Working with technology partners like Nexterra, Domtar is seeing promising results from a pilot plant project for lime kiln gasification at its Kamloops, BC, mill
June 2007
By Monica Shaw, Editor
With its easy access to wood waste, the pulp and paper industry seems a natural fit for gasification systems that produce biofuels, either for consumption or for sale. Very few North America mills, however, have embraced this advantage in a full scale or even pilot plant implementation.
Domtar's (formerly Weyerhaeuser's) Kamloops, BC, mill is bucking this trend by working with technology partners Paprican and Nexterra Energy Corp to build a pilot plant for using wood-based synthesis gas (syngas) to fuel the mill's lime kiln. The northern bleached softwood kraft (NBSK) mill produces enough excess electricity to sell, and the lime kiln remains the last opportunity to reduce fossil fuel use. In addition to cutting its natural gas consumption in half, Domtar also expects to make a positive environmental contribution by reducing CO2 emissions.
"Our mill has always been very focused on energy, seeing it as an opportunity for competitive advantage," says Bill Adams, manufacturing services manager for Domtar. "We also have a geographic advantage with regard to our proximity to several sawmills, which gives us competitive wood waste costs."
Current research is being conducted at Nexterra's pilot plant facility near Kamloops, BC. The commercial gasifier planned for the Domtar mill in Kamloops is expected to cost more than $8 million. This February, Sustainable Development Technology Canada, a non-profit foundation funded by the Canadian government, signed an agreement with gasification technology provider Nexterra to contribute $2.7 million to the demonstration effort. Natural Resource Canada also participates in the funding.
"Before committing to a major commercial installation, we must address all technological uncertainties with the lime kiln gasification project," Adams explains. "The project will involve careful, conservative work to make sure we are very confident with the business case and technology for a large scale, commercial implementation."
The Kamloops kiln
Located in British Columbia's southern interior, Kamloops is a two line NBSK mill with a 480,000 ad tonne/yr capacity. It sources 100% of its fiber from local sawmills and is electrical energy self sufficient, exporting up to 5 MW/hr to the market during peak pricing. The mill's two power boilers use 100% hog fuel.
Installed in 1972, the mill's F.L. Smidth kiln produces 400 bd tonnes/day of lime. Kamloops is one of the few North American mills to have installed a lime mud drying system (1998) that uses waste heat to dry lime mud entering the kiln, providing for a shorter drying zone that uses less natural gas.
Also, in 2006, the mill installed model predictive control, supplied by Norpac Controls, for its lime kiln operation. This advanced control system helps operators use the bare minimum of natural gas to produce high quality lime.
With all the improvements, the mill reduced the use of natural gas to 6.9 gigajoules (GJ)/bd tonne of lime. However, natural gas use still totals more than 1 million GJ/yr. At the mill's current NBSK production rate of 480,000 ad tonnes/yr, this places Kamloops around the average lime kiln energy consumption rate for Canadian mills of 2.08 GJ/tonne of pulp, according to Paprican.
Still, with natural gas prices spiking late in 2005 combined with the mill's desire to remain in the top quartile for NBSK, Kamloops (then owned by Weyerhaeuser) decided to look for ways to further cut its use of natural gas. In October 2005, the mill began examining gasification technologies for the lime kiln.
Gasification technology selection
In Scandinavia, gasification technology was used in the lime kiln in the mid 1980s. However, this technology was not as low cost as options today. The designs strived for 100% fuel substitution, which required the costly use of pre-dryers on the hog fuel and a wet scrubber to remove particulate. So, when the price of natural gas went back down, economics did not justify the technology.
Today, mills are again evaluating lime kiln gasification, and not just because of natural gas prices, but also because of the opportunity to reduce CO2 emissions, says Adams. In addition to looking for a 50% substitution of natural gas with syngas, a huge driver for examining lime kiln gasification at Kamloops was the goal of reducing CO2 emissions by 25,000 tons/yr.
While operating under Weyerhaeuser management in 2005, an extensive evaluation of existing gasification technologies from six different suppliers was undertaken. A number of factors went into the decision to partner with Nexterra, according to Adams, but the most important were: simplicity of design, technical expertise, and the capability for a very automated system that would not require a dedicated operator.
Pilot plan design
Nexterra's first commercial gasification plant started up in April 2006 at the Tolko plywood plant in Heffley Creek, BC. This application recovers the thermal energy from the gasification process and uses the heat to dry the plywood. The design for the Domtar lime kiln application requires the direct firing of syngas in the kiln.
With the support of SDTC's $2.7 million contribution, Nexterra initiated a direct fired program for lime kiln and boiler applications at its Product Development Centre near Kamloops. The pilot plant upgrade was completed in October 2006 and included a small kiln and burner to conduct direct firing trials of syngas.
In the Nexterra gasification process, hog fuel is fed into the middle of the gasification vessel, overflows a concrete "cone" in the vessel's center, and is injected with air to undergo partial oxidation at 1,500-1,800° F. This creates "smoke" or syngas that contains carbon monoxide, hydrogen, methane, nitrogen, water and carbon dioxide that exits the vessels at a temperatures of 400-700° F, depending on the moisture content of the incoming fuel.
