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1 ENERGETSKI ASPEKTI SUŠENJA DRVETA ENERGY ASPECTS OF WOOD DRYING Vuk MAROVIĆ, Damir ĐAKOVIĆ University of Novi Sad, Faculty of technical sciences, Novi Sad, Serbia, Danas je drvo primarni energetski izvor za vise od dve milijarde ljudi na svetu. Udeo drveta u ukupnoj primarnoj energiji je preko 14%. 21. godine proizvodnja drveta je bila preko 4 milijarde kubnih metara, od kojih je oko 6% bilo korišćeno u energetske svrhe. U Republici Crnoj Gori 211. oko 73 kubnih metara ogreva je upotrebljeno, a ukupni godišnji potencijal za drvnu biomasu skupljenu iz šuma je procenjen na preko milion kubnih metara godišnje. Bez obzira da li će drvo da se koristi u energetske svrhe ili za pravljenje nekog gotovog proizvoda, potrebno je da se osuši do određene vrednosti sadržaja vlage. Tip i kvalitet gotovog proizvoda određuje najveći dozvoljeni nivo sadržaja vlage u drvetu koje se koristi kao sirovina za dalje proizvodne procese. Ako je namera da se gorivo koristi kao ogrev ili da služi za dobijanje biomase, takođe postoje zahtevi u vezi najvećeg dozvoljenog sadržaja vlage kako bi se omogućio energetski efikasniji proces sagorevanja. Osim prirodnog sušenja (koje je nepredvidivo, može da bude veoma sporo i pomoću njega ne mogu da se ostvare vrednosti sadržaja vlage ispod određene granice), sušenje drveta je energetski zahtevan proces. Najveći deo od industrijskog sušenja drveta se obavlja u komornim sušarama. U radu je pokazano da je najviše toplote potrebno da bi se isparila vlaga iz drveta. Značajna količina energije je potrebna za pogon cirkulacionih ventilator. Postoji vise tipova sušara za sušenje drveta. U ovom radu naglasak je na konvencionalnim sušarama, kao i sušara kod kojih se koristi toplotna pumpa za dobijanje toplote i za odvajanje vlage od vazduha za sušenje. Osnovni cilj ovog rada je da uporedi ove dve vrste sušara sa tehničkog i ekonomskog aspekta. Očigledno je da je sistem sa toplotnom pompom energetski efikasniji u poređenju sa konvencionalnom sušarom, ali postoji i pitanje početnih troškova opreme i perioda povrata investicije. U radu je prikazana detaljna analiza ovih pitanja na primeru po jednog odabranog četinara i lišćara. Ključne reči: drvo; vlaga; sušenje; sušara; toplotna pumpa; energija Today, wood is primary energy source for more than two billion people in the world. In total primary energy, share of wood is over 14 %. In year of 21 wood production was over 4 billion cubic meters, of which around 6 % was used to produce energy. In Republic of Montenegro in the year of 211, around 73 cubic meters of firewood was consumed, and overall annual potential for wood biomass collected from forests has been estimated to over one million cubic meters per year. 523

