Efficiency Improvement in Boiler by Trimming Oxygen

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 11 | April 2017 ISSN (online): 2349-6010

Efficiency Improvement in Boiler by Trimming Oxygen Sumeet M. Sanghvi PG Student Department of Mechanical Engineering Faculty of Technology & Engineering , M.S. University of Baroda, Vadodara

Vijay M. Parmar Assistant Professor Department of Mechanical Engineering Faculty of Technology & Engineering , M.S. University of Baroda, Vadodara

Abstract Thermal efficiency of boiler can easily improve by number of methods. Some of them required big changes in the system, but some of the techniques are available by which it is very easy to improve the boiler efficiency without major changes. This paper focus on this issue that how the efficiency can improve of the three different boiler - two spreader stoker, one FBC of one of the power plant at leading chemical unit which have same steam producing capacity of 30 TPH at 56kg/cm2(g) pressure.

Management of the company argued that they want to improve the efficiency and reduce the cost with very less changes in the system. One of the best methods for that is trimming oxygen and reduces excess air with control the level of CO. An online flue gas monitoring system or flue gas analyzer is required to measure oxygen level, excess air, CO level and various losses in the boiler at economizer outlet, APH outlet, ESP outlet and ultimately find efficiency of the boiler. Controlling of oxygen is done by ID and FD fan damper.

Keywords: Boiler efficiency, Trimming oxygen, Excess air, online flue gas monitoring, Flue Gas analyzer _______________________________________________________________________________________________________ I.

INTRODUCTION

Boilers mix air with fuel to provide oxygen in the combustion process. For safety reasons, a small amount of excess air is always provided to assure that all fuel is burned inside the boiler. The level of excess air is dependent on fuel type, burner design and boiler design. By operating the boiler with a minimum amount of excess air, we can decrease stake heat losses and increase boiler efficiency. Given complete mixing, a precise amount of air is required to completely react with a given quantity of fuel.

When air is delivered for combustion, the nitrogen absorbs heat and is carried up the stack, resulting in energy losses. If there is excess air, the result is unused oxygen as well as even more nitrogen to absorb heat that is carried up the stack. As Nitrogen contributes nothing to the combustion process, it is essential that the excess air is kept to a level which gives maximum efficiency. This optimum level may vary considerably according to the firing rate of the boiler (boiler load) and is measured by the residual oxygen content in the flue gases.

II. OXYGEN TRIM SYSTEM Boiler efficiency can be improved by incorporating an excess air trim loop into the boiler controls. It is easy to detect and monitor excess air, as oxygen not used for combustion is heated and discharged with the exhaust gases. A stack gas oxygen analyzer can be installed to continuously monitor excess air and adjust the boiler fuel-to-air ratio for optimum efficiency. A carbon monoxide trim loop, used in conjunction with the oxygen analyzer, assures that incomplete combustion cannot occur due to a deficient air supply. Boiler efficiency can be increased by 1%for each 15% reduction in excess air or 40˚F reduction in stack gas temperature. An annual fuel savings of 5% is often obtained with tighter excess air control. We can periodically tune the boiler and manually optimize fuel-to-air ratios after measuring the oxygen in the flue gas with an inexpensive test kit. More expensive hand held computer-based analyzers display percent oxygen, stack gas temperature, and boiler combustion efficiency.

An automatic oxygen trim control system minimizes operating costs through ensuring that the proper fuel-to-air mixture is maintained at all boiler loads.

III. THERMAL EFFICIENCY BEFORE TRIMMING OXYGEN Thermal efficiency of all boilers before trimming of oxygen are find out by indirect method from measured data and its value are 70.12%, 72.39% and 77.79% for boiler 1,2 and 3 respectively as shown below.

Sr. No. 1 2

Table – 1 Thermal Efficiency of Boiler by Indired Method from Measured Data Various heat losses Boiler 1 Boiler 2 % L1-Heat loss due to Dry flue gas 13.34 12.52 % L2-Heat loss due to evaporation of water formed due to H2 in fuel 6.0 6.1

Boiler 3 8.48 4.38

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Efficiency Improvement in Boiler by Trimming Oxygen (IJIRST/ Volume 3 / Issue 11/ 011)

3 4 5 6 7 8

% L3-Heat loss due to evaporation of moisture in fuel %L4- Heat loss due to moisture present in combustion air % L5-Heat loss due to incomplete combustion % L6-Heat loss due to radiation & convection % L7-Heat loss due to un-burnt in fly ash % L8-Heat loss due to un-burnt in bottom ash % Total heat losses % Thermal Efficiency from Measured Data (Indirect Method)

