EFFICIENCY ASSESSMENT OF A DOMESTIC WOOD GASIFICATION BOILER

Transcription

1 U.P.B. Sci. Bull., Serie D, Vol. 77, I. 3, 015 ISSN EFFICIENCY ASSESSMENT OF A DOMESTIC WOOD GASIFICATION BOILER Paolo STEFANI 1, Stefano ANTOGNONI, Elia PIERATTI 3, Paolo BAGGIO 4, Simone ZANONI 5 Energy ource will play an important role in the world future. Energy i conidered a ignificant factor in economic development and wealth. In thi article the ue of bioma (wood) to produce energy by a thermochemical proce i dealt with. The purpoe of thi work i to evaluate the efficiency of the combution proce conidering variou flue ga outlet temperature (50, 100, 150, 00, 50, 300 C). The data to be analyed are referred to a combution tet, performed through a high efficiency wood tove applying a downdraft gaifying technology. The data obtained from the tet have alo been ued to calculate the energy loe, in order to evaluate the overall energy efficiency of the thermal converion tage. Keyword: bioma, combution, efficiency, gaification, wood tove. 1. Introduction Nowaday, the prime energy ource in the word, i repreented by foil fuel uch a oil, coal and natural ga. It i known that thee reource will not lat forever. The exceive ue of foil fuel i one of the primary caue of global warming, acid rain and urban mog, which have tarted to affect the earth climate, weather, vegetation and aquatic ecoytem [1]. Due to the continuou rie of the cot of the foil energy reource, there i a growing worldwide interet about the development of technologie allowing the exploitation of renewable energy ource, both for environmental and economic reaon. For that, bioma look a one of the mot promiing and viable alternative to recover energy [, 3, 4]. If bioma i adequately managed and exploited, it can be termed a a truly renewable energy ource, which can contribute to diplace the 1 Reearch collaborator, Department of Civil, Environmental and Mechanical Engineering, Univerity of Trento, Italy, paolo.tefani@unitn.it Reearch collaborator, Department of Civil, Environmental and Mechanical Engineering, Univerity of Trento, Italy, tefano.antognoni@unitn.it 3 PhD, Department of Civil, Environmental and Mechanical Engineering, Univerity of Trento, Italy, pieratti@ing.unitn.it 4 Prof., Department of Civil, Environmental and Mechanical Engineering, Univerity of Trento, Italy, paolo.baggio@unitn.it 5 Reearch collaborator, Department of Civil, Environmental and Mechanical Engineering, Univerity of Trento, Italy, imone.zanoni@unitn.it

2 18 P. Stefani, S. Antognoni, E. Pieratti, P. Baggio, S. Zanoni conumption of non-renewable foil ource [5]. Moreover, the expected environmental damage have tempted the world to try to reduce carbon emiion by 80% and hift toward utilizing a variety of renewable energy reource (RES) which are le environmentally harmful uch a olar, wind, bioma etc. in a utainable way [6, 7, 8]. Indeed, in the lat year, the depletion of foil fuel and the increaing of the world energy demand, ha foter the interet for renewable energie, including bioma for energy production [9]. The utiliation of bioma i a very important ource of energy in many part of the world, epecially for area remote from upply of high-quality foil fuel. Bioma energy converion technologie, epecially pyrolyi and gaification, have been ubtantially tudie to promote renewable energy utilization and olving partially the environmental iue [10, 11, 1]. The 009/8/EC Directive on the promotion of the ue of energy from renewable ource et the objective of reaching at leat 0% of the EU final energy conumption through renewable energy ource by 00. It et for each Member State mandatory national target for the overall hare of RES in gro final energy conumption a well a a mandatory hare of 10% in tranport. Currently, the energy produced and/or derived from ource infinitely renovated (hydro, olar, wind) or generated by renewable fuel (utainably produced bioma), which i defined a renewable energy, contribute to the total world energy conumption for le than 0% [13]. In the lat year, the bioma contribution at the world energy demand wa about 13% [14] and in European Union (EU7) in 01, the primary energy production from bioma wa 8.3 Mtoe (million ton of oil equivalent) [15]. On average, in the indutrialized countrie bioma contribute ome 9% to 14% to the total energy upplie, but in developing countrie thi i a high a one-fifth to one-third [16]. Bioma derive from plant and animal material uch a wood from foret, crop, eaweed, material left over from agricultural and foretry procee, and organic indutrial, human and animal wate [17, 18, 19, 0, 1]. It ha been found that utilizing bioma in boiler offer many economic, ocial and environmental benefit uch a financial net aving, conervation of foil fuel reource, job opportunitie creation and CO and NOx emiion reduction [6, 7]. The bioma carbon, when i burned or ued after converting it to other type of fuel (olid, liquid and gaeou), react with oxygen in the air to form carbon dioxide which i releaed into the atmophere. Therefore, if bioma i totally combuted, the amount of CO produced will be equal to the quantity that i taken from the atmophere during the growing tage. Indeed, plant ue unlight through photoynthei to metabolize atmopheric carbon dioxide. Hence, there i no net addition of CO and bioma can be regarded a a carbon ink. Thi proce i known a the carbon cycle or zero carbon emiion [5, 6].