2 Whether wood is used to produce energy, or to build some finished product, it has to be dried to some value of moisture content. Type and quality of finished product determine maximal moisture content allowed in wood used as raw material for further manufacturing processes. If wood is to be used as firewood or to serve for biomass production, there are also requirements concerning maximal allowed moisture content in order to make combustion process energy efficient. Besides seasoning by natural air drying (which is unpredictable, can be very slow and can t achieve values of moisture content below some point), wood drying is energy demanding process. Most of industrial wood drying today is carried out in wood drying kilns. It is shown in this manuscript that great quantity of heat is required in order to evaporate moisture from wood. Considerable amount of energy is required to power circulating fans. There are many types of kilns used for wood drying. In this manuscript emphasis is on conventional kilns and kilns that use heat pump to produce heat and separate moisture from drying air. Main purpose of this manuscript is to compare these two types of kilns from technical and economy aspects. It is obvious that heat pump system is more energy efficient in comparison with conventional kiln, but there is a question of initial costs of the equipment and investment payback period. This manuscript provides detailed analysis of these issues on the example of drying chosen type of deciduous and also of conifer. Keywords: wood; moisture; drying; kiln; heat pump; energy Introduction Wood and wood products are used by mankind since its origin. At first wood was used as firewood and for building tools and weapons. In time, wood application area expanded. Wood was used in masonry, in shipbuilding, for building first transport vehicles, and later in airplane industry. Even today wood is widely used: as firewood, in construction industry, in furniture industry and many others Today, wood is a primary energy source for over two billion people in the world, and in overall global production of primary energy, wood and charcoal are participating with over 14%. Overall wood production in the year of 21 was around 4 billion cubic meters of which 2, 4 billion m 3 was used to produce heat. That means that most of the wood produced in the world (around 6%) is used for energy purposes. Over 54% of total area of the Republic of Montenegro is consisted of forest land. With.9 ha of forest land per capita, Montenegro is in 8 th place in Europe by forest cover. For comparison Slovenia has.6 ha per capita, Croatia.47 ha per capita, and Serbia has.3 ha of forest land per capita. Overall wood supplies in Montenegrin forest are estimated to around 72 million m 3, and 81% of all forests are intended for wood production. Annual wood growth is estimated to around 1.5 million m 3, and annual cutting is around 7 thousand m 3, which means that there is great accumulation of wood biomass in Montenegrin forests. Real annual potential for wood biomass production in Republic of Montenegro is estimated to about 1.1 million m 3 per year [1]. 524

3 For heating purposes only in the Republic of Montenegro in the year of 211 it was spent m 3 of firewood, m 3 of large wood residues from industry, m 3 of small wood residues from industry (sawdust), 251 m 3 of residues from wine growing and fruit growing, 16 tons of wood, 692 tons of wood pellet, m 3 of wood residues from construction industry and 139 tons of charcoal [2]. About wood drying Reasons for wood drying While water is necessary part of alive wood, when wood is intended for further products manufacturing water is ballast. Since water is not quite adequate term considering its physical state, when wood is intended for drying moisture is the term that would be in further use. There are many reasons for wood drying. Dried wood is more expensive, it weighs less, it is machined better, it glues better, it finishes better On the other hand, if wood is not dried properly it cannot be used for building high quality finished products, there are conditions for parasites infestations, there are possibilities for dimension changes and many other negative effects Moisture in wood Moisture in wood is determined by moisture content on dry basis which is represented as: where: W - mass of moisture in wood [kg] G SM mass of wood dry matter [kg] G summary mass of matter intended for drying [kg] [4]. Whether wood is intended for building some finished products or for energy purposes, it requires to be adequately dried to some moisture content level. If wood is to be used for energy purposes it needs to be dried to around 15 2 % of moisture content for firewood, around % for wood pellet, and around 6 1 % of moisture content for wood briquette. Different types of finished products require different maximal moisture content. Some examples are shown in table below. Methods for wood drying There are many methods for wood drying. Some of them are: air drying, shed drying, solar kiln drying, microwave drying, vacuum kiln drying In this article is shown analysis of wood drying using conventional kiln, and wood drying using kiln with heat pump. 525

4 Table 1 Maximal allowed moisture content for different wood products Product Maximal moisture content [%] Masonry wood 2 Wood constructions 17 Wood for shipbuilding 15 Wood for furniture 1 Wood for musical instruments 8 Wood for transformer cores 5 A conventional kiln uses heat provided by boiler and transferred by either steam or hot water coils to heat the kiln chamber and remove moisture from the wood. The moisture removed from the wood is evaporated, and then exhausted from the kiln together with the heated air. This process requires large amount of heat and requires constant heating of air, so these systems are very energy demanding. Besides that, these kilns can provide very good quality of dried wood, as they allow all relevant parameters of drying air (temperature, relative moisture and airflow) to be controlled. Scheme of conventional kiln is shown in figure 3. A kiln with heat pump uses heat pump for both heating the air and removing moisture from air. In this type of kiln, heated air, is circulated over the wood load with circulating fans, evaporating the moisture contained in the wood. The hot, moist air then passes over a heat pump evaporator where it gets cooled down, moisture gets condensed and flushed out of kiln as stream of water. Heat removed from the air is brought back to it at heat pump condenser. Then heated dry air is used again for drying process. This way theoretically, there is no heat exchange with the surroundings, so it is clear that this type of kiln is more energy efficient in comparison to conventional kiln. Also, as conventional kiln it allows all important parameters of drying air to be controlled, therefore allows high quality drying. Scheme of kiln with heat pump is shown in figure below. Figure 1 Scheme of kiln with heat pump 526

