Radiant tubes connected to biological waste drying and gasifying chambers

Transcription

1 Radiant tubes connected to biological waste drying and gasifying chambers IOAN CĂLDARE*, CORNEL MUNTEA**, IOAN GIURCA** * Mechanic Engineering Department, ** Building Services Engineering Department Technical University of Cluj-Napoca Boulevard Labour, no.13-15, 4641, Cluj-Napoca Romania ioan.caldare@rezi.utcluj.ro Abstract: The modern recovery instalations of bioenergetic waste, wood chips or sawdust, cereal waste, other vegetal waste, have a low efficiency because of using a big heatflow for crossing the endothermic course of the drying-gasification phases. The using of some radiating sources for the supply of endothermic energies of the process brings a big advantage, on the one hand, ensuring the possibility of separating the phases of drying and gasification, on the other hand, completely detaches the source from the technological energy process. We achieve this by a system of intake energy with radiating tubes, which in high temperature assure through radiation the necessary heat contribution for gasification, and for the low temperature they assure the necessary heat contribution for the humidity elimination. The communication exposes the physic basis of this new and original technology. Key-Words: cereal waste, vegetal waste, gasification, woodchips, radiant tubes, radiation, sawdust, humidity, drying. 1 Introduction The modern recovery installations of bioenergetics waste, wood chips or sawdust, cereal waste, other vegetal waste, have a low efficiency because of using a big heat flow for crossing the endothermic course of the dryinggasification phases. The using of some radiating sources for the supply of endothermic energies of the process brings a big advantage, on the one hand, ensuring the possibility of separating the phases of drying and gasification, on the other hand, completely detaches the source from the technological energy process. We achieve this by a system of intake energy with radiating tubes, which in high temperature assure through radiation the necessary heat contribution for gasification, and for the low temperature they assure the necessary heat contribution for the humidity elimination. The communication exposes the physic basis of this new and original technology. This paper represents a follow-up of the research works presented in papers [1], [2], [3], [4]. The study was also a necessity, considering that our country is rich in biomass resources. 2 Technological Process The nowadays biomass gasification technological processes are exclusively based on the use of a superior contribution fuel which, using high-temperature heating gases produced by a burner, is heating, in the counter-stream, the biomass flow subject to the technological process. There are two major disadvantages of this technology: - on the one hand, the drying process of the usually highly humid biomass, as it is the case of the woodchips and wood sawdust, carries the water vapors in the carrying burning gases and in the gasification gases; - on the other hand, the gasification gases have a low caloric potency due to their content of water vapors and burning gases of the auxiliary fuel, and because of this reason, they cannot be used in co-generation cycle thermal engines, with high output. ISBN:

2 Our studies related to the technological applicability of the radiant tubes, a use field quite little explored by the nowadays technologies, led to a use scheme of the radiant tubes for the biomass drying and gasification process, where the drying process can be physically separated from the gasification process, and moreover, the heat contribution for the technological process is made using the thermal radiation and therefore it does not suppose a convective mixture of various thermal agents. In figure 1 we present the classical biomass drying and gasification process, using burning gases from an auxiliary fuel. caloric potency due to the water vapors, which are inert gases. This is why, in classical systems, the initial humidity of the biomass is limited to the value W = 12 %. We must also mention the difficulty of controlling precisely enough the convective heat transfer between the burning gases and the fuel layer. A low gas speed leads to an insufficient heat transfer, while a too high speed may lead to the mixture of the gases with solid particles. In figure 2 we present the biomass drying and gasification process using radiant tubes. Fig. 1 Classical biomass drying and gasification process using burning gases from an auxiliary fuel In figure 1 we notice the way in which the burning gases coming from the auxiliary fuel burner contribute with gasification heat in the 2 nd phase of the process, and they carry these gases towards the common exhaust. As carbon oxidation air is necessary for the coke gasification, the auxiliary fuel burning process must be made with an excess of air, in order to create the necessary technological air. The excess air must be extremely finely dosed because if the oxygen necessary to gasify the coke is exceeded, the reduction reactions 2 C + O 2 = 2 CO shall be continued with complete oxidation reactions 2 CO + O 2 = 2 CO 2, which decreases the caloric potency of the gas producer. In the 1 st phase of the process, the gas mixture is loaded with the water vapors coming from the biomass drying process. The result will be a more accentuated decrease of the Fig. 2 The biomass drying and gasification process using radiant tubes From figure 2 it results that the radiant tubes placed over the biomass layer are transferring heat through radiation towards this layer. This time, as compared to other uses of the radiant tubes, the fact that the radiant tube s casing has a higher temperature at the end located towards the burner and a lower temperature at the exhaust end is an advantage, because a higher temperature is needed for gasification, while a lower temperature is needed for drying. Also, from figure 2 one firstly notices that the track of the burning gases of the auxiliary fuel is completely separated from the track of the technological gases. In this way, the burning process corresponding to the heat contribution ISBN:

