Analysis of the Required Energy for Wood Heat Treatment Processes Using Superheated Steam

Transcription

1 Analysis of the Required Energy for Wood Heat Treatment Processes Using Superheated Steam Yonggun Park 1 -Yeonjung Han 1 - Jun-Ho Park 1 - Yoon-Seong Jang 1 - Sang-Yun YANG 1 - Hwanmyeong YEO 2* 1 Dept. of Forest Sciences, CALS, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul South Korea. 2 Research Institute for Agriculture and Life Sciences, Dept. of Forest Sciences, CALS, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul South Korea. hyeo@snu.ac.kr * Corresponding author Abstract The Heat Treatment (HT) is a method to change the physio/bio-chemical properties of wood to be better to human by heating at high temperature( ), which can modify its composition. Wood increases in hydrophobicity and dimensional stability, its inhomogeneous surface color becomes homogeneous, and its decay-resistance is improved due to HT. On the contrary, because of HT, wood tends to decrease in strength due to the development of many internal checks.and when it has not been adequately dried before HT, more defects such as checks, warping, and fractures occur during oven HT. In addition, there also exists the danger of fire during the oven HT process because wood can catch fire due to the effect of the high temperature. Consequently, this study seeks to develop an HT method, which uses superheated steam as a medium of HT for reducing fire hazard. By simultaneously drying green wood and exposing it to HT, controlling the process, analyzing the energy required for the entire process, and evaluating the energy efficiency, usability of superheated steam was evaluated in view of its significant for safety and energy efficiency. Generally, the thermodynamic energy required for eliminating moisture from wood in drying process can be classified into the heat energy (H 1 ) for increasing the temperature of wood, heat energy (H 2 ) for overcoming absorptive power, heat energy (H 3 ) to heat the residual moisture in wood, and heat energy (H 4 ) for heating and evaporating the moisture to be eliminated from wood. After calculating the total required energy (H Total ) including the for producing and handling the superheated steam (H 5 ), through comparing the totally required energy with the energy actually consumed in the superheated steam drying-ht process, the amount of lost energy and the energy efficiency of the process were evaluated. Through the results of this study, the eco-friendly effect of superheated steam HT method was proved quantitatively. Keywords:Superheated steam, drying, heat treatment, required energy, efficiency, water movement, defect-free Paper PS-81 1of 8

2 Introduction Wood has contributed to survival of humanity and civilization and developed variously from simple living tools of primitive man to engineering wood of civil man. By being interested in the safety of residential living and environmental conditions and preferring eco-friendly materials, the demand of wood has been increased. Wood is regarded as ecofriendly material which can enhance the living standards. The wood has not only aesthetic benefits as materials that are consistent with people s emotions like a variety of colors and patterns, smooth touchand visual sensation that cannot expected in other materials, and but also physical benefits like great strength and stiffness per unit weight and, good thermal and electrical insulation, and the various extractives in the wood can be used as raw materials for chemical products and drugs. But, the wood has some disadvantages like uneven dimensional stability and durability by each water sorption state. To overcome and/or minimize these disadvantages some treatments like heating, compressing and injecting chemical agents have been used. The Heat Treatment (HT) is a method to change the physio/bio-chemical properties of wood to be better to human by heating at high temperature ( ), whichcan modify its composition. Because of increasing the concerns about environmental pollution of chemical preservativeinsecticide and its health hazard,the method is attracting attentions as artificial preservative-free treating technique. Militz (2002) had compared some HT processes each other and evaluated the effects of Finland (Thermo Wood process), Netherlands (Plato process), France (Rectification process and Boisperdure process) and Germany (OHT-Oil Heat Treatment process).and he reported that increasing dimensional stability and durability is possible if the processes control properly depending on species in the temperature range of 160~260. Kamdem (2002) reported that French HT (for Pine, Fir and Beech) improves durability of wood but decreases its bending strength. Also, Esteves et al. (2007) reported that dimensional stability of Eucalyptus was enhanced but its strength was decreased by mass loss and equilibrium moisture content decreased after heat treatment at 170~200 On the other hand, it has been reported that the heat treatment without pre-drying causes serious cracks. Also, Conventional oven HT process is known to have a higher risk of fire. The cautions on the risk are needed. Superheated Steam, steam with a temperature above the boiling point, began to be used at the drying in the early 1900 and was used at the drying industry in Germany in the 1930s (Mujumdar, 1990). Drying method by superheated steam has many advantages. The method can save net energy because it is easy to condense the used vapor and recover energy. Andthe method can diminish amount of hazard substances emitting to the outside and have additional effects on the sterilization and deodorization. Also, the method is safe, because there is no oxidation or combustion reaction, and its drying rate can be higher than kiln drying rate. In this study, green lumber was dried and heat-treated with superheated steam for preventing the occurrence of drying check and diminish the fire risk. And energy efficiency of the treatment was evaluated. Paper PS-81 2of 8

3 Materials and Methods Materials. Pitch Pine (Pinusrigida) tree produced in Korea was cut and it was sawed and planned.lumbers with dimension of 150mm (width) 50mm (thickness) 600mm (length) were prepared. Superheated steam heat treatment (SHS HT) Superheated steam HT was performed in the following order. A green lumber was put into reactor and 150mL water was supplied into the reactor.the reactor was sealedtightly. (The water which is poured into reactor is changed to superheated steam rapidly and helps a fast heat transfer from steam to wood at initial stage of drying process.)next, the air in sealed reactor was drawn off with vacuum pump, then control temperature (about 255 ), inner surface of reactor, was adjusted to heat to target temperature (220 ). Water in reactor was evaporated;the reactor became filled up with superheated steam. Asliquid water in reactor and water in wood being evaporated, vapor pressure in reactor increased. If vapor pressure in reactor reached target pressure, pressure controller was operated to maintain target pressure. Normal heat treatment. Normal oven HT was performed as a control to compare its required energy and its energy efficiency with those of superheated steam HT. Target temperature and processing time were same as superheated HT, butinitial moisture content of lumber was different. Pre-dried specimen of about 8% MC was used. Reactor was not sealed and air in reactor was heated at atmospheric pressure. Evaluation of water movement in wood during superheated steam HT process. To evaluatemoisture movement in wood during superheated steam HT, the average moisture content (MC) of lumber and sectional specimen s MCs along longitudinal and transverse direction after 4, 6, 8, 10 and 12hour treatments. Figure 1 shows the specimenswhich were classified to 3 parts - End, Quarter, and Middle. Left and right three parts in figure were specimens for evaluating moisture distribution along longitudinal direction and transverse direction, respectively. Each specimen of transverse direction was cut to 25 parts and classified Outer, Inner and Core (Figure2). Figure1.Location of specimens prepared for evaluating moisture distribution along longitudinal and transversal direction during SHS heat treatment process Outer Inner Core Figure2. Sectioning specimen to evaluatemoisture distribution along transverse direction Paper PS-81 3of 8

4 Evaluation of required energy and energy efficiency. Generally, the thermodynamic energy required for eliminating moisture from wood in drying process can be classified into the heat energy (H 1 ) for increasing the temperature of wood, heat energy (H 2 ) for overcoming absorptive power, heat energy (H 3 ) for heating the residual moisture in wood, and heat energy (H 4 ) for heating and evaporating the moisture to be eliminated from wood. This classification can be applied to evaluate the energy consumption in heat treatment process. Heat energy (H5) for generating superheated steam (forsuperheated steam HT),and heating air in reactor (fornormal HT) could be added to the above 4 kinds of heats. Table 1 summarizes the equations for calculating each heat-energy. The energy efficiency was defined as the ratio of entire required energy to actual amount of energy used in each process. Table 1.Summary for equations calculating each heat-energy Heat energy for rising wood temperature (H 1 ) Heat energy for overcoming hygroscopic force (H 2 ) Heat energy for heating remaining water in wood (H 3 ) Heat energy for heating and evaporating water removed from wood (H 4 ) Heat energy for generating superheated steam or heating air in reactor (H 5 ) W 0 = oven-dry weight of wood (g) C wood = specific heat of wood(= cal/g ㆍ ) C water = specific heat of water(=1cal/g ㆍ ) T 0 = initial temperature of wood before heating ( ) T 1 = final temperature of wood after heating ( ) T 2 = temperature of wood when dry ( ) H de = heat of desorption per unit weight of wood(= ) (cal/g) M R = residual moisture content (fraction) Δmc = changes of moisture content (fraction) ΔT = difference temperature between vapor(air) and outer air when oven-dry ( ) h water = Latent heat of vaporization(= ) (cal/g) Results And Discussion Evaluation of water movement during superheated steam HT. Table 2 showsthe moisture content before and after superheated steam HT at each processing times. Table 2.Moisture content before and after superheated steam HT at each processing times Processing Time (hour) Initial Moisture Content (%) Final Moisture Content (%) Paper PS-81 4of 8