5 Energy and financial parameters of wood drying Examples of drying oak and spruce boards are shown separately in this article. In both cases dimensions of boards, their quantity, initial moisture content, and final moisture content are identical and are 68% and 12 % respectively. Load and kiln dimensions The boards that are dried are 2 m long,.2 m wide, and.5 m thick. Load consists of 2 52 boards. Volume of a board is V d = =.2 m 3. Overall volume of boards in drying load is: V D = = 5.4 m 3. The boards are sorted in 18 stacks. Stacks are sorted in kiln chamber: 3 by height, 3 by depth and 2 by width. Number of boards in each stack is 14 pieces. By literature recommendations [3] the boards in a stack are separated from each other in vertical order using slats that are.25 m thick and.35 m wide. Stacks are separated from each other in vertical order using beams that are.75 m thick and.85 m wide. Height of chamber is: H = 4.65 m and width of chamber is: B = 5.5 m. Having all the above considered, air inlet area has been calculated to: A 1 = m 2. In the picture below front view of drying kiln is shown and air inlet area is visible. Figure 2 Front view of drying kiln Drying parameters calculation In the figure below is shown side view of wood drying kiln with scheme of drying parameters. 527

6 Figure 3 Side view of conventional drying kiln In the first step drying regime is determined using method recommended by institute FPL MEDISON, because it gains the best quality of dried lumber. According to this method, drying is performed in multiple phases of which everyone has its determined air temperature and relative air humidity [3]. By using corresponding tables in literature [3] temperature and relative humidity of inlet air has been determined for each drying phase. In the table below, values of air temperature and relative air humidity for each phase of drying are shown. Also, by using nomogram and diagram from literature [3], duration of every drying phase and overall drying duration are determined. Those times are also shown in tables below. Phase Table 2 Air parameters and durations by phase for oak drying Moisture content range [%] Air temperature [ С] Relative air humidity [%] Phase duration [h] Overall drying duration [h] , , , , ,

7 Phase Table 3 Air parameters and durations by phase for spruce drying Moisture content range [%] Air temperature [ C] Relative air humidity [%] Phase duration [h] Overall drying duration [h] Next step is calculation of mass and energy balance for each phase separately. For calculation of air parameters in states 2 and m and for calculating air and moisture flow rates equations from literature [4] are used. State 1 is referred to temperature, absolute humidity and enthalpy of air in the inlet of the load; state 2 is referred to same air parameters on the outlet of the load, state is referred to parameters of the air surrounding the kiln, and state m is referred to parameters of mixture of surrounding air and state 2 air before mixture enters air heater. Final energy balances for each phase are: h 1 and h m [kj/kg] - enthalpies of air in states 1 and m ; ζ = 1,15 losses coefficient, adopted from general recommendations, in lack of information about real losses; 529

8 Thermal power [kw] Phase Figure 4 Thermal power required by phases 8 Heat [kwh] Phase Figure 5 Amount of heat required by phases Required boiler thermal power is determined by maximal thermal power required in drying process and overall heat consumed in drying process is calculated by summing heat consumed in each phase. These values are shown in charts below. Boiler thermal power [kw] Figure 6 Boiler thermal power required 53

9 2 Heat [kwh] Figure 7Amount of heat required for drying process In order to calculate drying costs using conventional kiln, wood briquette is adopted as main boiler fuel with heating value of H l = 18 MJ/kg [1] and cost of 134 euros per ton. In order to calculate drying costs using kiln with heat pump, power required for running heat pump compressor and fans, which is in this case 1 kw [5], is multiplied by overall drying duration in hours. Cost of.1 eurocents per kwh is adopted as it is approximate real price of power in the Republic of Montenegro. Comparison of drying costs using conventional kiln and kiln with heat pump is shown in chart below. Drying costs [ ] Conventional kiln Kiln with heat pump Figure 8 Comparison of drying costs of conventional kiln and kiln with heat pump As it is obvious kiln that uses heat pump is more energy efficient and cheaper to run, but it is necessary to consider higher investment cost (which is approximately 1 euros per cubic meter of wood load for kiln with heat pump, in comparison to approximately 8 euros per cubic meter of wood load for conventional kiln) for evaluating economy benefits of purchasing kiln with heat pump. 531