In addition to producing clean, low temperature syngas, the gasifier's design provides the following: very low particulate emissions (< 100 mg/dscm), free flowing ash that is removed by an automated grate; a turn down ratio of 5:1; idling capacity of 48 hours; hog fuel moistures of 10-55%; and automated operation.
In the design for a full-scale commercial system at Kamloops, syngas exits two gasification vessels through ducting and is blown by a fan into the kiln, where it is mixed with natural gas for kiln flame combustion. Figure 1 depicts this proposed direct-fired design for Kamloops. The design is in contrast to the Tolko Heffley process where syngas is oxidized in a separate chamber and then sent through heat exchangers to dry plywood.
Nexterra and the Kamloops mill have identified three major challenges for the lime kiln gasification project. The following sections discuss these challenges and how they are addressed.
Challenge 1: Flame temperature
Moisture levels of the waste wood supply for the Kamloops mill are very low due to the very dry climate in the region and typically range 25-50% moisture The syngas fuel values derived during early testing have ranged 120-150 Btu/dscf. Lower syngas fuel values restrict the natural gas substitution rate, as the minimum flame temperature must be maintained to ensure lime quality. Lower flame temperature could affect lime quality enough to impact pulping operations, negating gasifier benefits.
Challenge 2: Gas volume
As mentioned in the previous section, syngas has lower fuel value and higher moisture than natural gas. This creates higher mass flow that may affect the kiln operation. The Kamloops mill investigated the potential impact of higher gas volume on the kiln operation by modeling the kiln. The mill worked with Process Simulations Limited (PSL), based out of the University of British Columbia in Vancouver (UBC), to create detailed computational fluid dynamics modeling for its kiln, both with and without lime kiln gasification. The company was chosen due to expertise in kiln work throughout many industries. This modeling provided a three-dimensional look at the lime reactions and all energy balances. It also assisted in evaluating syngas blending ratios and fuel values.
The software model uses finite element analysis on 1.5 million variables and requires several days to run each trial on PSL's computer, one of North America's fastest, according to Adams. Data from a software model showed that while the gas temperature profile using natural gas in the 13 ft x 300 ft kiln was hotter at the outset, the basic temperature performance using 55% syngas was similar enough through the length of the kiln to ensure productivity.
According to Adams, the software "perfectly models" the kiln and has already provided valuable insight into its current operation. In addition to offering information on syngas substitution, the model isolated a problem with the surplus oxygen sensor and has resulted in replacement of some of the lime kiln's instrumentation.
"Before, the mill was reluctant to increase the lime production for quality reasons, but this modeling showed us we could, so we've actually increased our lime production, which has been a savings for us," says Adams.
Challenge 3: Non-process elements
Because the gasification design from Nexterra would fire syngas directly into the Kamloops lime kiln, the mill needed to know what impact this would have on the amount of non-process elements (NPEs) in the mill's liquor and lime cycles. For example, the mill wanted to know if there would be NPE concentrations that might have a detrimental effect, such as fouling the evaporators, producing poor white liquor, or reduced recovery boiler capacity.
"We didn't want to run into a situation where the gasification system worked well for a few days or weeks, after which we started having problems with lime quality due to buildup," Adams explains. "We wanted Paprican to measure every flow out of the mill and how syngas will touch it, using a mathematical accumulation model to quantify buildup of elements like silicon, aluminum, calcium, and magnesium."
To asses the NPE situation, Paprican performed a study that involved measuring current NPE concentrations, constructing a mass balance of the mill, and predicting the impact of ash in syngas on accumulation levels. Paprican tested for a baseline for the current Kamloops operations, just as it had done for other mills in Canada.
To estimate NPEs in a 55% syngas fueled lime kiln, Nexterra provided gases for analysis from its pilot plant using regionally appropriate chips from Kamloops. Samples were also taken from the Tolko Heffley plywood plant, which burns wood sources similar to the mill.
Paprican data compares Kamloops' baseline NPE concentrations in white liquor with an estimated maximum using syngas, as well as with other Canadian mills. Paprican estimates show that NPE concentrations in this stream are in line with the current ones at Kamloops, and are even less than in other Canadian regions in some cases.
Also from the Paprican analysis, Figures 2a-b show data that detail the predicted axial profile in the kiln with the two fuel sources. "Basically," says Adams, "Paprican determined there were no operations risks associated with NPEs in the syngas we would use."
Such testing is expensive, according to Adams, but the testing and analytical resources are free to Paprican member companies. This data is critical to a lime kiln gasification assessment for any mill. Among other reasons, this is because there is "variation in syngas, depending on its wood source," comments Adams.
"I've worked at mills throughout Canada, and I've come to appreciate how those regional differences in NPEs can cause issues," he notes.
Next steps
Domtar is planning to make a decision about project funding for full scale implementation of lime kiln gasification by this November. Nexterra will continue with its direct fired program at its Product Development Centre pilot plant for several more months to increase syngas fuel values and ensure it understands all the operational issues associated with the direct fired application. Nexterra is also working with other forest products companies, but has agreed that Domtar Kamloops will be its first commercial lime kiln gasification system in the industry.
"Nexterra has been very thorough with its research and wants to be very confident that the lime kiln application of its technology will be successful at Kamloops," says Adams. "The pulp and paper industry is watching this project, and neither Nexterra or Domtar want to see this project do anything but succeed from the start."
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