3.2 0.41 2.61 0.61 0.15 3.2 29.88 70.12

3.3 0.27 1.8 0.8 0.15 2.67 27.61 72.39

4.72 0.19 1.96 0.8 0.10 1.58 22.21 77.79

Boiler 1 before Trimming the Air Flow for Combustion Initial readings taken on boiler 1 at economizer inlet, convection bank outlet, ESP outlet and APH outletare shown below in the table. Note that numbers of readings are taken at above locations at different O2% before and after trimming oxygen but only some of them are shown in this paper. Almost 35 reading are taken on boiler 1, 24 readings on boiler 2 and 18 readings on boiler 3 at different conditions like before and after soot blasting, peak load condition, before and after trimming oxygen and reduction in steam demand but only 12 readings of boiler 1 is shown in this paper.

Reading 1 2 3 4 5 6 Location ESP outlet ESP outlet Economizer outlet Economizer outlet Economizer outlet Economizer outlet O2% 12.6 12.1 11.4 9.3 9.1 8.8 CO ppm 41 45 0 0 10 10 Stack Temperature C 143 140 223 221 216 214 Ambient C 35 34 34 34 34 32 Excess air % 126 112 119 79 76 72 Total Heat Loss 19.7 18.8 30.0 28.8 28.3 27.7 Efficiency at Economizer/ESP outlet 80.3 81.2 70.0 71.2 71.7 72.3

High O2% about 12% at reading 1 and 2 of ES Poutlet taken before consistent readings 3 to 6 at economizer outlet and also numbers of same reading taken at APH outlet seen higher O 2% indicates possibility of combustion air leak in APH or ESP lowering the stack temperature, indicating an incorrect higher efficiency of 81%. As the design efficiency of the boiler is 80%, but here the efficiency at ESP and APH outlet is more than design value and O 2% is also very high as compare to economizer outlet. So there is a possibility of air leakage at APH or ESP outlet. So flue gas analysis readings at Economizer outlet to be more reliable than at APH outlet of Boiler 1.From calculations it is found that heat recover at economizer is about 10.5% and at APH is 0.052kg/kg of fuel.

Boiler 1 after Trimming the Air Flow for Combustion Air flow rate in boiler is reduced by adjusting FD fan output as well as ID fan damper. The best oxygen level to get higher efficiency and reduces total heat losses is in the range of 7 to 8% with acceptable level of CO below 150 ppm and lower un-burnt in bottom ash. After closing dampers of both FD and ID fans, O 2% showed a reduced trend and number of readings taken on boiler 1. At 7.3 O2% total heat losses, CO level is comfortably controlledon boiler 1.

Reading 7 8 9 10 11 12 Location Economizer outlet Economizer outlet Economizer outlet Economizer outlet Economizer outlet Economizer outlet O2% 8 8.1 7.3 7.8 8.1 8.1 CO ppm 5 25 25 30 2.5 2.5 Stack Temperature C 217 216 219 222 222 221 Ambient C 35 35 36 36 37 37 Excess air % 62 63 53 59 63 63 Total Heat Loss 27.0 26.8 25.9 26.8 26.9 26.8 Efficiency at Economizer outlet 73.0 73.2 74.1 73.2 73.1 73.2

Summary of Readings for Boiler 1,2 and 3 In given below table there is an operating efficiency of Boiler 1,2 and 3 on the basis of average readings taken and heat losses at economizer outlet before trimming and after trimming is shown. Following conditions are considered to take readings.

1) After trimming the air flow rate 2) During peak load when the steam pressure had fallen and steam output was not delivered.

3) When CO was high with O2 below 6% 4) Heat recovered at APH and Economizer is also accounted.

Averaged Readings and Heat Loss at Economizer outlet of Boiler Type of boiler O2% CO ppm Stack Temperature C Ambient C

Boiler 1 Stoker fired 7.5 50 217 34

Boiler 2 Stoker fired 8.3 150 221 36

Boiler 3 FBC 7 145 218 35

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Efficiency Improvement in Boiler by Trimming Oxygen (IJIRST/ Volume 3 / Issue 11/ 011)

