circulating Fluidized Bed Gasifier (~1990–1997). ABB's process for black liquor gasification centered on an atmospheric air-blown circulating fluidized bed operating at roughly 700ºC [3,5]. Preheated air was both the fluidizing medium and the oxidant for gasification. ABB operated a pilot unit in Sweden for several years. Performance was reported to be good, with less than 1% unburned carbon in the solids and high sulfur reduction efficiency (>95%). The fuel gas was run through a cyclone and bag filter to remove entrained particles, and had a heating value of roughly 4 MJ/Nm3. Agglomeration of the bed did not cause severe problems. This may be due to the fact that the pilot runs were generally short, less than a day. But it also may be due to high fluidization velocities and associated attrition keeping Fuel gas agglomeration in check. ABB Cyclones discontinued development in 1997 due to lack of funding and lack of a Gas cooler clear market. Babcock & Wilcox Circulating Scrubber fluidized bed Bag house Fluidized Bed Gasifier (1989– gasifier 1998). Babcock & Wilcox's low temperature gasifier design centered on an air-blown, bubbling fluidized bed, operated either at Air atmospheric or elevated Bed pressure [6]. The system design Blower Air solids incorporated a fluidized bed preheater Black liquor combustor to preheat the air and Mix Weak wash tank Green liquor fully convert unburned bed material and tars not destroyed in the gasifier. A bench-scale system Figure 2. Fluidized bed gasifier similar to ABB process

3 was built and tested at atmospheric pressure, with bed temperatures ranging between 482–538ºC [7]. The testing proved that black liquor could be gasified in a low temperature, bubbling bed gasifier. However, serious problems associated with carbon conversion and condensation of tars were encountered, and it was concluded that unless these problems can be solved, the process will not be a viable commercial alternative. The program was discontinued in 1998. High Temperature Gasification Technologies While many companies have pursued low temperature gasification, at least as many have pursued high temperature gasification. The mix of approaches for high temperature gasification is much more varied, and the problems encountered are of a much different nature.

NSP Cyclone Gasifier (1971–1985). The Swedish Pulp and Paper Research Institute (STFI) worked for a decade and a half on a new black liquor recovery process ("Ny Sodahus Process," NSP, in Swedish) that involved gasification. Black liquor was sprayed into a downwards-sloping cyclone chamber and partially combusted (dried and pyrolyzed) at approximately 950–1050ºC with the help of an oil-fired burner. The condensed material was forced to the outside of the cyclone where char gasification took place, leaving a molten smelt that was tapped Flue from the bottom of the cyclone. The combustible gas gas that resulted was combusted with air in a subsequent chamber and heat recovery would proceed in much the same manner as a conventional boiler [8]. Early efforts involved conceptual studies Air Air and modeling of the process. Encouraging results from this study led several Swedish and Finnish companies and governmental organizations to team Black up and build a pilot unit at a mill in Sweden. The liquor pilot was operated during 1983–1984 and ran with a Smelt maximum capacity of roughly 78 tds/day [9]. The pilot efforts focused on the black liquor feed system and identifying suitable operating conditions. The NSP concept was proven, but problems were encountered with black liquor feed and severe Figure 3. NSP gasifier system corrosion of the cyclone. The project was discontinued in 1985 due to technical difficulties and lack of financial support.

Champion/Rockwell Molten Salt Gasifier (1978–1985). In the late 1970's Rockwell International Corporation began investigating gasification of black liquor by molten smelt. The results from these tests were excellent, and Rockwell subsequently teamed with Champion International Corporation (then St. Regis Corp.) to further develop the technology with the support of DOE [10]. The resulting system was quite novel, and involved a two-zone process in which black liquor is was dried and pyrolyzed in the upper part of a reactor as it flowed counter-current to hot product gases. In the bottom of the reactor, the char was gasified in a pool of molten smelt that resulted after full conversion. Sub- stoichiometric air fed into the bottom of the reactor, some into the smelt and some above the smelt, provided the heat necessary for the process. Molten smelt was allowed to overflow into a quench dissolving tank to form green liquor. Gasification took place at 900–1000°C, and commercial installations were envisioned to operate at elevated pressure, allowing integration with a gas turbine.

4 Black liquor A bench-scale unit with a capacity of feed nozzles 3.5 kg/h black liquor was constructed and tested with a variety of liquors at several conditions. These tests were successful enough that the project was taken to the next Product phase of development. A 350 kg/h pilot unit gas outlet Castable was constructed (Figure 4), and tests at refractory atmospheric pressure showed positive results. Black liquor conversion was essentially complete, sulfate reduction was over 95%, Insulation and a fuel gas with a heating value of 4.5 MJ/Nm3 was produced. Heat loss from Fuse cast the reactor was significant, which alumina bricks Upper air necessitated a higher air/fuel ratio than would nozzles (x6) be the case in larger, commercial plant. This Lower air resulted in unrepresentatively low fuel gas Smelt nozzles (x2) heating value and sulfate reduction. Funding withdrawal port for the project ran out in 1985. Air nozzle and oil gun port VTT Entrained Flow Gasifier (1989– Air nozzle and 1992). In addition to their fluidized bed liquor drain port gasifier, VTT worked on a concept for high temperature gasification of sulfate soap and Figure 4. Champion/Rockwell pilot gasifier black liquor. Gasification took place in an atmospheric pressure, air-blown entrained flow reactor at approximately 1000ºC [4]. A development reactor was built at the Äänekoski, Finland test facility, and was operated for a short period during 1992. Development ceased when funding for the program ended at the end of 1992.

Ahlstrom Gasifier (~1989–1993). Finland's Ahlstrom Machinery, a division of Ahlstrom Corporation, was actively pursuing a gasifier concept in the early 1990's. Few details of the process were ever made available, but it was known that it was a high temperature system that operated under pressurized conditions [4,5]. Ahlstrom had a pilot unit at the Äänekoski test facility where the VTT reactors were located, and tests were carried out here during 1991. These tests went long over schedule, suggesting that problems were encountered getting the system to function [4]. This may explain the lack of further development, which ceased some time around 1993.

Tampella Entrained Flow Gasifier (1989–1994). Tampella's system was based on high temperature gasification in a refractory-lined entrained flow reactor, similar to the Chemrec system described later [3]. Gasification took place at 900–1000°C, could be either air- or oxygen-blown and could be either atmospheric or pressurized. In contrast to Chemrec's water quench system which cools and separates the gas and smelt, the Tampella process employed a high temperature cyclone for separation followed by a fluidized bed gas cooler/air preheater. Tampella operated an atmospheric, air- blown pilot unit in Karhula, Finland between 1991 and 1994, and testing focused largely on removal of sulfur and alkali from the product gas [11]. A serious problem encountered in this system was that the high temperature cyclone was prone to corrosion by the molten smelt. Largely due to this, Tampella ceased development of their gasification system in 1994.

Noell Entrained Flow Gasifier (~1995–2000). The German company Noell developed a high temperature, pressurized process for black liquor gasification which was based on their coal gasification technology [12]. In many ways, the Noell system was similar to the Chemrec system and had an entrained flow gasifier with a water quench system underneath. But rather than lining the gasifier with

5 refractory, the walls of the Noell reactor are made of a membrane of water-filled tubes covered with a protective material. A layer of frozen black liquor smelt develops on the wall, protecting it from corrosion by the molten smelt. Trials in Noell's pilot facility demonstrated this concept for black liquor. In 2000, Babcock Borsig Power, the mother company of Noell, became the majority shareholder in Chemrec AB, and the Noell technology became part of Chemrec's development efforts. Other Black Liquor Gasification Technologies Several other pyrolysis/gasification technologies have been developed in addition to those described above. These include the following: • SCA-Billerud pyrolysis-based process • Weyerhaeuser dry pyrolysis process • St. Regis wet pyrolysis process • SKF plasma gasification process These technologies reached various stages of development, but generally did not advance as far as the processes described in the previous sections.

CURRENT STATUS OF BLACK LIQUOR GASIFICATION COMMERCIALIZATION Though there has been considerable interest in black liquor gasification over the past two decades, today just two companies, MTCI and Chemrec, are actively pursuing commercialization and represent the state of the art in the technology. MTCI/StoneChem Technology MTCI, based in Baltimore, Maryland, has been working on development of a black liquor gasification process since 1987. MTCI is independently owned and markets their process in North America and world-wide through two subsidiary companies, StoneChem and ThermoChem.

Process Description. The MTCI process centers on an indirectly heated, bubbling fluidized bed steam reformer () that operates at 580-620ºC, well below the melting temperature of the inorganic material in the black liquor. Black liquor is sprayed into the bed where it coats the bed material (primarily sodium carbonate) and quickly dries and pyrolyzes. The remaining char reacts with steam to form a medium heating value, hydrogen-rich fuel gas that passes through a cyclone at the reactor's exit to remove entrained solids. Sulfur in the liquor exits the reactor as hydrogen sulfide. Sodium remains in the bed in the form of carbonate. The fluidized bed is indirectly heated by Product Gas several bundles of pulsed combustion burners that burn a portion of the fuel gas generated in the bed. Each bundle contains a large number of burner tubes which provide a large surface area between the combustors and the bed to enhance heat transfer. The combustion tubes are of a very special design, and resonate as the fuel gas is burned inside. This Black Liquor PulseEnhanced™ combustion system is proprietary Flue Gas technology of MTCI and provides very efficient heat transfer between the combustion gases and the walls of Product Pulsed Combustors the combustor tubes. The exhaust gases from the Gas + Air combustors pass through a superheater to raise steam, Bed Solids Fluidizing Steam which in turn is used as the fluidizing medium and reactive gas in the gasifier. Figure 5. MTCI steam reformer

6 The gaseous product from the reactor contains roughly 73% H2, 14% CO2, 5% CO, 5% CH4 and 3 3% C2 and higher hydrocarbons [13]. It has a higher heating value of roughly 13 MJ/Nm . Upon leaving the reactor, the gas passes through a heat recovery steam generator, a venturi, a gas cooler and finally an H2S absorption system. It is then scrubbed with a sodium carbonate solution to remove H2S. A portion of this clean gas is used in the pulsed combustion heaters in the bed. The remainder can either be burned in an auxiliary boiler, pressurized and fired in a gas turbine or used in a fuel cell to generate electricity. In February 2001, MTCI demonstrated the integration of a steam reformer and fuel cell. Syngas from MTCI's test rig in Baltimore was fed to a solid oxide fuel cell and generated 161 watts of power [14]. A portion of the bed material is continuously removed and dissolved in water to form a sodium carbonate solution. This stream is treated to remove unburned carbon, and then is passed to the causticizing section of the mill. Part of the sodium carbonate solution may be used to scrub H2S from the fuel gas.

Status of the Technology. MTCI has operated a 0.25 tds/day process development unit at their facilities in Baltimore since 1990. In 1992, a 12 ton/day steam reformer for processing sludge waste was installed at Inland Container's Ontario, California recycled paper mill. In 1994, a 50 tds/day demonstration unit was constructed at Weyerhaeuser's New Bern mill in North Carolina. After a break-in period and some modifications to the reactor, the system was able to achieve 500 hours of continuous operation [15]. MTCI test systems have also been installed in Spain, Egypt and India. MTCI recently reported that they have five commercial projects in various stages of development [13]. These range in size from 61 to 454 tds/day and cover several different types of black liquor. The most visible of these projects is a 181 tds/day installation planned for Georgia Pacific's Big Island mill in Virginia. The mill currently uses "smelters" to process the black liquor, an unattractive method from both environmental and efficiency standpoints. Installation of a steam reformer is a very fitting solution, and promises to greatly improve operations. Construction is expected to be complete next year.