10 Number of drying cycles and working hours required to payback the investment of replacing the existing conventional kiln with kiln with heat pump and also to payback the investment of choosing new kiln with heat pump over new conventional kiln are shown in the charts below Cycles [-] Replacement of the existing kiln Purchasing new kiln Figure 9 Investment payback period in drying cycles 6 Working hours [h] Replacement of the existing kiln Purchasing new kiln Figure 9 Investment payback period in working hours Remarks Previously shown numbers should not be taken literally as some assumptions and approximations were used in lack of concrete information and in order to simplify calculations. These simplifications are: kiln was considered to be adiabatic; kiln preheating has not been considered; boards dimensions were identical; tree bark was removed; parameters of surrounding air were considered to be constant (average values for city of Kolašin for month May of 214 [6]); 532

11 fixed initial and final moisture content for both wood types were adopted and considered constants; losses coefficient was adopted as ζ = 1.15; when calculating drying energy consumptions of heat pump, it was only considered power required for running compressor and heat pump fan multiplied by drying duration recommended by literature; costs of running circulation fans, labor costs, maintenance costs and all other expenses were not considered; variation of factors like fuel costs, power costs and all other variations were not considered; density of absolutely dried wood was adopted to be 545 kg/m 3 in calculations which is averaged value; initial investment costs shown are only approximate values and are considered constant for both wood types, although it is clear from calculation that kiln intended to dry only spruce requires much larger boiler in comparison with kiln intended to dry only oak. Conclusion In this article it was shown that there is a great potential for wood use in the Republic of Montenegro, both for energy and other purposes. It is also important to mention that there are no many alternatives for wood biomass use in Montenegro since there are no sources of natural gas, or pipeline for its import. Because of the vastness of wood area, especially if considered per capita, Republic of Montenegro probably has greatest potential for wood biomass use in the entire region. One of the most important element in making wood products is its drying, whether its intended for energy purposes or for production of some finished product and it was shown in this manuscript, wood drying is energy demanding process. Calculations show that in some circumstances, using kiln with heat pump instead of conventional kiln can be profitable. In this manuscript, fuel cost calculations were based on wood biomass market values, but having considered that owners of wood drying industry probably have their own sources of biomass in form of residues or waste, or that they may have access to purchasing wood biomass for prices that are below market prices, it is possible that choosing drying kiln with heat pump instead of conventional drying kiln would be unprofitable in those cases. Intentionally in this article were shown examples of drying two completely different species of wood: oak, which requires mild drying regime (smaller temperatures and higher relative humidity) and long drying time in comparison to spruce which is resistant to harsh drying regime (higher temperatures and lower relative humidity) and requires much shorter drying time. Calculations showed that spruce drying require considerably bigger boiler which would, for sure, lead to bigger initial investment, but also that spruce drying is overall energy less demanding process than oak drying due to shorter drying times. Also it is visible that in case of spruce drying it requires shorter payback period of investment for purchasing kiln with heat pump than in case of oak drying. 533

12 References [1] Glavonjić, B., Drvna goriva: vrste, karakteristike i pogodnosti za grejanje, Podgorica: SNV Montenegro, 211. [2] Glavonjić, B., Krajnc, N., Stijović, A., Zvizdojević, J., Peković, D., Savićević, M., Raičević, B., Ristović, D., Pavlović, M., Potrošnja drvnih goriva u 211. godini u Crnoj Gori, Zavod za statistiku, Podorica, Crna Gora, 213. [3] Tomić, B., Sušenje drveta, Zavod za udžbenike i nastavna sredstva, Beograd, SFRJ, [4] Đaković, D., Đurić, S., Tehnika sušenja sa primerima skripta, Fakultet tehničkih nauka, Novi Sad, Srbija, 213. [5] (accessed on 2th august 216). [6] Micev, S., Mijanović, T., Drljević, M., Kuč, T., Alilović, N., Popović, M. Adžić, I., Obradović, D., Godišnjak meteoroloških i hidroloških podataka 214. Zavod za hidrometeorologiju u seizmologiju Crne Gore, Podgorica, Crna Gora,