3 is independently adjusted and the burning gases do not dilute the gasification gases. Secondly, the drying process area with its water vapor emissions may be separated inside the furnace, and the water vapors may be separately exhausted to the chimney. This water vapor separation process allows us to use the biomass as raw material, irrespectively of its initial humidity. The gases from the gasification area compartment are separately collected and transformed into energy. Their characteristics are typical to the air gas if one introduces only air through the coke s gasification air blasting valve, or the characteristics typical to the mixt gas, if one introduces a mixture of air and water vapors through the coke s gasification air blasting valve. 3 Dimensioning Radiant Tubes The temperature of the heat receiving surface and the temperature of the gasifying biomass layer respectively vary along the layer. We take into account the following heat flows necessary in the biomass gasification process [1]: - biomass heating up to the temperature of 15 o C: cp = 2.72 kj/kg/k; - water vaporization: r = kj/kg; - biomass heating up to the temperature of 28 o C: cp = 2.96 kj/kg/k; - exhaust of volatile gases: rv = kj/kg; - coke s heating up to the temperature of 6 o C: cp =.921 kj/kg/k; - coke s gasification - exothermal process. With these heat flows, taking as an example the wood waste bearing the atmosphere balanced humidity, it results the gasification temperature row along the layer presented in figure 3. Fig. 3 Gasification temperature row along the layer In the TEMTUB [4] calculation software, which calculates the temperature of the radiant tube s casing if the heat receiving surface is a plane surface with environmental temperature, we performed the necessary modifications in order to take into account the high and variable temperature of the heat receiving surface of the gasification layer. We took into account three air excesses (alpha = 1,1, 1,2 and 1,3) used for the burning process of the radiant tube s burner. We notice, as expected, that the lower is the air excess, the higher is the temperature of the radiant tube s casing. Although the temperature of the heat receiver Ts varies quite a lot along the technological process, the temperature of the radiant tube Tr does not registers big differences. This phenomenon is due to the fact that in the heat transfer through radiation the 4 th powers of absolute temperatures of the two heat exchanging bodies interfere, which makes the influence of the low temperature body neglectable: Q RT = c o S T ε T S [(T T /1)4 (T S /1) 4 ] (1) The S T surface is the diametric projection surface of the radiant tube while ε T S is the reciprocal emission coefficient of the two surfaces. ISBN:

4 In figure 4 we present the temperature of the radiant tube s casing depending on the relative length on the axis of the radiant tube, in the burner - gas exhausting direction. We also notice that in case of variations of burning air excess leading to variation of the initial temperature of the radiant tube close to 2, the exhausting temperature of the burning gases registers very small differences, therefore being little influenced by the excess of burning air. Temperature of the radiant tube s casing, in [oc] Relative length of the radiant tube, in [m] Tper, alf = 1,1 Tper, alf = 1,2 Tper, alf = 1,3 temperature, we performed the necessary modifications in order to take into account the high and variable temperature of the heat receiving surface of the gasifying layer. In order to exemplify this, we have chosen as a gasification material the wood waste with 12% humidity. The heat flows necessary for the gasification of the material mass unit are the following: - material heating from 2 o C up to 15 o C; kj/kg; - humidity vaporization of 12 %; kj/kg; - (dry) material heating from 15 o C up to 28 o C; kj/kg; - volatile gas emission (38%) and material heating from 28 o C to 38 o C; kj/kg; - coke s heating (33.1% ) from 38 o C up to 6 o C; 21.4 kj/kg; - over 6 o C the processes are exothermal. Temperature, in [oc] Fig. 4 Temperature of the radiant tube s casing depending on the relative length on the axis of the radiant tube in the direction of the burner towards the gas exhausting Heat Flows along the Radiant Tube The calculation of the heat flows transmitted by the radiant tube to the receiving surface (the gasifying biomass layer) is extremely important, on the one hand because it established the areas of the various phases of the process, and on the other hand, from the total thermal balance, the necessary speed of the conveyor belt shall result. In the FLUXTUB [4] calculation software, which calculates the heat flow transmitted by the radiant tube s casing if the heat receiving surface is a plane surface with environmental Relative length of the radiant tube [L/Lmax * 1], in [m] Tper, alf = 1,1 Tmat Fig. 5 Temperature variation of the radiant tube and of the gasification material The results of the calculation program processing are presented in: - figure 5, which presents the temperature variation of the radiant tube and of the gasification material; ISBN:

5 - figure 6, which presents the unitary heat flows transmitted by the radiant tube to the working surface. Unitary heat flow received by the material, in [W/m2] [1] Antonescu, N. et.al. Physical model of wood burning with gasification. Conference of the Classic and Nuclear Thermo-mechanic Equipment Department. Politehnica University of Bucharest, July 26. [2] Antonescu, N., et.al, Experimental results concerning wood burning with gasification. Conference of the Classic and Nuclear Thermomechanic Equipment Department. Politehnica University of Bucharest, July 26. [3] Antonescu, N., N., et. al, Experimental researches with visualization concerning wood burning in furnaces with gasification. Scientific Session Constructions-Installations CIB 27, Braşov, November 27. Published in the volume Works of the Scientific Session Constructions-Installations. The Publishing House of Transilvania University from Braşov, 27. [4] Cǎldare, I., Contributions to the study of heating with low temperature radiant tubes. Doctoral Thesis. Technical University of Cluj- Napoca, Cluj-Napoca, 25. Relative length of the radiant tube [L/Lmax * 1], in [m] q, in [W/m2 K] Fig. 6 Variation of the transferred heat flow 5 Conclusion The increased use of radiant tubes in technological processes leads to the substantial improvement of the processes where the primary thermal agent must be separated from the technological material due to quality and energy saving reasons. The exemplification made in this research paper concerning the biomass gasification systems is probative. The way of approaching this issue, as exposed in the paper, may be applied to many technologies where one can use the radiant tube as a primary heat source. References: ISBN: