The Nitrogen Monoxide Generation and Diminishing to the Co-combustion Corn-coal

Transcription

1 The Nitrogen Monoxide Generation and Diminishing to the Co-combustion Corn-coal IONEL PΪÃ* Bucharest Polytechnical University, 313, Splaiul Independentei, , Bucharest, România The study analyses the nitrogen monoxide generation and reducing during the co-combustion of biomass (corn-cob) in a power plant fired with pulverised coal. The physical and mathematical model shows the nitrogen monoxide generation and the reducing of its concentration by the air staging and by the combination of the primary methods. Keywords: biomass, combustion installations, co-combustion, pollution The developing of the sources of renewable energy is a top priority of the European Union. Within this framework, the biomass area will bring a major contribution in solving the request of energy for the future due to the biomass potential and to the available technologies. One of the opportunities of development the biomass is its combustion together with the coal. In this paper, the author studied the co-combustion process and the generation and reducing the concentration of nitrogen monoxide during the combustion of the coal-corn mixture (the corn is used as fuel under the form of cob, being crushed and then ground in the coal mill). In the physical and chemical processes from the furnace, the appearance of the thermal decomposition and the combustion of the released volatiles lead to a developed mathematical model, in order to be closer to the analysed real processes. In comparison with previous models [1], this paper takes into account the fact that not all the nitrogen quantity is transformed in NO. This decrease is studied in different types of equations in [2]. The exposed model considers the NO concentration decreasing caused by the heterogeneous reaction of the nitrogen monoxide on carbon surface. The NO real concentration is thus obtained. The purpose was to calculate the nitrogen monoxide concentration and its main influences, in order to decrease the nitrogen monoxide concentration. The nitrogen monoxide results from the nitrogen oxidation existing in the fuel organic matters and from the oxidation of the nitrogen introduced together with the combustion air. The influence of these characteristics was quantified after the mathematical model was obtained and applied. This paper paid attention to the primary air excess coefficient, to the initial diameter of the particles of equivalent fuel and to the gravimetric participation of the corn to the mixture. The internal recirculation of the combustion gases at the burning inlet, the recirculation of the combustion gases at the furnace end-part, the value of the air excess coefficient and the introduction time of the secondary air have been considered constants. The method of successive air injections (air staging) at burning inlet (or after an established law) was considered the primary reducing method (basic). Other primary methods were, also extended, in the same time, in order to obtain, if it is possible, a NO concentration lower then the value allowed by environment protection norms. Thus, the secondary methods, which are expensive and request supplementary locations, are eliminated. The mathematical model The co-combustion total time was divided into three distinct stages and the secondary air introduction was considered to be done according to a linear time low. The co-combustion took place by an infinite number of stages, as it was shown in [1]. This co-combustion is called also co-combustion with continue access of secondary air. Thus, the equation structure [3], which forms the mathematical model, was changed. The spray corn-coal co-combustion layout, completed by the interfering parameters, is shown in figure 1. Also, the mathematical model functions have a dynamics related to the period of co-combustion. This dynamics is presented in figure 2. The co-combustion time was divided in three stages, Z 1, Z 2, Z 3, resulting, for each one of them, the characteristic mathematical model for their own physical and chemical processes. The mathematical model for Z 1 This stage corresponds to the interval 0 < τ < τ 1, where τ 1 is the moment when the introduction of the secondary air starts. It mentions that, τ 1 is immediately after τ aprv, volatiles ignition time. The equations for Z 1 are: - the released volatiles dynamics: - the volatiles combustion dynamics: [kg/(kg.s)]; [kg/(kg.s)]; Fig.1. The sprayed coal combustion layout (1) (2) * ipisa@caz.mecen.pub.ro 698

2 Fig.2. The combustion dynamics of the solid fuel particle (qualitative presentation) - the equivalent fuel temperature dynamics: (3) Q k is calorific power of coke (heating value), kj/kg; c k - specific coke heat, kj/(kg.k); - the powdered equivalent fuel apparent density, kg/ m 3 ; ρ k - coke density, kg/m 3 ; T fl - flame temperature, K; a k - coke energy emission coefficient; α g - thermal convective coefficient [4], kw/(m 2.K); V ie - initial volatiles volume of the equivalent fuel, kg volatiles /kg fuel ; - the combustion gases temperature dynamics: (4) where : GFL is the thermal radiation coefficient, between the radiant bodies within the furnace and the burning powder jet; V gz1 - specific combustion gases volume of Z 1 stage, referring to one kg of dust equivalent fuel; V 0 - theoretical wet air volume requested for the aum combustion process, Nm 3 /kg;v v - specific volatiles volume, Nm 3 /kg; Q v - calorific power of volatiles, kj/kg; c CO2, c O2 - specific volatiles heat of carbon dioxide, respectively of oxygen kj/(kg.k). The computing relation for V gz1 is: (5) [Nm 3 /kg]; V O2 is the specific oxygen volume required by the cocombustion process, as follows: C f index referring to 1 i kilo fix equivalent fuel combustion and v index referring to 1 kilo of volatiles; V go - resulted combustion gases volume, on the same reference base; ie C f - the initial content of fix equivalent fuel; λ i,λ f,λ ev - air excess coefficient at the burning inlet, at the furnace end-part, at the discharge (economiser area), respectively the primary air excess coefficient; r evp, r i, r f the recalculation degree from the discharge (combustion gases for primary inlet), from the burning inlet, respectively from furnace end-part; ζ - combustion gases rising volume coefficient. - the diameter dynamics of the equivalent fuel particle is described by the relationship: [m/s] (6) 699