5 After 4 hours, average moisture content (AMC) of wood was dropped to half of initial moisture content.after 6 hours, AMC was dropped to below 10% MC. Although there is a little time variation depending on the initial moisture content, wood became completely dried (oven-dried) after 8hours treatment. Figure 3showsthe changes of the temperature of wood and the temperature and pressure of vapor during 8hours, 10hours and 12hours treatments respectively. 3 graphs in the Figure 3 show the temperature rise of wood at between 6hours and 8hours. The temperature rise time could be determined to the time to reach oven-dry state of wood. Figure3. Graph of changes of temperature in wood and temperature, pressure of vapor and power with processing time (left: 8 hours, center: 10 hours, right: 12 hours) Figure 4 shows the distribution of moisture content in lumber along longitudinal direction. After 8hour superheated steam HT, the wood became oven-dry, all parts of End, Quarter and End are almost 0% MC. At the case of superheated steam HT for 6hours difference between End part and Middle part is about 5%;it means that moisture gradient in longitudinal direction is very small. Figure4. Distribution of moisture content along longitudinal direction in lumber after 4, 6, 8, 10 and 12hour SHS HT Figure5shows moisture contentsof outer part and core part in lumber after 4 hours (left) and 6 hours (right) SHS HT. In both case, moisture content of Outer part is lower than Core part. In case of 4 hours, the graph shows that MC difference between Core and Outer is 40%MC.It looks very large. But MC of Outer part is greater than 10%. It is a relatively high MC value at high temperature drying and HT process.this relatively high MC of outer part seems to prevent drying check occurrence during SHS HT. In case of 6 hours the MC difference between Outer and Core, is about 10%MC. This figure shows that moisture content gradient was small enough and time to dry was short enough to prevent a drying check occurrence. Paper PS-81 5of 8

6 Figure 5.Moisture content of outer and core parts in lumber during SHS HT (left: 4hours, right: 6hours) Evaluation required energy and energy efficient. The followings are the steps for calculating required energy and energy efficiency of superheated steam HT (0.5MPa, 220 ). a. Heat energy for rising wood temperature (H 1 ) The oven-dry weight of wood was g.Average specific heat of wood, depending on moisture content of wood, was calculated 0.40cal/g with equation in the Table 1. Initial temperature of wood before heating was 11, and final temperature of wood was 226. So, heat energy for rising wood temperature was kcal. b. Heat energy for overcoming hygroscopic force (H 2 ) The heat of desorption per unit weight of wood from above fiber saturation point to ovendry depends on residual moisture content (M R ). After superheated steam HT, wood became oven-dry, so the heat of desorption per unit weight of wood was 17.12cal/g. And heat energy for overcoming hygroscopic force was 35.53kcal. c. Heat energy for heating remaining water in wood (H 3 ) After superheated steam HT, the wood became oven-dry. So, the residual moisture content was 0. Therefore, heat energy for heating remaining water in wood was 0kcal. d. Heat energy for heating and evaporating water removed from wood (H 4 ) Fig. 6shows the changes of temperature of wood, vapor s temperature and pressure and the power used at the processing time. As mentioned above, after around 8 hourssuperheated steam HT, the wood specimen became completely dried. In case of superheated steam HT at 0.5MPa and 220 target conditions, the real temperature of vapor, the temperature of outer air and the latent heat for vaporization of water were 227, 25 and cal/g respectively. Therefore heat energy for heating and evaporating water removed from wood was calculated to kcal. e. Heat energy for generating superheated steam (H 5 ) The process of water of 0.1MPa, 20 superheated steam of 0.5MPa, 227 can be classified and analyzed to water of 0.1MPa, 20 water of 0.1MPa, 100 saturated vapor of 0.1MPa, 100 superheated steam of 0.5MPa, 227. Required Paper PS-81 6of 8

