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Debarking enhancement of frozen logs. Part II: infrared system for heating logs prior to debarking.

Abstract

Log volume losses, remaining bark on logs after debarking, and bark content in wood chips are significantly higher in winter than in summer for northern sawmills. It is, therefore, beneficial to raise the temperature of the log prior to debarking. Heating logs before debarking in the winter could generate an estimated savings of up to half a million Canadian dollars for a sawmill processing half a million cubic meters of wood annually. In the past, sawmills used water soaking to thaw logs, but most have stopped this practice due to new environmental regulations that increase water treatment costs. The goal of the project described in this paper was to demonstrate, on a semi-industrial prototype, the applicability of using infrared radiation to preheat black spruce logs. The main objectives were to evaluate specific energy consumption and the profitability of the technology. If all of the economic considerations of bark content in woodchips for the pulp and papermill are considered, the return on investment of an infrared system to preheat frozen logs is believed to be less than 1 year.

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In Canada, as well as in any northern country producing paper and lumber, debarking of wood logs during the winter months is a source of concern. The colder the logs, the greater the debarking problems due to stronger bark cohesion and higher wood adhesion (Berlyn 1965, Laganiere and Bedard 2009). The force applied through the debarking tool tips must, therefore, be augmented to reduce the amount of bark in the wood chips. This, in turn, accelerates wear of the tool tips and increases the rate of damage to the debarking system. More importantly, it increases the amount of wood fiber that is lost with the removed bark. In winter, up to half of the disposed bark volume can be wood fiber. Moreover, wood tear-out causes lumber downgrading because of changes in sawing patterns and loss because some boards must be trimmed.

In very cold conditions, increasing the force applied through the debarking tool tips does not solve the problem of residual bark in wood chips. For an independent sawmill, high bark particle content in the chips may lead to rejection of the chips by the pulpmill. The presence of bark in wood chips also leads to lower digester productivity and to the presence of pitch in the pulp. Furthermore, the presence of a high bark content in the wood chips favors the occurrence of sclerite in the paper sheet, which increases the web break rate at the paper plant, and ultimately printing flaws at the printing press (fish eye).

This situation generates important economical losses. A study by Laganiere (1998) confirmed that log volume losses, remaining bark on logs after debarking, and bark content in wood chips are significantly higher in winter than in summer for northern sawmills. The study also showed that the soaking of logs in cool (around 8[degrees]C) water for 20 minutes was sufficient to substantially improve debarking performance. The study showed that, for a northern sawmill processing half a million cubic meters of wood annually, preheating the logs during the winter could save half a million Canadian dollars. Therefore, strong advantages are linked to the preheating of logs prior to debarking. In the past, Quebec sawmills used water ponds, but most of them have abandoned this practice due to new environmental regulations that involve costly water treatment procedures. Also, water ponds are very energy intensive and have traditionally exhibited poor energy efficiency.

This situation calls for the emergence of new technological solutions. Bedard and Poulin (2000) demonstrated experimentally the applicability of the infrared (IR) technology for log preheating purposes. Later, as part of the MRNF (Quebec Minister of Natural Resources and Wildlife) Electrobois program, a pilot system was designed, constructed, and implemented at LTE (Laboratoire des Technologies de l'Energie d'Hydro-Quebec). The project was jointly conducted by the LTE and Forintek (1). A final report was published in 2004 (Bedard and Laganiere 2004).

New solution: Infrared heating

Infrared technology offers an opportunity to heat, without contact, a given product by electromagnetic radiation within the 1 to 20 micron wavelength range. At the base of an infrared unit is a resistor coil which is brought to an elevated temperature. Infrared radiation is "light" beyond the visible range and is generally well-absorbed by organic materials. Unlike microwave or high-frequency heating, which is characterized by a good penetration (volume-heating), infrared is absorbed at the surface and is characterized by negligible penetration depth. Infrared was preferred over microwave or radio-frequency mainly because of the simplicity of the concept and its low cost.