3 Efficiency aement of a dometic wood gaification boiler 183 For upgrading the energy in term of more eaily handled fuel, in the pat decade there ha been a high interet for thermochemical converion of bioma. Among the mot widely known method to convert bioma into available form of energy, there i the thermal converion (combution, gaification, and pyrolyi) []. The bioma gaification repreent an alternative to improve innovative energy procee, in particular for mall rural communitie. In fact, the large amount of wate bioma, widely pread on rural territorie, look promiing for ynga production a fuel for electric power generation by direct utilization in conventional internal combution engine or, after clean up and reforming tage, for innovative generation ytem a fuel cell [3]. Gaification i a proce of converion of olid carbonaceou fuel into combutible ga by partial combution. The reulting ga i a mixture of carbon monoxide, hydrogen, methane, carbon dioxide and nitrogen. The produced ga i more veratile in it ue than the original olid bioma. It can be burnt to produce proce heat and team or ued in ga turbine to produce electricity [3, 4, 5]. The aim of thi tudy i to evaluate the thermal efficiency of a dometic gaification boiler, conidering different exhauted ga outlet temperature (50, 100, 150, 00, 50, 300 C). Notion of combution theory were ued to ae the quantitie neceary to calculate the proce heating loe and the Lower and Higher Heating Value (LHV, HHV).. Material and method.1 Revere flame boiler The experimental invetigation ha been carried on a gaifying wood tove, whoe thermal power i between 11.0 kw and 5.9 kw. The chematic diagram of the ued intrumentation i reported in Fig. 1. A reported in [6], the tove i divided in three ection. In the upper part, the feedtock i initially dried during the heating up of the ytem. Then the temperature grow up to the operating temperature ( C) and the pyrolyi and char oxidation procee take place. The produced ga flow through a grid in the combution chamber, placed below the previou one, in which two procee take place: firtly, the ga i partially oxidized in ubtoichiometric condition by mean of the primary air tream; econdly, by mean of the econdary air, the effective combution proce take place. The third ection of the tove i jut a chamber where ah i collected.

4 184 P. Stefani, S. Antognoni, E. Pieratti, P. Baggio, S. Zanoni 1 Thermotatic valve Boiler enor 3 Safety group 4 30V power upply 5 Heating flow 6 Heating return 7 Feed water temperature enor 8 Check valve 9 Electric motor-driven pump 10 Three-way mixing valve 11 Thermotatic valve 1 Buffer 13 Circulating pump 14 Temperature enor 15 Convector heater 16 Boiler 17 Flue outlet 18 Digital control panel 19 Cloed membrane expanion tank (4 l) 0 Ball valve 1 Thermometer Preure gauge 3 Regulating valve (manual) 4 Automatic vent valve 5 Outlet Fig. 1. Schematic diagram of the dometic gaification boiler An electrochemical analyzer wa ued to meaure the ga compoition at the tack, for analyzing the flue ga concentration and combution data uch a O, CO, NO and SO. Subequent calculation are made conidering the tationary phae of the combution proce. So no tranient phae wa conidered. The tet duration wa about.5 hour, of which 1 hour wa aumed a tationary phae. For the experimental run 7.4 kg of beech wood wa ued. It relative chemical-phyical characteritic i reported in Table 1. Table 1 Chemical-phyical characteritic of beech wood (percentage value, ma and mole fraction). TS: Total Solid; VS: Volatile Solid Element % kg j /kg fuel kg j /kg fuel kmole/kg fuel C H O N E-03.60E-04 S E E-06 H O Ah Moiture TS VS 88.80

5 Efficiency aement of a dometic wood gaification boiler 185 In order to obtain the HHV, it wa ued the Dulong formula [7]: [ O] HHV = 7,831[ C] + 35,93 [ H] +,1[ S] + 1,187[ O] + 578[ N] [ kj kg VS ](1) 8. Stoichiometric and non-toichiometric combution Known the ma fraction of the element that make up the fuel and auming that the atmophere i an ideal ga mixture of two component with ideal behaviour, it i poible to evaluate the toichiometric oxygen (Eq., Eq. 3) and the combution air (Eq. 4, Eq. 5) neceary for the proce. The atmophere i aumed di be compoed of 1% oxygen and 79% equivalent nitrogen with molar ma repectively 3 kg kmole -1 and 8.16 kg kmole -1. mo =.67[ C] + 8 [ H] + [ S] [ O] kg / O kg fuel () nh no no nc ns kmole / O kg fuel = (3) [ m air ] = 4.31 O / m kg air kg fuel (4) [ n air ] = 4.76 O / n kmole air kg fuel (5) Where the ubcript indicate the toichiometric quantitie and n the number of mole of the element j. It i important to highlight that the oxygen, which make up the fuel, i ubtracted for obtaining the remained amount of oxygen neceary for the proce. Similarly, i poible calculate the toichiometric concentration of other element (N, CO, H O and SO ) preent in the exhaut ga, through the following equation: m 8.16 N = 0.79 [ ] [ ] / 8.97 mair + N kgn kgfuel (6) m CO = 3.67 [ C] kg / CO kg fuel (7) mh 9 [ ] [ O = H + HO] kgh / O kg fuel (8) m SO = [ S] kg / SO kg fuel (9) In order to evaluate the non-toichiometric ma fraction (the real compoition of exhaut ga), it i neceary to calculate the exce air, which i ued during the combution proce. For thi tep the exce air (Eq. 10) and the mole fraction of moke (Eq. 11) are defined a:

6 186 P. Stefani, S. Antognoni, E. Pieratti, P. Baggio, S. Zanoni [ O] [ O air ] [ O ] E = (10) 0.79 [ C] + [ S] + [ H] + 0.1[ N] 0.79[ O] [ ] kmole Exhaut ga Exhaut ga = (11) 0.79 kg 0.1+ v fuel CO v O A it can be een from the lat equation, it i neceary to introduce the real mole fraction [kmole j kg -1 fuel ] and the volume fraction v j of the molecule, which are in the bioma and in the exhaut ga. Thee value were obtained from an electrochemical analyer, which wa ued to meaure the ga compoition at the tack. From the equation 11 and by data in Table 1 and the mean value of volume fraction of component meaured by the electrochemical analyer it i poible to calculate the concentration of exhaut ga. The next tep i to calculate the exce air. It i poible to write the Eq. 10 in a different way, obtained through the following equation: [ H ] O = C + S + [ O] (1) vco [ H ] O Exhaut ga = air vo + C + S + [ O ] (13) N + N = [ Exhaut ga] v air N (14) [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] The new equation that defined the exce air i the following. vco [ Exhaut ga] vo E = 0.1 v [ Exhaut ga] vn [ N ] [ Exhaut ga] vo 0.79 CO (15) Known the real ma of air introduced in the combution proce that wa extracted from Eq. 15, it i poible to calculate the real concentration of component (O, N, CO, H O and SO ) preent in the exhaut ga. The relation for CO, H O and SO (Eq.7-9) do not change, wherea for the oxygen and nitrogen the following equation are ued: mo = 0.3 ([ m ] [ ] ) / air mair kg O kg fuel (16) m 8.16 N = 0.79 [ ] [ ] / 8.97 mair + N kgn kgfuel (17)

7 Efficiency aement of a dometic wood gaification boiler Heating loe of combution proce and thermal efficiency The thermal efficiency i calculated according to the LHV and HHV. For thi reaon, it i neceary to determine the enible heat (Q) (which i the energy lot by hot exhaut ga), the heat lot by the unburned material (Qu) and the latent loe due to the evaporation of water (Q l ). The enible loe are determined conidering different flue ga outlet temperature by the following equation: t Q = mc i pi g ( t t g ta) (18) a Where m i i the ma fraction of the generic element of the exhaut ga and C pi i the pecific heat at contant preure calculated between the temperature of exhaut ga (t g ) and the ambient temperature (t a ). The ga outlet temperature evaluated are 50, 100, 150, 00, 50, 300 C and the recorded ambient temperature wa 3.3 C. To obtain the pecific heat at contant preure it wa ued the Eq. 19, where the C pi baed on the t 0 =0 C wa derived by the [8]. tg ta Cpi ( tg t0) Cpi ( t t a t0) g t0 t0 Cpi = (19) ta tg ta The energy lot by the unburned material and latent loe are obtained, repectively, by the product of the ma fraction of carbon monoxide and it LHV and the ma fraction of water preent in the exhaut ga with the latent heat. Note the heating loe, it i poible calculate the thermal efficiency by the Eq. 0 and Eq. 1 which are baed, repectively, on HHV and LHV. Q + Qu + Ql PHHV = 1 HHV (0) Q + Qu PLHV = 1 HHV Ql (1) 3. Reult and dicuion In order to obtain the HHV from the data in Table 1, it wa ued the Dulong formula (Eq. 1), which ha reported a higher heating value of about 19,710 kj kg VS -1. In the econd tep, the toichiometric ma fraction of the oxygen and air and the ma fraction of molecule that exit in the tack ga were calculated. Eq were ued. In the Table the value are reported.

8 188 P. Stefani, S. Antognoni, E. Pieratti, P. Baggio, S. Zanoni Table Stoichiometric ma and mole fraction of O, N, CO, H O, SO, exhaut ga and air Component kg j /kg fuel kmole j /kg fuel O N CO H O SO 3.55E E-06 Exhaut ga Air In the Table 3 the mean value and the tandard deviation of the exhaut ga, recorded by the electrochemical analyer, during the table phae are reported. The concentration of N wa calculated a the remaining proportion of the volume not occupied by other gae. Table 3 Mean value (v) and tandard deviation (td) of volume fraction of component, meaured by electrochemical analyer, of exhaut ga Component v [% in volume] td [% in volume] O CO CO E-0 NO E-03 SO 7.8E-05.91E-04 N From the equation 11 and by data in Table 1 and the mean value of volume fraction of component meaured by the electrochemical analyer (Table 3) it i poible to calculate the concentration of exhaut ga, that i equal to 0.88 kmole kg fuel -1. The next tep wa to calculate the exce air, which wa poible through the Eq. 15. The exce air calculated i 0.564, which coincide with a nontoichiometric ma fraction of 8.37 kg air kg fuel -1. Known the real ma of air introduced in the combution proce, it i poible to calculate the real concentration of component (O, N, CO, H O and SO ) preent in the exhaut ga. The relation ued are Eq and Eq In the Table 4 the reult obtained are decribed.

9 Efficiency aement of a dometic wood gaification boiler 189 Non-toichiometric concentration of component of exhaut ga Component kg j /kg fuel Exhaut ga O N CO H O SO 3.55E-04 CO Table 4 A decribed above, the thermal efficiency i calculated according to the LHV and HHV. The value of Q, relating at each component of exhaut ga and the total enible loe for different outlet temperature, are reported in Figure. Fig.. Senible heat (Q) relating at each component of exhaut ga (O, N, CO, H O, SO ) and total loe obtained from different outlet temperature The energy lot by the unburned material i kj kg fuel -1, which i the product of the ma fraction of carbon monoxide and it LHV, aumed equal to 10,100 kj kg CO -1. The latent loe, which are 1, kj kg fuel -1, were obtained

10 190 P. Stefani, S. Antognoni, E. Pieratti, P. Baggio, S. Zanoni multiplying the ma fraction of water preent in the exhaut ga with the latent heat, aumed equal to,453.3 kj kg HO -1. The thermal efficiencie, evaluated by Eq. 0-1, are reported in the Table 6. It i poible oberve a the P LHV i alway higher than P HHV. Thi due to the definition of LHV and HHV. Indeed, the HHV explain the energy produced by the complete combution where all product are cooled down to the ambient temperature o the water vapour formed during combution i condened, wherea the econd aume the water at the vapour phae. Furthermore, it highlight a the thermal efficiency decreae with increaing of temperature; thi i due to greater enible loe at the exhaut ga. In the end, uppoing a linear relation between t g and PLHV (Figure 3) and calculating the mean outlet temperature in the table phae during the tet, equal to 91 C, thi ha allowed to determine the efficiency value equivalent at 83%. Fig. 3. Thermal efficiencie of air gaifying wood tove conidering different flue ga outlet temperature 4. Concluion In thi paper, a theoretical approach, utained by an experimental combution proce through a gaifying wood tove, wa applied to beech wood. The tet wa conducted in a laboratory cale tove with a thermal potentiality between 11.0 kw and 5.9 kw, in order to be aimilated with the reult from a dometic heating ytem. The data obtained have highlighted a the greater i the performance of cooled ytem, the greater i the thermal efficiency and the leer are the enible loe. It can be een that the enible heat loe depend nearly linearly on the temperature of the flue ga. The reult obtained are

11 Efficiency aement of a dometic wood gaification boiler 191 in line with the mot recent literature on the gaification tove on the market emitting exhaut ga between 70 C and 300 C. Following the introduction of UNI EN 303-5:01, the energy recovery ha become a key iue in order to reach the new and more retrictive efficiency tandard for the dometic boiler of new contruction. The challenge lie in the recovery of the latent heat through the cooling of the flue ga below the dew temperature of the water vapor. Some condening heat exchanger for mall cale (< 35 kw) bioma boiler exit: thee are either integrated into commercially available tove or can be retrofitted to exiting one. So a it i deirable for Government to take meaure to timulate a renewal of boiler fleet by mean of incentive to purchae high efficiency boiler, alo capable of limiting the emiion of pollutant into the atmophere. Hence, verified the increaing demand of energy, epecially in the reidential heating, the ue of thi bioma combution ytem can be conidered a uitable and enhanced technology to improve the utilization of thi renewable energy reource. Acknowledgement Thi tudy wa done within the BIQUEEN project (Quality Bioma for Efficient Energy production). R E F E R E N C E S [1] P.N. Sheth, B.V. Baby, Production of hydrogen energy through bioma (wate wood) gaification, International Journal of Hydrogen Energy, 35, 010, pp [] M. Baratieri, P. Baggio, L. Fiori, M. Grigiante, Bioma a an energy ource: Thermodynamic contraint on the performance of the converion proce, Bioreource Technology, 99, 008, pp [3] E. Girelli, M. Ragazzi, E. Malloci, E.C. Rada, L. Paternoter, Agricultural bioma availability for energy converion in Italy, UPB Scientific Bulletin, erie C, 74(1), 01, pp [4] R. Patraşcu, E. Minciuc, D. Tuţică, M. Norişor, G. Ionecu, P. Stefani, Reducing environmental impact through efficient utiliation of bioma in a cogeneration plant. Cae tudy Energy upply of an indutrial company through bioma utiliation in a cogeneration plant with internal combution engine, Quality acce to ucce, 15(1), 014, pp [5] S. Mohapatra, K. Gadgil, Bioma: the ultimate ource of bio energy, International Journal of Renewable Energy Reearch, 3, 013, pp [6] R. Saidur, E.A. Abdelaziz, A. Demirba, M.S. Hoain, S. Mekhilef, A review on biomaa a fuel for boiler, Renewable and Sutainable Energy Review, 15, 011, pp [7] E.C. Rada, M. Ragazzi, E. Malloci, Role of levoglucoan a a tracer of wood combution in an alpine region, Environmental Technology, 33(9), 01, pp [8] R.D. Ionecu, M. Ragazzi, L. Battiti, E.C. Rada, G. Ionecu, Potential of electricity generation from renewable energy ource in tandard dometic houe, WIT Tranaction on Ecology and the Environment, 176, 013, pp

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