7 energy at the process of water of 0.1MPa, 20 water of 0.1MPa, 100 is calculated to 12kcal by the equation of and required energy at the process of water of 0.1MPa, 100 saturated vapor of 0.1MPa, 100 is calculated to 80.74kcal by equation for latent heat of vaporization of water in Table 1. The enthalpy of saturated water at 0.1MPa, 100 conditions is J/g and the enthalpy of superheated steam at 0.5MPa, 227 conditions is J/g. So, the difference of two conditions is J/g. The Volume of reactor is 15L and the specific volume of superheated steam at 0.5MPa, 227 conditions is 0.45L/g. So, the weight of superheated steam in reactor when the process is over is 33.21g. Therefore required energy at the process of saturated vapor of 0.1MPa, 100 superheated steam of 0.5MPa, 227 is calculated to 1.89kcal. As a result, heat energy for generating superheated steam was calculated to 94.62kcal. Figure6.Wood temperature in wood, Vapor s temperature and pressure and the power used at processing time f. Energy efficiency The actual amount of used power was monitored and recorded by and/or in a data acquisitioncomputer system connecting to reactor.the total amount of energy consumed during superheated steam HT was kcal. And the total heat energy required for HT is kcal. Therefore the energy efficiency of superheated steam HT is calculated to 68.08%. The required energy and the energy efficiency of normal HT were calculated in the same way and shownin Table 3. Table 3. Required energy and energy efficiency for normal and superheated steam HT Required Energy (kcal) H 1 H 2 H 3 H 4 H 5 H Total Actual Energy Consumption (kcal) Lost Energy (kcal) Energy Efficiency (%) Normal HT SHS HT The energy efficiency of superheated steam HT was higher than the efficiency of normal HT. In case of normal HT, Lostenergy was much higher than superheated steam HT because high temperature air in reactor leaks free. Paper PS-81 7of 8

8 Conclusion Bythe 4 hour heat treatment with 0.5MPaand 220 superheated steam, moisture content of 50mm thick green lumber was dropped down to a half of the initial MC of the lumber (63.50%MC to 32.58%MC).And within only 8 hours the lumberreached oven-dry state. During superheated steam HT moisture gradient along the longitudinal direction was maintained small. Moisture content gradient was small enough and time to dry was short enough to prevent a drying check occurrence. Also, the energy efficiency of superheated steam HT (68.08%) was much higher than normal HT (27.46%). The defects-free superheated steam heat treatment process developed in this study would be very useful for treating green wood without pre-drying with high energy efficiency. References Cooper, P High Temperature Treated Wood. Value to Wood Research Program Poster Presentation. Deventer, H. C., Industrial Superheated Steam Drying,TNO-report, R 2004/239. Esteves, B., I. Domingos and H. Pereira Improvement of technological quality of eucalyt wood by heat treatment in air at 170 ~ 200. Forest Products Journal 57(1/2): Kamdem, D.P., A. Pizzi and A. Jermannaud Durability of heat-treated wood. HolzalsRoh- und Werkstoff 60(1) : 1-6. Militz, H Heat treatment technologies in Europe: Scientific background and technological stateof art. Conference on Enhancing the durability of Lumber and Engineered Wood Products, Forest Products Society, Madison, USA. Mujumdar, A. S CEA Report. 816 U 671. Montreal. Canada. Og Sin Kim, Dong Hyun Lee and Won Pyo Chun Eco-Friendly Drying Technology using Superheated Steam. Korean Chemistry Engineering Research, 46(2): Paper PS-81 8of 8