Excess air % L1 Heat loss % due to dry flue gas L2 Heat loss % due to evaporation of water formed due to H2 in fuel L3 %Heat loss due to evaporation of moisture in fuel L4 % Heat loss due to moisture present in combustion air L5 % Heat loss due to incomplete combustion L6 % Heat loss due to radiation & convection L7 % Heat loss due to un-burnt in fly ash L8 % Heat loss due to un-burnt in bottom ash Total Heat loss Boiler 1 Efficiency: 100 – Total heat loss

59 10.6 4.3 3.2 0.35 2.3 0.65 0.1 4.6 26.1 73.9

65 12.2 4.71 3.2 0.35 1.29 1.75 0.4 1.6 25.5 74.5

50 8.8 2.9 1.3 0.42 0.9 1.3 0.08 4.3 20 80

From above reading, analysis and summary it can be seen that the efficiency of boiler 1,2 and 3 can be improved from 70.12% to 73.9%, 72.39% to 74.5% and 77.79% 80% respectively by trimming the excess air flow rate in the boiler.

Fig. 1: Summary of Readings for Boiler 1,2 and 3

For Boiler 1 GCV of coal = 4,311 kcal / kg Enthalpy of superheated steam at 56 kg/cm2and 460 ºC: 797 kcal / kg Enthalpy of feed water with condensate: 50 kcal / kg Base steam generation from boiler 1 =500 MTPD 4,311 kcal / kg x 70% Steam to fuel ratio at 70% efficiency= -------------------------------- =4.04 kg steam per kg of fuel 797 – 50 kcal / kg Present fuel consumption for 500 MTPD=500/4.04=124 TPD Steam to fuel ratio at 73.9 % TPD =4.2 Saving in fuel per day=6 TPD Annual saving potential=Rs.54 Lakhs.

Similarly for boiler 2 and 3 savings in fuel per day are 2.8 TPD and 1.2 TPD and annual savings are Rs. 25 lakhs and Rs. 11.6 lakhs respectively.

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Efficiency Improvement in Boiler by Trimming Oxygen (IJIRST/ Volume 3 / Issue 11/ 011)

Fig. 2: Average Value after Trimming Oxygen

IV. CONCLUSION 1) Design thermal efficiency of all three boilers are 80% while actual efficiency of the boiler from measured data before any trimming of air are 70.12%(boiler 1), 72.39%(boiler 2) and 77.79%(boiler 3).

2) Average oxygen level to reduce the excess air and improve efficiency is 7.5%(boiler 1), 8.3%(boiler 2), 7%(boiler 3).

Excess air reduce to 59%, 65% and 50% respectively in boiler 1,2 and 3.

3) Efficiency of the boiler after trimming of oxygen improved to 73.9%, 74.5% and 80% respectively in boiler 1,2 and 3.

4) Heat recover at economizer is about 10.5% and at APH is 0.052kg/kg of fuel in boiler 1 and 2.

5) Savings in fuel per day for boiler 1,2 and 3 are 6 TPD, 2.8 TPD and 1.2 TPD respectively.

6) Annual savings potential for boiler 1,2 and 3 are Rs. 54 lakhs, Rs. 25 lakhs and Rs. 11.6 lakhs respectively.

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Vikram Singh Meena, Dr. M.P Singh. “Efficiency Assessment and Improvement of Boiler at Super Thermal Power Station”. IIARET-Volume 3, Issue IV, April 2015, ISSN 2320-6802 P.Ravindra Kumar, V.R.Raju, N.Ravi Kumar, Ch.V.Krishna. “Investigation of Improvement in Boiler Efficiency through Incorporation of Additional Bank of Tubes in the Economizer for Supercritical Steam Power Cycles”. IJERD-eISSN : 2278-067X, pISSN : 2278-800X, Volume 4, Issue 8 (November 2012), PP. 94-100 Rahul Dev Gupta, SudhirGhai, Ajai Jain. “Energy Efficiency Improvement Strategies for Industrial Boilers: A Case Study”. Journal of Engineering and Technology. Vol 1. Issue 1. Jan-June 2011 V. K. Gaudani, Energy Efficiency in Thermal System. Vol. III. IECC Press. Delhi 2009 Kevin Carpenter, Chris Schmidt and Kelly Kissock. “Common Boiler Excess Air Tends and Strategies to Optimized Efficiency”. ACEEE Sumer Study On Energy Efficiency In Buildings. 2008,3,page 52-63 J. Gladstone Evans, C. Damodaran. “Modern Trends in Boilers and Efficiency Improvement Programmes”. Avant-Grade Engineers and Consultant. Techpapers.

British standards, BS845:1987 ASME Standard:PTC-4-1 power test code for steam generating units

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