In the case of wood debarking, the region to heat is the bark itself and the interface between the sapwood and the bark (the cambium). The negligible penetration depth of infrared radiation into organic matter such as bark leaves heat conduction for the transfer of heat between the surface and the deeper layers. Thermal conductivity of frozen logs is relatively high and with an appropriate time delay, the temperature at the cambium is raised sufficiently to ease debarking.

Heating of the external surface calls for exposure of the entire circumference of the logs to infrared radiation. Thus, the proposed system consists of a ceiling of infrared panel heaters above a bed of parallel logs moved transversally by a pusher chain conveyer and rotated by an inverted tooth chain or rack (Fig. 1).

Prototype and test procedure description

Description of the prototype

The prototype (Fig. 2) built to test the concept included a conveyer system with an "open" section and a section covered with infrared panel heaters. The conveyer system consisted of four edge-wise chains: two chains with pusher brackets to move the logs transversally and two inverted tooth chains to rotate the logs while positioned below the infrared panel heater ceiling. This set-up simulated translation conditions for a long, continuous infrared heating system.

Infrared units are quartz composite panels made of coils embedded in a black cemented quartz fabric. Electronic control enabled the user to vary the temperature of the coil and, consequently, the radiant heat flux emitted by the panel heaters. The infrared panel heaters were kept at a relatively low temperature (a few hundred [degrees]C), so the emitted infrared radiation was the long wavelength type. Below the log level, a metallic plate reflected the infrared radiation passing between the logs back to the logs or the infrared heaters, and consequently enhanced the system efficiency.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Log supply

Black spruce (Picea mariana [Mill.] B.S.P.) logs were taken from the logyard of a Kruger sawmill (Gerard Crete et ills sawmill at the time) at St-Severin de Proulxville, Quebec, Canada, during the winter. The chosen logs were representative of the typical state of logs prior to the debarking process. Close attention was paid to the condition of the bark. The logs were cut to 1.2-m lengths and were kept frozen in a cooler, maintained at a temperature between -20[degrees]C and -30[degrees]C, for a few days before performing the experiments on the infrared prototype. All of the experiments involved black spruce logs with a diameter between 13 cm and 21 cm.

Instrumentation

The parameters measured during testing included the electric power of the infrared panel heaters, the bark external surface temperature, and the temperature within the first layers inside the log. The internal log temperature was measured through thermocouples inserted into holes drilled along the length of the logs, up to mid-length. The thermocouples were connected to the data logger via a thermocouple slip ring to allow rotation of the logs. The depth of each thermocouple junction was unknown at first. The instrumented log was cut after the experiment to obtain an accurate distance of the thermocouples from the surface as well as bark and sapwood thickness (Fig. 3). Thus, the instrumented log was used only once and discarded afterward.

The bark surface temperature was measured using infrared sensors located underneath the conveyer and oriented toward the surface of the instrumented log: one infrared sensor positioned in the open section of the prototype, the other in the infrared panel heaters section.

[FIGURE 3 OMITTED]

Procedure

Three logs were removed from the cooler and transported to the prototype area. The logs were positioned in the open section of the pilot and laid down on the inverted tooth chains. The middle log was instrumented with thermocouples, while the other two logs were used to simulate a real situation in which logs on each side are partially hiding a given log from radiation emitted by the above infrared panel heaters. The pusher bracket chains were then used to convey the logs beneath the infrared panel heaters, which had been previously warmed. These chains were then stopped, but the inverted tooth chains ensuring rotation were allowed to operate. The logs were rotated and exposed to infrared radiation for a certain period of time, varying between 3 and 6 minutes (2), and then moved back to the open section for another period, for a total period of 15 minutes. The level of infrared power density, the period of exposure to infrared (heating period), and the diameter of the instrumented log and of the neighboring logs were the main variable parameters of the experiments. Variability of the log characteristics (such as water content and bark morphology) were uncontrolled parameters.