Technical Challenges. All other attempts at fluidized bed gasification of black liquor have had problems achieving good carbon conversion, necessitating high temperatures, while avoiding bed agglomeration, which requires low temperatures. MTCI faces the same challenge. Condensation of "tars" is a notorious operational problem in biomass gasification, and low temperature fluidized beds are particularly vulnerable. Chemrec Technology Chemrec AB is based in Stockholm, Sweden, and has been pursuing development of black liquor gasification since 1985. Chemrec began as an independent company that was sold to Kamyr (later Kvaerner) in the early 1990's. Recently, Kvaerner sold a majority stake to the German company Babcock Borsig Power [16]. Chemrec is pursuing two variants of their system, an air-blown, atmospheric "booster" version aimed at providing incremental black liquor processing capacity and a more advanced oxygen-blown, pressurized "BLGCC" version targeted as a replacement for recovery boilers.

Booster System Description and Commercial Status. The heart of the Chemrec process is a downflow entrained flow reactor, similar to a Texaco gasifier. Gasification takes place at 950–1000°C, well above the melting point of the inorganics in the liquor. Black liquor is fed through a burner in the top of the reactor, forming small droplets that are partially combusted with either air or oxygen. The heat generated by this combustion drives the gasification reactions, and the liquor is rapidly converted to fuel gas and molten smelt. After exiting the reactor, the smelt is separated from the gas and falls into a sump where it dissolves to form green liquor. The raw fuel gas is fed to a cooling and scrubbing system where water vapor condenses out and H2S in the gas is removed.

7 The booster system generally offers about 350 tds/day extra black liquor processing capacity, corresponding to 15–20% of an average mill's total throughput. The booster produces a low heating value gas (HHV ~2.8 MJ/Nm3) that is suitable for firing in an auxiliary boiler. The overall thermal efficiency is quite low (approx. 45%), but the focus of the booster is to provide an inexpensive solution for increasing capacity. Chemrec has built two booster systems. A 75 tds/day demonstration system was constructed in Sweden in the early 1990's. It operated for a couple years, but was then taken off-line since the mill no longer needed the capacity. In 1996, a 330 tds/day commercial booster was built in North Carolina at a mill owned by Weyerhaeuser. Both of these systems had technical problems, many of which have been solved. New Bern in particular has been plagued by problems with the refractory lining in the reactor. Nonetheless, Chemrec feels that they have this issue well enough under control that they are now offering the booster commercially as a means of increasing black liquor processing capacity.

BLGCC System Description and Development Status. The BLGCC system (Figure 6) is similar to the booster system, but operates at approximately 30 atmospheres and has a more advanced gas cleanup system. After the syngas exits the reactor, it is quenched, separated from the smelt and sent to a gas cooler. In the cooler, water vapor in the gas condenses and flows counter-current to the gas, helping to clean it of fine particles and condensed hydrocarbons. An absorber/stripper system removes hydrogen sulfide from the gas and generates a sulfur- S-rich Gas rich gas stream that can be used Oxygen Gas to prepare advanced pulping Gas Turbine Black Cooler solutions. The clean, sulfur free Liquor gas has a heating value of roughly 7.5 MJ/Nm3 and is fired in a gas turbine to generate Gasifier Fuel electricity. The turbine exhaust Gas is passed through a heat Steam Turbine Quench recovery steam generator to Steam Condensate raise steam. A portion of this is Water run through a turbine to generate additional electricity. Green Absorber / Stripper The combined cycle system is Liquor HSRG Stack able to achieve thermal efficiencies (steam + power) in Figure 6. Chemrec BLGCC system schematic excess of 80%. Chemrec operated a 10 tds/day pressurized pilot plant between 1994 and 2000, and demonstrated that pressurized, high temperature gasification is feasible [17]. The plant produced a medium heating value gas that was within gas turbine specifications for particulate matter. The design of the system went through several iterations, and the final, optimized configuration will be used in Chemrec's upcoming development plant, scheduled to start up summer 2002. The development plant will be larger than the pilot and will operate under higher pressure. Chemrec plans to use the data from the development plant to design commercial systems, and hopes to have a commercial demonstration unit within 5 years.

Technical Challenges. As with all high temperature gasification systems, the largest technical challenge for Chemrec is finding a reactor material that can withstand the corrosive smelt. Chemrec's experience at the New Bern mill highlights the seriousness of this issue. Condensation of tars in the

8 system may also create difficulties during long-term operation. Integration of the gasifier with the gas turbine and the rest of the mill will inevitably require a few reworks to get the bugs out, as well.

CONCLUSIONS Over the past few decades, no less than a dozen companies have tried to develop a process to gasify black liquor. Many efforts have been discontinued due to overwhelming technical difficulties or lack of funding, but two companies are still actively pursuing commercialization of black liquor gasification. About half the companies attempting black liquor gasification have gone with a low temperature process, where the reactor operates below the melting point of the inorganic material in the liquor. All of the low temperature efforts have employed fluidized beds, either bubbling or circulating, and all but one have been air-blown. Generally, these technologies have had problems maintaining good carbon conversion while avoiding bed agglomeration. Condensation of "tars" also created operational problems for many of the technologies. Several companies have tried to develop black liquor gasification processes that operate at high temperature (above the melting point of the black liquor inorganics). Most of these have used entrained flow reactors, though some much more novel approaches have been explored, as well. The most significant challenge with the high temperature systems is finding a means of limiting or avoiding corrosion by the high temperature smelt. This has proven to be a showstopper for many of the technologies. Despite setbacks in the development of the technology, it appears that black liquor gasification is on the brink of commercialization. MTCI has five commercial projects in progress for their low temperature steam reforming process. The first of these should start up within the next couple years. Chemrec has already sold one of their atmospheric pressure, high temperature booster systems, and is offering that system commercially. No black liquor gasification combined cycle systems have yet been sold or demonstrated. But pilot work on the BLGCC system has been successful and development is continuing. If development continues to be successful, paper mills will should have a proven alternative to the recovery boiler within the next decade.

REFERENCES

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7. McILROY, R.A., KUCHNER, R.A., MONACELLI, J.E., JOHNSON, D.W., "Black liquor gasifier," U.S. Patent #5,634,950 (1997). 8. MAGNUSSON, H., WARNQVIST, B., "The NSP project: an alternative to the conventional recovery furnace," Chemical Engineering Progress 76(2):54-46 (1980). 9. LEKSANDER, K.-E., WARNQVIST, B., ROOS, T., BJÖRKLUND, H., "NSP Ny Sodahusprocess, Slutrapport," (NSP Final Report, in Swedish). Stockholm, Sweden (1985). 10. KOHL, A.L., "Black liquor gasification," Canadian J. of Chem. Eng. 64(2):299-304 (1986). 11. SALMENOJA, K., "Black liquor gasification: Theoretical and experimental studies," Bioresource Tech. 46(1-2):167-171 (1993). 12. LORSON, H., SCHINGNITZ, M., WHITE, V.F., DEAN, D.R., "Black liquor recovery by pressurized oxygen-blown gasification," Proc. 1996 Engineering Conf., 16-19 Sept. 1996, Chicago, IL, pp. 557-565 (1996). 13. ROCKVAM, L.N., "Black liquor steam reforming and recovery commercialization," 2001 Int'l Chem. Recovery Conf., 11-14 June 2001, Whistler, Canada (2001). 14. MTCI News Release, February 2001. 15. "PulseEnhanced™ steam reforming and recovery of kraft spent liquor: Weyerhaeuser New Bern demonstration final report," MTCI (1997). 16. BROWN, C., LANDÄLV, I., "The Chemrec black liquor recovery technology — A status report," 2001 Int'l Chem. Recovery Conf., 11-14 June 2001, Whistler, Canada (2001). 17. WHITTY, K., NILSSON, A., "Experience from a high temperature, pressurized black liquor gasification pilot plant," 2001 Int'l Chem. Recovery Conf., 11-14 June 2001, Whistler, Canada, pp. 281-287 (2001).

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