3 where M O2k is the quantity of oxygen required by coke cocombustion, kgo 2 / kg coke. The model takes into consideration the nitrogen monoxide generation from air and equivalent fuel: - kinetics of the nitrogen volatilisation of the equivalent fuel organic mass: [kg/(nm 3.s]; (7) N ie is the concentration of the atomic volatilised nitrogen existing in the initial equivalent fuel organic mass, kg nitrogen / kg fuel ; γ - fraction of the initial nitrogen which can be volatilised. - the dynamics of the molecular nitrogen generation from the volatilised nitrogen: [kg/(nm 3.s]; - the generation velocity of the released nitrogen monoxide from the nitrogen of the equivalent fuel organic mass, taking into account its heterogeneous reduction on the coke basis surface negative term is expressed by: (8) [kg/(nm 3.s]; (9) - the nitrogen monoxide generation from the nitrogen of the combustion air, according to the Zeldovici s low: (10) This stage includes the initial moment, τ = 0. At this moment, the oxygen concentration resulted from primary air combustion, from combustion gases recirculated at the furnace end-part and from the combustion gases recirculated at the steam generator end-part, is: (11) where M O2 is the quantity of oxygen required for combustion, C f index refers to the combustion of 1 kilo i of fix equivalent fuel, while v index refers to the combustion of 1 kilo of volatiles. In this stage, the concentration of the molecular nitrogen resulted from air, is: (12) The mathematical model for Z 2 This stage corresponds to the interval τ 1 τ τ 2 (fig. 2). After the volatiles were ignited (W e > 0) the secondary air and the recirculated combustion gases are introduced. The equations of this stage are as follows: - equation (1) for the released volatiles dynamics; - equation (2) for the volatiles combustion dynamics; -equation (3) for the equivalent fuel temperature dynamics; - for the gaseous phase temperature variation: (13) c am is the specific heat of mixed air and combustion gas at discharge, kj/(nm 3.K); t am - the secondary mixing temperature at the burning inlet, C; c g - the specific combustion gas heat, kj/(nm 3.K); V gz2 - gaseous phase volume, corresponding to Z 2. V gz2 is computed by the formula: [Nm 3 /kg]; (14) which shows the linear ascending (in time) of secondary air introduced in jet. - the diameter variation of the equivalent fuel particle is presented in relation (6); - equation of nitrogen volatilisation dynamics is: [kg/(nm 3.s)]; (15) The other equations are identical with the ones of Z 1, while for Z 2 the difference is given by oxygen concentration, which is calculated by the relationship: (16) 700

4 while the concentration of the molecular nitrogen resulted from the air is: (17) The Z 2 stage end, corresponding to τ 2 = τ 1 + τ, is considered at the moment when the volatiles were burned. Also, at the moment t 2 the secondary air introduction ended. The Z 2 can have variable length, depending of the fuel type. The time interval t is established at the beginning of the computation. In this case, the optimal value was considered 0.2 s. The mathematical model for Z 3 The Z 3 stage starts at the moment τ 2 and ends at the moment when the coke basis has burnt. The Z 3 stage lies in the interval τ 2 τ t τ a, where, the total burning time of equivalent fuel particle is τ a. Z 3 equations, that set the difference between this stage and the other two, are presented below: - gaseous phase temperature variation: (18) - nitrogen existing in equivalent fuel volatilisation dynamics: [kg/(nm 3.s)]. (19) For this stage the gaseous phase volume is calculated by: [Nm 3 /kg]; (20) - current oxygen concentration: (21) while the current nitrogen concentration is: The equations contain the following symbols: δ, δ 0, - current, respectively initial diameter of the equivalent fuel particle, µm; τ, τ a, τ 1, τ 2 - current, respectively burning time, at the ends of Z 1 and Z 2, s; kdv 0, kav 0, k on, k or, k oon - preexponential factor of volatiles releasing velocity, volatiles combustion, nitrogen volatilisation, molecular nitrogen recombined in atomic nitrogen and nitrogen oxidation; E dv, E av, E vn, E r, E on - activation energy of these reactions, kj/ kmol; K - global coefficient of equivalent fuel particle combustion velocity [3], m/s. Results and discussions The system for different variants has been solved with the aim to make comparisons between them starting from (22) the equation representing the dependence of the value of the NO concentration upon different independent variants considered for different variable parameters: (23) δ 0 is the initial diameter of the equivalent fuel particle; λ p the primary air excess coefficient; g p - the gravimetric participation of the corn to the mixture. To solve the system the following fuels with the elementary analysis at the primary stage have been selected: Based on the data above and depending on the gravimetric participation of the corn, the analysis of the equivalent fuel is determined. The results are presented in table 1. It is mentioned that the maximal gravimetric participation of the corn, g p, has been considered as 30%, because over this value there are problems related to the obstruction of the grounding mills. The data in table 1 allow the calculation of the values needed for solving the equations system (example:v o, aum V o, etc.). The system has been solved, in different variants, g for the following cases: δ 0 = 60; 90; 120; 160 mm; λ p = 0.20; 0.30; 0.40 and, obviously, g p = 0.10; 0.20; The results are presented in the graphs in figures 3-6 (the NO concentration is expressed in mg/nm 3 and the diameter of the particle of equivalent fuel in µm). 701

5 Table 1 THE COMPSITION OF THE EQUIVALENT FUELS Fig..3.The variation vs NO=f (δ 0 ) ; g p =0.00 Fig..4.The variation of NO vs f (δ 0 ); g p =0.10 Fig..5.The variation of NOvs f (δ 0 ) ; g p =0.20 Fig..6.The variation of NOvs f (δ 0 ) ; g p =0.30 Conclusions The physical model proposed, namely the linear introduction of the secondary air divides the combustion time in three bounded stages, like: the first one from the initial moment until after the volatile substances ignition, the second one ending at the moment of total introduction of the secondary air and the third one at the complete combustion of the particle of equivalent fuel; - the combustion of the mixture corn-coal leads to reducing the total combustion time by 10-20% compared with the combustion of the coal due to the higher content of volatile substances of the corn than of the coal; - the value of the nitrogen monoxide concentration in the mixture of fuels depends, among others, on the initial diameter of the particle of equivalent fuel, δ 0, on the primary air excess coefficient, λ p, and on the gravimetric participation of the corn in the mixture, g p ; - the influence of the corn combustion over the concentration of NO is a complex and contradictory one, thus: the higher content of nitrogen in the organic mass leads to the increase of NO c ; the higher calorific power leads to the increase of the theoretical combustion 702 temperature, thus favouring the thermal NO; the high content of oxygen in the composition of corn leads to reducing the volume of oxygen introduced, a fact that diminishes the concentration of NO; - the value of NO concentration increases with the increase of the initial diameter of the particle of equivalent fuel up to about 120 µm after which it becomes constant, and after 160 µm a decreasing tendency can be seen; - also, the value of NO is influenced by the quantity of air introduced under the form of primary air (the concentration of NO diminishes with the increase of λ p ) and by the gravimetric participation of the corn in the mixture (the concentration of NO increases with the increase of g p ). These influences meet the expectations because by the increase of g p the content of volatile substances of the equivalent fuel (NO increases) increases and this increase of V ie entails a increase of λ p (NO diminishes), being known that one of the conditions for obtaining a low NO concentration is the observance of the condition, λ p V i e [ 5 ]; - by the combustion of the mixture coal-corn under the above mentioned circumstances, its have been obtained values of the NO concentration between 350 and 530mg/

6 Nm 3, values close to the limit values of emission. Anyhow, the decision to implement the corn (or any other type of biomass) in producing energy implies the analysis of the economic factors (the production and the transport cost for the corn, the benefit on the market of green certificates, etc.). References 1. NEAGA, C., PΪÃ, I., The influence of the linear introduction of the secondary air over the dynamic of formation of thermal NO, Energetics, nr.1, Bucharest, 1999, p PΪÃ, I., Researches on the generation and reducting the nitrogen oxides in the process of combustion of powered coal, Doctorate Dissertation, p. 150, Bucharest, PΪÃ, I., The Flue Gas Recirculation to Suppress Nitrogen Monoxide Formation in Combustion Installations, Rev. Chim. (Bucureºti), 57, nr. 4, 2006, p EPPLE, B., Modellbildung und Simulation von Stromungs-Reactionsund NO Bildungsvorgangen in technischen Feuerungen, Univ.Stuttgart, 1993, p PΪÃ, I., Generation and reducing NO X - during the coal combustion in boilers, PERFECT Publishing House, Buc., 2003, p. 218 Manuscript received: