CHAPTER 6 BOILER EFFICIENCY

Transcription

1 CHAPTER 6 BOILER EFFICIENCY 6.1 Introduction Boiler operation is very complex and plays very important role in sugar mill. Sugar mill requires steam for the process and electric power for auxiliary consumption. Most of the sugar mills have cogeneration plant. Bagasse being by-product of sugar mill is used as a fuel to fire boilers in sugar mills. The heat generated by the combustion of fiiel in furnace of the boiler produces required steam for process and power generation. Boiler is single most expensive item in a sugar mill. Because of high operating temperature and pressure it is imperative to control it precisely to ensure safe, efficient and 'clean operation'. In the past the fuel requirements for most of the sugar mills were easily met by the bagasse available in house. In most of the cases bagasse used to be surplus after meeting needs of the mill. However, with increase in cost of fossil fuel and electricity coupled to economic growth resulted in to the increased demand for bagasse as an alternative fuel. As a result more mills find themselves a shortage of bagasse and have to resort to alternative fuel in the form of coal. This development led to an increasing awareness for energy conservation and obviously focus shifted to enhancing boiler efficiency. Boiler operation is very complex as it is linked to many process parameters, its configuration and on the physical and chemical properties of the fuel. The combustion process produces required heat for steam conversion and releases gases in the form of flue gas. Stoichiometric or Theoretical Combustion is the ideal combustion process where fuel is burned completely. The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct when all fuel and air burned is consumed without any excess left over. The complete combustion of bagasse releases gases like Carbon dioxide, Water vapor. Nitrogen oxide and Oxygen. Combustion process in the boiler depends on many process parameters like moisture content in fuel, proper fuel to excess air ratio and ambient temperature of air and fuel. Any imbalance in these process parameters will lead to incomplete combustion. Incomplete combustion will release 128

2 gases like CO2, Carbon and soot particles when flue gas is released in air. Therefore boiler operation needs to be properly controlled and needs to be handled carefiilly. The scope of the research is limited to theoretical analysis of enhancing thermal efficiency of boiler by optimizing calorific value of fuel, predicting combustion temperature and excess air required for complete combustion. However, general aspect like boiler Configuration contributing to over all operational efficiency is covered in short. 6.2 Boiler Configuration The main components of a modem boiler are: the grate, fuel feeders, combustion chamber or furnace, water or mud drum, steam drum, main bank, super heater, economizer, air heater, scrubber, induced draught (ID) fan, forced draught (FD) fan, secondary air (SA) fan, boiler feed water pumps etc. The area of research is to enhance calorific value of fuel by pre drying using microwave dryer. The thermal efficiency of the boiler is analyzed. 6.3 Boiler Efficiency Boiler efficiency is very complex function and it is linked to many process parameters Moisture Content in fuel, (Bagasse) Ambient temperature of fuel and excess air Ratio of excess air to the theoretical quantity of air The furnace temperature Coefficient taking in account losses due to radiation-p Coefficient taking in to account unborn solids-a Coefficient taking in to account incomplete combustion-t] Heat losses in Flue gases 6.4 Stoichiometric combustion or Theoretical combustion It is ideal combustion process where fuel is burned completely. A complete combustion is a process burning all Carbon (C) to CO2, All Hydrogen (H) to H2O and Sulfur (S) to SO2 if any. Sugarcane bagasse does not have sulfur content. With unbumed components in the exhaust gas (Flue gas) such as C, CO, H, the combustion process is incomplete and not stoichiometric. 129

3 The combustion process can be expressed as follows: [C + H (fuel)] + [O2 + N2 (Air)] < (Combustion Process) > [CO2 + H2O + N2 (Heat)] (6.1) Where C = Carbon; H = Hydrogen; O = Oxygen; N = Nitrogen To determine the excess air or excess fuel for a combustion system the stoichiometric air-fuel ratio is considered first. The Stoichiometric combustion is shown in Fig. 6.1 [153]. The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct and where all fuel and air used is consumed without any excess left over. In practice process heating equipment are rarely run that way. 'On-ratio' combustion used in boilers and high temperature process furnaces usually incorporates a modest amount of excess air - about 10 to 30% more than what is needed to bum the fuel completely. If insufficient amount of air is supplied to the burner, unbumed fuel, soot, smoke and CO exhaustsfi"omthe boiler - resulting in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion. To avoid inefficient and unsafe conditions boilers normally operate at an excess air level. This excess air level also provides protection fi-om insufficient oxygen conditions caused by variations in fuel composition and 'operating slops' in the fuel-air control system. If air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean. If air content is less than the stoichiometric ratio - the mixture is fuel-rich. Excess Fuel Excess Air Fig. 6.1: Stoichiometric combustion 130

4 6.5 Fuel The composition of fiiel plays a major role in assessing its calorific value and calculating thermal efficiency of the boiler. To optimize boiler efficiency it is essential to analyze physical, proximate and ultimate analysis of fuel. 6.6 Fuel Properties The fuel components are usually divided in to ash, moisture and other constituents. In spite of diversity of milling plants and machines employed, the physical composition of bagasse varies between rather narrow limits. The most important property of bagasse; from the point of view of steam production is its moisture content. Physical analysis describes the fuel component in terms of their quantities. It is used to calculate the calorific value. Physical and chemical composition of bagasse are given Table 6.1 and Table 6.2 respectively. Bagasse constituents Fiber (F) Ash (E) Moisture(wet)(w) Total Bagasse % Table 6.1: Physical Composition of bagasse Bagasse constituents Carbon Hydrogen Oxygen Ash Bagasse- % Table 6.2: Chemical Properties of bagasse Value N.C.V of bagasse is lower than G.C.V. Another formula to determine G.C.V of bagasse is suggested by Hessey [6th Congr. I.S.S.C.T; p 1054] is as follows. This formula is determined experimentally and verified G.C.V. = *s-83.45*w (6.2) N.C.V. = *s-88.27*w (6.3) 131

5 Where w = moisture % bagasse. s = sucrose % of bagasse. Another formula to determine G.C.V is as follows: G.S.V = 8280(1-w) in BThU/lb (6.4) G.S.V = 4600(l-w) in kcal/kg (6.5) Where w = moisture content of bagasse relative to unity For example when bagasse is having 45% moisture then w =. [55] Net Calorific Value (N.C.V.) of wet bagasse is given in Table 6.3 N.C.V m.units kcal/kg *w Br.Unit BThU/lb *w Table 6.3 Net calorific value of wet bagasse 6.7 The Ultimate analysis The uhimate analysis is the analysis of the fuel into its basic chemical elements. This analysis is used to get the theoretical air needed for combustion based on stoichiometric equations of various elements. It also provides a means to determine the quantity and combustion of the flue gases. It forms the comer stone for deriving the boiler efficiency using loss methods. 6.8 The Proximate Analysis: The proximate analysis is defined as the part of the fuel that gasifies below 750 C, called volatiles, in relation to the fixed carbon. It provides an indication of its combustion properties especially combustion stability. This property is particularly important for coal and used to a lesser extent for bagasse. 6.9 Calorific Value It is the heat produced by unit mass of bagasse when used as fiiel to fire boilers. Calorific value of bagasse depends on its moisture content. Fig. 6.2 shows the improvement in calorific value of bagasse with reduction in its moisture content. The net saving of bagasse is also shown. As bagasse is having large percent of moisture 132

6 present, its N.C.V is relatively low. Higher percentage of moisture may lead to low furnace temperature hence lower steam production per unit consumption of bagasse. It also results in to incomplete combustion resulting in emission of CO and shoots particulate in the air. Combustion temperature T in the bagasse furnace is readily calculated from the fact that the heat developed in combustion is recovered in the gases passing from furnace to the boiler. For wet bagasse as fuel, it is necessary to take in to account the heat losses due to water vapor produced by combustion of the hydrogen constituent of bagasse and water vapor originating due to moisture in bagasse. Net Calorific Value of bagasse takes in to account the latent heat of vaporization of water due to Hydrogen content of bagasse as well as heat losses due to moisture content in bagasse, from the point of view of steam production e ^ ^ > g u 0 - Caloriflc value of bagasse l_jcalorific Value i Incremental calorific _ fl value n Moisture % (Wet basis) -1 r 60 >" t > - 30 ^ J U 5 0 Fig. 6.2: Calorific value of wet bagasse. After taking in to account the heat lost in converting water in to water vapor, the following losses are to be taken into account: Sensible heat lost in the flue gases Losses by radiation Losses in unburned solids Loss by incomplete combustion of carbon giving CO instead of CO2 133

7 6.10 Sensible heat loss (q) Net Calorific Value of bagasse, already takes in to account the loss of heat of water vapor passing with flue gases to the chimney. Out of the losses to be accounted, the most important loss is the sensible heat lost in these flue gases. Table 6.4 gives the sensible heat lost at different moisture % and at different excess air ratios for 1kg of bagasse. The composition of flue gases and the specific heat of its component gases are known. From these inputs corresponding heat loss can be obtained. Flue gases during bagasse combustion at 50% (wet basis) moisture content and 45% (wet basis) moisture content are given in Table 6.5 and Table 6.6 respectively. The mean specific heat of the flue gases is between 32 F (0 C) and the flue gas temperature, which varies only slighfly because this temperature itself is very less. In modem installation, with economizer or air heater, it is easy to obtain flue gas temperature below 190 C. There is little interest going below C which may be considered as the lower economic limit. Conversely, it would be only a very old or inadequate installation which would allow the gases to leave at temperature greater than 284 C. Fig. 6.4 shows flue gases during bagasse combustion as a function of excess air % and fig. 6.5 shows Carbon dioxide and Oxygen % in flue gases as a function of excess air %. The weight of flue gases depends on moisture content in the bagasse as well as on quantity of excess air in the boiler. For 1 Kg of bagasse, when burnt, the following constituent of flue gases is obtained by weight. Weight of bagasse: 1kg, moisture =0, excess air % m=1.5 w = moisture % m = excess air Vo Pg = weight of flue gas- kg 0 n 5J1 Boiler efficiency: All the above factors affect the efficiency of the boiler. Optimizing bagasse fired boiler efficiency is shown in Figure 6.6 and optimizing combustion temperature by controlling bagasse moisture is shown in Figure

8 moisture (wet basis) w% 50 Excess air m% moisture w.r.t unity( w) Excess air,(m) 1 Flue gas temperature t C 165 q- sensible heat lost Kcal/Kg Table 6.4: Sensible heat lost at different moisture % and at different excess air ratios q-sensible heat loss A * Fig. 6.3: Sensible heat lost -Moisture %(Wet basis) m % Excess air t C Flue Gases -q-sensible Heat lost kcal/kg

9 M Excess Pg w N2 02 H20 C02 C02% 02% N2% air% Table 6.5: Flue gases during bagasse combustion Flue gases-bagasse combustion H20 C02% 02% N2 «> > > I Excess air % (m) 120 Fig. 6.4: Flue gases during bagasse combustion as a function of excess air%

10 Bagass weight= 1 kg, w= m Excess Air% Pg w N2 02 H20 C02 C02% 02% N2% , Table 6.6: Composition of flue gases at different excess air % (Bagasse moisture 45% (wet basis) Bagasse combustion-excess air % (C02,02) Moisture (w =) on unity basis. C02 02 Fig. 6.5: Carbon dioxide and Oxygen % in flue gase as a function of Excess air % 137

11 Optimizing boiler efficiency moisture % (wet basis) t= c Flue gas Temp c % saving bagasse m % Excess air Excess air q Sensible Heat lost k cal/kg '% Boiler Efficiency ^ K i50 A IDO ii ido A 150 A IDO A IDO A s Fig: 6.6 Optimizing bagasse fired boiler efficiency by controlling its Process parameters Optimizing combustion temperature h- Moisture % (Wet basis) H^ Excess air% h Increase in combustion temp% )( Boiler efficiency % g^ 50»"T^ a^ \(^n r m^^r^j - 1 T- T ^ ds 1560 C Fig: 6.7 Optimizing combustion temperature of boiler and its efficiency by controlling moisture content in bagasse ns

12 6.11 Conclusion The efficiency of bagasse fired boiler is very complex, as it depends on many process parameters as well as on physical, thermal and Ihemical properties of the fuel. Theoretical analysis is carried out to find effect of moisture content in bagasse on boiler efficiency. Excess air ratio plays very important role in ensuring complete combustion process resulting into clean boiler operation. If an insufficient amount of air is supplied to the burner, unbumed fuel, soot, smoke and carbon monoxide exhausts from the boiler resulting in heat transfer, surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion. The presence of higher excess air results into lower combustion temperature, lower Carbon Dioxide % and excess Oxygen % in the flue gases. This results in to excess energy consumption for air handling equipments and overall boilers efficiency rolls off. [Refer Figure 6.5] Boiler efficiency greatly depends on moisture content in bagasse. Higher percentage of moisture may lead to low ftimace temperature hence lower steam production per unit consumption of bagasse. It also results in to incomplete combustion resulting in emission of CO and shoots particulate in the air. T = t + *^o Governing equation [54] for combustion temperature. From the governing equation of combustion temperature it is clear that combustion temperature increase when ambient temperature of air and fuel increases. The combustion temperature is directly proportional to N.C.V of bagasse, which is linked to its moisture content. Higher moisture will lower combustion temperature greatly on account of the additional water vapor present and more so since the specific heat of water vapor is nearly double that of other gases present in flue gas. Combustion temperature decreases as excess air increases. [Refer Figure 6.7] It is possible to optimize the boiler efficiency by modelling N.C.V of bagasse, excess air used and initial temperature of fuel and air entering boiler. 139

13 By controlling process parameters the complex boiler efficiency can be optimized as shown in the graph [Fig. 6.6]. By bringing down moisture content in bagasse from initial 50% to 30% and reducing excess air from 150% to 125%, the overall efficiency improved by 30% (from 50% to 83%). During this process the combustion temperature has increased by 37.77%. It is important to note that the flue gas temperature also has gone down fi-om 162 C to C. It means nearly 16.4% reduction in flue gas temperature coupled with 42% saving in bagasse consumption and reducing power energy by 25 % reduction in excess air. Beyond these points efficiency improvements slow down as moisture evaporation process slow down. Therefore there is no point in drying bagasse beyond 30% moisture. Further improvements in boiler efficiency takes place at very slow rate hence not viable. [Refer Figure 6.6] By optimizing process parameters in the sugar mill the boiler efficiency improves significantly ensuring complete combustion and hence 'clean operation', reduced discharged flue gas temperature and nearly 42-45% saving in fuel is possible. The efficiency of bagasse fired boiler is very complex as it depends on many process parameters as well as on physical, thermal and chemical properties of the fiiel. 140