RETROFITTING AND OPTIMISATION OF BAGASSE BOILERS

Transcription

1 RETROFITTING AND OPTIMISATION OF BAGASSE BOILERS P.A. Beaton, a A.L. Brito,a* J. Ballester, b C. Dopazo b 'Energetic Efficiency Study Center. University of Oriente. ISPJAM. Ave. de Las Americas y Casero, s/n Santiago de Cuba. Cuba b* Fluid Mechanics Group/LITEC, Universíty of Zaragoza Zaragoza, Spai n Combustion systems with low efficiency are traditionally employed in the Cuban suga r industry, where about 60% of 860 boilers installed have a horseshoe furnace with a n efficiency of about 65%. A program aims to replace obsolescent boilers with others designe d in Cuba with the help of Russian specialists. In some sugar milis the old boilers were Babcock and Wilcox, War, and Martin furnace s with loads of t/h of saturated steam and gross efficiency of 40 60%. They wer e replaced by boilers type RETO (CV-25-18) with a load of 25 t/h, steam pressure of 1.8 MPa, steam temperature of 593 K, gross efficiency of 80% and combustion on dumper grate. Modifications of the new furnaces have improved boiler performance. This paper describes retrofitting of a bagasse-fired boiler RETO (CV-25-18) in the sugar factory " Amancio Rodriguez" located in Las Tunas province. The new configuration has a higher gas velocity and a more homogenous temperature fleld in the furnace. Heat and mas s transfer are much improved and more steam can be produced. The retrofitting increase d steam generation, gross efficiency and combustion efficiency. 1. INTRODUCTION The need to better use bagasse-fuel and to make good use of the potential for electricit y cogeneration in the Cuban sugar industry has led to a study of combustion systems that us e sugar cane bagasse as a fuel. ' Older systems need to be replaced by new systems through retrofitting of the installe d furnaces. The principal disadvantages of these traditional combustion systems well known in Cuba and referenced by Norton 2 and Watkins 3 are the following : high maintenance cost for furnace brickwork, necessity to maintain a high excess air in the furnace to avoid a high level of heat losse s due to incomplete burning from chemical and mechanical causes. This results in low gas temperature inside the furnace, and consequently in a poor heat exchange by radiation. high heat losses in the flue gases, owing both to the poor heat exchange inside the furnac e and to the increased gas flow rate caused by the high excess air. An increased powe r consumption of forced and induced draught fans is observed.

2 Figure 1. Cuban boiler for buming bagasse in suspension. 1. Swirl zone. 2. Straight llame zone. 3. Furnace exit. 4. Steam superheater. 5. Convective zone. 6. Boiler exit. 7. Forced draught fans. As a result of the retrofitting the nominal steam generation was increased from 25 t o 35 t/h owing to a better use of the furnace volume. The combustion in suspension allows more bagasse to burn 10.0 t/h before, 14.0 t/h after retrofitting without efficiency losses 80% before and 82.17% after the retrofitting. This means that 40.3% more heat i s yielded by the bagasse-fuel in the retrofitted combustion chamber. Table 1 Parameters of the boiler RETO for different loads. (i) Design parameters before retrofitting (BF) and (ii-iv) measured values after the retrofitting (AR) BR AR Unit (i) (ü) (iii) (iv) Nominal capacity t/h Real load t/h Moisture content in bagasse % Ash content in bagasse % Superheated steam pressure MPa Superheated steam temperature K Exit gas temperature K Oxygen content in outlet gases % Carbon dioxide content in outlet gases % Carbon monoxide content in outlet gases % Carbon content in ash % Carbon content in slag %

3 Table 2 Heat losses and efficiency for the boiler RETO at different loads. (i) Design parameters before retrofitting (BF) and (ii-iv) measured values after the retrofitting (AR) Unit (i) (ii) (iii) (iv) Excess air ratio in outlet gases (aex ) Heat losses with outlet gases (q2)+ % Heat losses by incomplete burning from chemical causes (q 3)+ % Heat losses by incomplete burning from chemical causes ++ % Heat losses by incomplete burning from mechanical causes (q4)+ % Heat losses to the surroundings (q 5)+ % Gross efficiency (rl g) % Combustion efficiency(rl e) + % Combustion efficiency ++ % Parameter calculated by the Normative Method. ++ Parameter calculated by the method proposed by Nussbaumer. 7 The heat losses by the physical heat of slag (qe) is not considered Impact of the retrofitting on the efficiencies Different efficiencies are defined in the bibliography reviewed : 6 ' 7 gross efficiency, ne t efficiency, and combustion efficiency. The first two efficiencies are used more frequently to describe the level of usage of th e heat released by the fuel in a furnace. The efficiency test of the steam boilers can b e determined by direct or indirect methods. The direct method is rarely employed for bagass e boilers due to the difficulty in continuous weighing of large quantities of fuel. The equation for efficiency calculation by the indirect method is as follows. 6 v r1=100 Zqi (1 ) += 2 where : rl- gross efficiency of the boiler, %. qi- Different heat losses in the furnace that including the processes of combustion and hea t transfer (see Table 2). The experimental values of the excess air (a"ex) in the outlet gases are 1.6 to 1.62 higher than assumed in the design calculations (1.54). This and the flue gas's exit temperatur e cause the high value of the loss q2 ( %) (Table 2). The gross efficiency obtained di.iring the tests varíes in the range % and ma y be increased by reducing the loss due to the latent. heat of the outlet gases, q 2 (Figures 2-5). Nussbaumer 7 observes that the combustion efficiency is the main parameter to describe th e performance of a biomass furnace. Therefore a correct determination of the combustio n efficiency is essential for a comparison between furnaces.

4 , Ñ 14.4! / \ 6 O.to I o Betbre rctrotitti n Load, (ton/hr) 13.2 L_ Figure 2. Variation of q2 vs. load. o ój L M u a' EO m o 4 U U 0. 3 v oo 0.2 i - N U o E r O 0. 1 U 4r W f o r Betore retrofittin g ' Load, (ton/hr ) Figure 3. Varíation of q 3 vs. load. 83 rt - F c\ Zr N Uj N 3 Betore retrofitting Before retrofittin g Ú u 2 R U U N y ó E ro E 2 i \ -Load, (ton/hr) A-. A Load, (ton/hr ) Figure 4. Variation of q 4 vs. load. Figure 5. Variation of rl g vs. load. There are several calculation methods to determine the mechanical and chemical losses o f heat. 6 ' 7 The complete combustion of a fuel is determined by two losses : mechanical and chemical (2). *7. =100 (q 3+ q4) Tice = Combustion efficiency, %. Q 3 = Chemical heat losses in the flue gas, %. q4 = Mechanical heat losses in the ash and slag, %. (2) Chemical heat losses occur when the air in the furnace is insufficient to burn all th e volatile gases released by the fuel or there is not good mixture between oxygen and fuel.

5 1 522 Mechanical heat losses consider the incomplete combustion of the fuel owing t o insufficient residence time for the range of particle size of the fuel. It is determined by th e existente of carbon in the slag and ash. 4 ' 5 The results in Table 2 display an increase in chemical losses (calculated from the amount of CO in the flue gas) as the load is raised. The calculations also indicate that the chemica l losses are 36.4% higher for the retrofitted furnace with respect to the original configuration. The higher generation of CO in the new geometry can be attributed to different effects : (i) 33.3 % more bagasse is burned in the combustion chamber (ii) the air is supplied only for th e lower zone of the furnace (iii) the diminishing of the excess air coefficient for the load o f 35 t/h until 1.6. The chemical losses have been also calculated by the method proposed by Nussbaumer. 7 The results displayed in Table 2 indicate that small deviations are obtained between thi s method and the Nussbaumer method. Therefore, it cara be concluded that the metho d proposed by Nussbaumer can be used for rapid calculations without a significant loss in th e accuracy of the results. High levels of mechanical losses (up to 8% have been measured) have been commonl y observed in other bagasse-fired furnaces. Hence, these losses affect combustion efficiency and must be an ingredient of the method. Following the Normative Method (Table 2), th e mechanical losses were assumed to be q4 = 4% for the original boiler. As a result of the following, the value was reduced down to q4 = 2.05%. In the retrofitted boiler this parameter was reduced to 2.05%. The balance test indicates that only 6% of the total solid residues was collected on the grate. The carbon content of this residues is very low (1.7 < Cs i ag < 3.86), due to the recycling of the larger particles in th e lower zone of the furnace. The remainder of the solid residue, i.e., 94%, exited the furnace in the forro of fly ash particles. Their carbon content is in the range < C ash < 21. The reduction of the mechanical losses after the retrofitting is the main factor that improved th e combustion efficiency at nominal loads (see Table 2). Similar results have been obtaine d following the Nussbaumer method. 5. CONCLUSIONS 1. A significant increase in combustion efficiency (from 95.5 to ) was obtaine d as a result of the retrofitting. The reduction of the mechanical losses (from 4% for the original boiler to 2.05% for the new geometry) is considered the main factor. Chemica l losses were only slightly modified by the retrofitted (0.5% for the original boiler, an d % after the modifications). 2. The new combustion technology markedly increased the fraction of solid residue s emitted as fly ash particles (50% for the original boiler, and 94% after retrofitting). Thus an additional system is needed to separate collected solid particles before the flue gase s are discharged into the atmosphere.

6 REFERENCE S 1. Situacion actual y proyeccion del Programa de Desarrollo de las Fuentes Nacionales d e Energía en la Agroindustria Azucarera (1995). MINAZ. La Habana. Cuba. 2. Horton (1980). Development in boilers design and installation, International Suga r Journal, 82, pp Watkins (1993). Boiler design, maintenance and operation, International Sugar Journal, 95, pp Dixon, T. F. (1983). Combustion characteristics of bagasse suspension boilers, Proceedings of Australia Society of Sugar Cane Technologists, Mackay Conference. Brisbane Q. Watson Ferguson and Company, pp Oliva, et al. (1991). Horizontal furnace from bagasse suspension burning, Proceedings o f the XI `h Brazilian Congress on Mechanical Engineering. Vol XIII. Serie C. Sao Paulo. Brazil, pp Trembovlia, B., et al. (1991). Tenlotexnihceskie ispitania papobix kotlov, 2 "d issue. Moscow. Energoatomizdat (in Russian). 7. Nussbaumer, Th. and J. Good (1998). Determination of the combustion efficiency i n biomass furnace, Proceedings of Biomass for Energy and Industry, Germany, pp