Flammability testing

The use of an infrared heat source raised the question of flammability. At low levels of radiant heat flux as used in the preheating experiments (maximum 12.3 kW/[m.sup.2] of input power density, delivering approximately 9.4 kW/[m.sup.2] of radiant power density), it is widely believed that entire logs will not ignite when exposed to such radiant heat flux for several minutes (Richardson 2003). Exposure of wood and bark pieces and particles under the same levels of heat flux, however, represents a more critical situation. To evaluate the fire hazards, standard tests were performed by FPInnovations' Fire Protection Group, as well as tests using the prototype unit at the LTE. The tests performed at the LTE consisted of laying down bits of bark and sawdust on the metallic plate facing the infrared panel heaters and exposing it to different levels of radiant heat flux for prolonged periods of time to simulate an actual situation as closely as possible.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Results and comments

Evolution of the temperature

Figure 4 illustrates a typical test result for the bark external surface temperature as measured by the two infrared sensors. The white circles correspond to the heating period and the grey circles to the relaxation period, outside the infrared heaters section. The two dotted vertical lines represent the instant of the introduction ('In IR') and the instant of removal ('Out IR') of the logs from the heating section, delimiting the heating period.

Figure 5 shows an example of the typical temperature evolution in the log. As expected, thermocouples located deeper show a delay in temperature rise.

Figure 6 illustrates, for the same test, the internal log temperatures as a function of the relative position along the radius (r/R). Here, the time since the introduction of the logs in the heating section is shown as a parameter. The graph illustrates the temperature gradient at different intervals of time. The black circles represent the situation at the end of the heating period. Diamonds represent the situation 600 seconds (10 min) after the beginning of the heating period, corresponding, for the illustrated case, to less than 6 minutes after the end of the heating period. The vertical dashed line represents the cambium location.

One can notice the heat transfer toward the interior of the log. Figure 4 shows a surface temperature of more than 40[degrees]C at the end of the heating period, while Figure 6 shows that the interface between the bark and the sapwood (the cambium) at around the same moment (258 s) is -3.4[degrees]C (by extrapolation). After the end of the heating period (in the so-called 'thermal relaxation' period), energy accumulated in the bark thickness is partly lost to the surroundings, and partly transferred more deeply into the log (underneath the cambium). The sapwood becomes warmer, while bark cools off at the end of the heating period. It is interesting to note that the temperature at the cambium seems to remain unchanged before and after the thermal relaxation period. While this is not always exactly the case as in the present example, most test results show a similar phenomenon.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

It must be mentioned that, during the relaxation period, the logs were still gaining heat from the relatively warm (20[degrees]C to 25[degrees]C) environment inside the laboratory room. Normally, the air inside a sawmill building is very cold, but the presence of an infrared preheating system delivering hundreds of kilowatts would keep it warmer as well.

All of the test results show roughly the same temperature rise patterns. Examination of all the graphs lead to the following conclusions:

* the bark external surface temperature rises rapidly after the log is moved under the infrared heaters and stabilizes between 40[degrees]C and 50[degrees]C;

* after the end of the heating period, the bark surface temperature decreases rapidly and stabilizes between 5[degrees]C and 10[degrees]C;

* after the end of the relaxation period, the temperature gradient within the sapwood is often quasi-linear; and

* "temperature versus time" plateaus are noticeable at 2[degrees]C or 3[degrees]C below normal (0[degrees]C) freezing point, this being in conformity with values in the literature (the presence of minerals in the water lowers the freezing point of the water in the wood).

[FIGURE 7 OMITTED]

Energy consumed vs. cambium temperature

The cambium temperature was chosen as the main criteria when considering debarking, since the cambium is the interface between the sapwood and the bark to be removed. It should be emphasized, however, debarking may be affected not solely by temperature at the cambium, but also by the temperature throughout the thickness of the bark, especially the humid inner bark. As this temperature gradient was difficult to assess, the cambium temperature was nevertheless considered as the main criteria.

Figure 7 describes the gain in temperature at the cambium after the same total ('heating period' plus 'relaxation period') length of time (600 s) as a function of the energy consumed (power multiplied by time of exposure) by the infrared heaters. Consumed energy values correspond to the actual average infrared input power and actual time of exposure of the log to infrared heat, divided by the length of the logs. The level of radiant heat flux seems to be not as much of a determinant factor as the energy delivered: a higher heat flux during a shorter amount of time has the same impact as a lower heat flux and a longer time of exposure, the total energy consumed being the same. This is relevant at least for the chosen range of levels of radiant heat watt densities (6.0 to 9.4 kW/[m.sup.2]).

It is important to note that the consumed energy (Fig. 7) was also divided by the diameter of the log: this operation led to a significantly lower dispersion of the points on the graph. It is interesting to note that the mean temperature gain ([theta]) vs. time ([tau]) of a cylindrical body (density [rho]; specific heat [C.sub.p]) submitted to a constant beat flux (q") over the entire external surface (Neumann boundary condition) is inversely proportional to its diameter (D):

[bar.[theta]] = 4/D q" [tau]/[rho] [C.sub.p] [1]

Infrared heating from above an actual log flanked by two others and containing frozen water that melts and different layers having different physical and morphological characteristics is a situation far from this theoretical problem. But, the results, nevertheless, tend to indicate that the amount of energy required to obtain a given temperature at the cambium is proportional to the log diameter.

Figure 7 also shows that for a given amount of energy (kWh per meter of log length and per meter of log diameter), the gain in temperature at the cambium is significantly higher when the initial log temperature is lower. This can be explained by the fact that the water within the logs at an initial temperature of -30[degrees]C changes from a solid to a liquid state later than in logs initially at -20[degrees]C. Since ice has a thermal conductivity four times higher than liquid water, this helps transfer the heat more deeply into the wood, resulting in a lower temperature and less losses at the surface. Also, during the relaxation period, heat gain from the surroundings (at 20[degrees]C to 25[degrees]C) is higher for logs initially at a lower temperature.

Figure 8 shows the energy (kW/m/m) consumed as a function of the cambium final temperature (proportional adjustments on the temperature have been made to group experiments with logs at an initial temperature a few degrees away from the -20[degrees]C or -30[degrees]C parameter values). It is difficult to distinguish between values with different initial temperatures. Figure 8 reveals a cloud of points rather than a well-defined curve. It should be noted that all of the experiments are represented here. Each instrumented log was used only once: hence, no repeatability test was performed on the same log. Data scattering is likely due to wood morphology and variability of physical characteristics (water content, density, bark thickness).

The presence of bark scale and of some ice and snow on the log are also factors that strongly influence the results. Black spruce occasionally presents bark characterized by scales loosely bound to the log, or even superimposed one over the other. This implies the presence of airspaces that constitute a thermal barrier to the penetration of heat inside the log. This calls for the eventual use of a brushing device prior to the introduction of the logs into the IR preheating system. Also, the presence of ice or snow over the area of the thermocouples quite naturally led to a significantly higher consumed power as compared to the general tendency: the two points identified with an arrow on Figure 8 represent situations in which some ice or a loose scale was present in the area of the thermocouples. The rest of the points in Figures 7 and 8 correspond to logs having a wide variation in bark thickness and water content (ratio of bark thickness to log radius of 3% to 8%, bark water content of 65% to 149% dry base), but with bark morphology less marginal and without ice or snow on the surface.

In a real situation, the brushing device used to remove loose scales would also help remove the snow. The presence of ice over the bark is definitively a demanding condition, as it is for a traditional water soaking system.

Flammability testing

Occurrence of smoke release and small embers was observed depending on the level of radiant heat flux imposed on wood and bark pieces and particles. At 7.6 kW/[m.sup.2] radiant power density or below, no embers were detectable, even after long periods of time.

Cost assessment

Average energy required.--The results in Figure 8 show the amount of energy per log meter diameter required to obtain a given cambium temperature. In an actual situation, approximately the same amount of energy will be spent per log when logs of different diameters travel at an equal distance beneath a large ceiling of infrared heaters. This means that smaller diameter logs will have a higher cambium temperature than larger logs. Also, given the relative scattering of the points, logs of the same diameter will not all have the same cambium temperature for a given amount of energy per log. In this context, the system designer has to choose a goal cambium temperature, identify the amount of energy to meet this goal temperature requirement for most of the logs, and set the maximum log diameter to be processed.

[FIGURE 8 OMITTED]

The debarking shear force vs. temperature, obtained following the tests conducted by Laganiere and Bedard (2009), shows that a -5[degrees]C bark temperature is the point to reach to achieve similar debarking efficiency to 20[degrees]C. A higher temperature barely provides better debarking results, and would require more energy, which has a cost. Thus, a cambium temperature of -5[degrees]C was chosen as the goal temperature. From Figure 8, an upper limit in energy spending of 1.2 kWh per meter of log diameter and per meter of log length was also chosen at this temperature. If one considers an upper limit of 20 cm (0.2 m) as a maximum log diameter for a northern sawmill (the average black spruce log diameter in northern Quebec is typically less than 12 cm at the small end), this represents an energy expenditure of 0.24 kWh per log length (1.2 kWh/m/m multiplied by 0.2 m). This is the estimated required energy for logs with an initial temperature between -30[degrees]C and -20[degrees]C. Elsewhere, logs at an initial temperature of -5[degrees]C would obviously require no energy. No tests have been performed on logs at an initial temperature between -18[degrees]C and -5[degrees]C, so it is arbitrarily estimated that the required energy decreases smoothly as initial log temperature goes from -20[degrees]C to -5[degrees]C. This assumption is used to estimate the power demand of an infrared log preheating system considering initial temperature of the logs, which is dependant upon the weather during the previous days in the area of the sawmill.

Cost of the electric energy.--Infrared heat can be delivered using electricity or a combustible gas. As the northern sawmills in Quebec seldom have access to natural gas or propane as an energy source, efforts have concentrated on the assessment of unit cost (cents per kWh) of electric energy, in the hypothesis of the installation and use of an electric infrared system to preheat the entire production of logs during the coldest days of winter. The addition of an infrared system power demand over and above existing power demand, during a relatively short period of winter, implies penalties that translate into extra cost on the marginal energy cost. To exactly determine this extra cost, one must consider the present situation (contract power, cost of peak kW, cost of kWh) and add the power and energy demand related to the infrared system on a day-to-day basis. Moreover, the infrared system power demand depends on the temperature of the logs to be debarked, which is dependant of the atmospheric temperature of the previous days. A study based on the daily average temperature in three different northern regions (Abitibi-Temiscamingue, Lac-St-Jean, Cote-Nord), as supplied by the meteorological service of Hydro-Quebec, was conducted. Energy demand was evaluated from the temperature history of a typical year and by using the previously mentioned relation between log temperature and energy demand. To assess the new subscribed power, extra power demand of the infrared system was evaluated based on specific energy demand (kWh/log) and on the sawmill log capacity (logs/h). This was used to determine the marginal cost of the electric energy (cents/kWh). For the nine major sawmills studied, the average energy cost was found to be 1.8 cents per log meter for the period covering December through February (year 2008 pricing).

It must be mentioned that infrared heating systems fuelled by gas are a possible alternative to electric devices. To produce the low levels of radiant heat flux required by the application, however, one will have to use IR gas catalytic heaters.

Cost of an infrared system.--One of the main reasons for choosing infrared heating technology is the relatively low cost of the technology. A more important cost is related to the conveyer. The conveyer must impose a translation movement to the logs and a simultaneous rotation, ideally with a minimum space between each log, and with mechanisms ensuring full loading of the conveyer and elimination of the risk for piling-up and damaging of the infrared heaters.

Period of return on investment.--Considering only the cost of the equipment, the energy cost and the cost savings related to higher volume of lumber and chips sold the return on investment (ROI) is estimated to be 2.7 to 6.5 years for an independent sawmill. Note that price of lumber is particularly unsteady from year to year and is key to the economic calculations. Also, preheating logs leads to lower debarker maintenance costs, both in terms of used parts as well as manpower, thus providing extra savings.

Elsewhere, it has been identified that a cambium temperature target of -10[degrees]C instead of -5[degrees]C has a strong impact (some sawmills believe -10[degrees]C is a satisfactory condition for good debarking): the ROI is then estimated to be between 1 and 2 years.

Moreover, if one considers the consequences of the presence of bark in the woodchips, i.e., the rejection of woodchips by the pulp and papermill, the use of chemical products used by the mill to dissolve and remove bark, the paper web breaks and the rejection of paper roll by the printing house because of the occurrence of printing flaws, the ROI is believed to be less than 1 year (as for an integrated sawmill and papermill company).

Conclusions

The main result of the study described in this paper confirms the applicability of the infrared technology to preheat logs prior to debarking. Infrared heat is absorbed by the surface of the bark and transferred by thermal conduction inside the log. After a few minutes of exposure to a relatively low level of infrared heat flux and a thermal relaxation period, temperature gain in the bark and at the cambium is sufficient to ease debarking.

The presence of superimposed scales on black spruce logs leads to higher energy losses. Removal of these scales increases the probability of obtaining good uniformity of log infrared preheating.

The infrared solution appears profitable in terms of ROI, especially when considering all economical impacts in the papermill and in the printing house. But irrespective of ROI, as the pulp and papermill requirements in terms of bark content in wood chips become stiffer, the need for a preheating system will become crucial for sawmills. Since water soaking has been abandoned because of environmental concerns, the infrared concept could become an interesting and viable alternative, at least in areas where electricity costs are relatively low.

Acknowledgments

The authors wish to thank Jean-Paul Bernier (LTE), Raynald Brassard (LTE), and Francis Tanguay (FPInnovations) for granting the project with their multiple skills and their efforts, Leila Chami-Boudjerida and Christian Nobert (Hydro-Quebec) for their assistance with this project, and James Kendall (LTE) for help in correcting the English text. The authors also acknowledge the contribution from Les Richardson (FPInnovations) concerning the flammability tests. Finally, the authors gratefully acknowledge the Quebec Minister of Natural Resources and Wildlife (MRNF) for financial support (through the Electrobois Program) for this research which is at the source of this article.

Literature cited

Bedard, N. and B. Laganiere. 2004. Infrared log thawing system prior to debarking. Rept. LTE-RT-0464-confidential. Joint report of LTE of Hydro-Quebec and Forintek Canada Corp. 69 pp. (in French).

--and A. Poulin. 2000. Infrared heating system applied to log thawing--Donohue Forest Products--Lab. tests. Rept. LTEE-RT-0219/2000.33 pp. (in French).

Berlyn, R.W. 1965. The effect of variations in the strength of the bond between bark and wood on mechanical barking. Wood Res. Index No. 174, Pulp and Pap. Res. Inst. of Canada, Montreal, QC, Canada. 22 pp.

Laganiere, B. 1998. Debarking efficiency (winter/summer). Forintek Canada Corp., Sainte-Foy, Quebec, Canada. 42 pp.

--and N. Bedard. 2009. Debarking enhancement of frozen logs. Part I: Effect of temperature on bark/wood bond strength of balsam fir and black spruce logs. Forest Prod. J. 59(6):19-24.

Richardson, L.R. 2003. Ignition of log debris by infrared heating preparatory to sawmill debarking. Internal Rept. 555-3338. Forintek Canada Corp. 11 pp.

(1) European Patent EP 1 670 622 B1.

(2) This period of time was adjusted from test to test, based on the results of past experiments and the intensity of the infrared radiation chosen.

The authors are, respectively, Research Scientist, Laboratoire des Technologies de l' Energie (LTE, part of Hydro-Quebec's Research Inst.), Shawinigan, QC, Canada (bedard.normand@lte.ireq.ca); and Research Scientist, FPInnovations--Forintek Division, Quebec, QC, Canada (benoit.laganiere@fpinnovations.ca). This paper was received for publication in September 2008. Article No. 10532.
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Author:Bedard, Normand; Laganiere, Benoit
Publication:Forest Products Journal
Geographic Code:1CANA
Date:Jun 1, 2009
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