Conversion of Furnace Oil Fired Boiler to Biomass (Gliricidia) Fired (External/Internal) Furnace Boiler
A dissertation submitted to the
Department of Energy Technology, Royal Institute of Technology, Sweden for the partial fulfilment of the requirement for the
Degree of Master of Science in Engineering By
Channa Gaya Siriwardhana Kahandawa Arachchilage
Department of Energy Technology Royal Institute of Technology,
Stockholm, Sweden
Conversion of Furnace Oil Fired Boiler to Biomass (Gliricidia) Fired (External/Internal) Furnace Boiler
by
Channa Gaya Siriwardhana Kahandawa Arachchilage
Supervised by Dr. Primal Fernando
Declaration
The work submitted in this thesis is the result of my own investigation, except where otherwise stated.
It has not already been accepted for any other degree and is also not being concurrently submitted for any other degree.
Channa Gaya Siriwardhana Kahandawa Arachchilage
Date
We/I endorse declaration by the candidate.
Dr. Primal Fernando
Conversion of Furnace Oil Fired Boiler to Biomass (Gliricidia) Fired (External/Internal) Furnace Boiler
Abstract
In the present era, with the prevailing competition, the cost of production plays a vital role. As the price of petroleum oils, especially diesel and furnace oil are growing at a steeper rate than solid fuel price, finding a substitute for furnace oil is one of the alternative available.
1 litre of furnace oil used in boilers can be totally substitute by 3.5 kg of biomass on the basis of calorific value. This may results in saving of more than 60% of operating cost and would have at- tractive payback period of 6-8 months.
Sri Lanka has large agriculture base and very common of having Gliricidia as an under-grow. The other biomass fuels such as paddy husk, saw dust, firewood are also available in large quantities around the country.
Objective of this article is to study the conversion of presently running furnace oil fired boiler, which is located at Ambilipitiya paper factory, Sri Lanka, to biomass fired external furnace boiler namely water-wall boiler. Further a case study, which was done previous to this study and running successfully, is described to show the viability of the conversion using the internal furnace method.
This case study was done at a Textile factory called Brandix Finishing, Siduwa, Sri Lanka. The re- sults will be reducing the operating cost of the boilers and reduction of green house gas emissions.
Opportunities for rural people to get extra income by farming Gliricidia, extra income from saw dust, paddy husk, and firewood are indirect benefits of the project. This report gives details of technical, environmental and commercial aspects of this unique opportunity.
Acknowledgments
I am heartily thankful to my project supervisor, Dr. Primal Fernando, whose encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the subject.
My sincere thanks go to KTH, Royal Institute of Technology, Stockholm, Sweden for giving an opportunity to follow the M.Sc. in Sustainable Energy Engineering. I must thank the project office of KTH & post graduate division of ICBT, Sri Lanka for helping in many ways to do my M.Sc.
Lastly, I offer my regards and blessings to all of those who supported me in any respect during the completion of the project.
Channa Gaya Siriwardhana Kahandawa Arachchilage
List of Abbreviations
A Area of cross-section of radiant heat absorption surface, m2
F View factor
GHG Green house gas
NPC National paper cooperation MDC Multi cyclone duct collector CDM Cleaner development mechanism
ID Induced draft
ppm Parts per million GCV Gross calorific value
FO Fuel oil
BHP Boiler hose power
F and A From and at
P and I piping and instrumentation
USD US dollars
VFD Variable frequency drive
FD Forced draft
x Fraction of heat radiation
T1 Absolute temperature of flame, K
T2 Absolute temperature of radiant heat absorbing surface, K Qrad Radiation heat transfer rate
Nu Nusselt number
Pr Prandtl number
Re Reynolds number
Ve Inlet velocity
Vax Axial velocity
ua Velocity at wall
ui Inner flow velocity ws50 Settling velocity di* Inner flow cut off de* Wall cut off
Dp Pressure drop
eta Total efficiency
Greek symbols
1
εf Flame emissivity
Θf1 Flame temperature 5.67 X 10-8 W/m2K4
Φeff Coefficient of thermal efficiency of water-wall λ Friction factor
Subscripts
Inc Incident
ID Inner diameter
refl Reflection
dep Deposition
rad Radiant
x Element of oxygen
2 Element of oxygen
ax Axial
a At wall
i Inner
Conversion of Furnace Oil Fired Boiler to Biomass (Gliri- cidia) Fired (External/Internal) Furnace Boiler
1 Introduction ……… 9
2 Objective……… 10
3 Energy use ... 11
3.1 Energy usage in the paper factory ... 11
3.2 Energy usage in the Textile Factory ... 12
4 Methodology ... 13
4.1 External furnace method - paper factory ... 13
4.2 Internal furnace method - textile factory ... 13
5 Technical details of steam boilers ... 14
5.1 Technical details of boiler at the paper factory... 14
5.1.1 Burner system ... 14
5.1.2 Burner control system ... 15
5.1.3 Boiler shell ... 15
5.1.4 Fuel and electricity consumption ... 15
5.1.5 Efficiency of the boiler ... 16
5.1.6 Calculation of boiler efficiency ... 16
5.2 Technical details of the boiler at textile factory ... 16
6 Pre-requisite of the boiler conversion ... 17
7 Retrofitting of the existing boilers ... 18
7.1 Technologies available ... 18
7.1.1 Internal grate with Induced draft (ID Fan) ... 18
7.1.2 External grate firing ... 18
7.2 Designing the water-wall membrane for paper factory boiler 19
7.2.1 Introduction to water-wall technology ... 19
7.2.2 Designing the water- wall membrane ... 19
7.2.3 Shell modification, lifting and interconnecting the shell and the water-wall ... 22
7.2.4 Insulation of the water wall membrane ... 23
7.2.5 Advantages of water-wall boiler ... 24
7.3 Conversion of the textile factory boiler – Internal grate .. 24
7.4 Fuel feeding system and fuel storage ... 26
7.5 Sizing the induced draft fan (ID Fan) ... 28
7.6 Designing the cyclone separator ... 29
7.7 Water scrubber and mist eliminator ... 31
8 Ash and dust collection ... 32
8.1 Ash and dust collection from water-wall boiler ... 32
8.2 Ash and dust collection from textile factory boiler... 32
9 Cost benefit analysis ... 33
9.1 Water-wall boiler ... 33
9.2 Textile factory boiler ... 34
10 Clean development mechanism (CDM) application ... 35
11 Economics of fuel conversion ... 36
12 Case study done for paper factory boiler ... 37
13 Case study done for textile factory boiler ... 38
14 Conclusions ... 39
15 References ... 40
Annexure A- Boiler efficiency calculation ... 41
Annexure B -Saving calculation due to boiler conversion ... 42
Annexure C – Existing shell at paper factory ... 43
Annexure D- Cyclone calculation results ... 45
Annexure E – Running water-wall boiler at paper factory-Walachchena, Sri Lanka 46
Annexure F – Running water-wall boiler at Hindustan Coca-Cola, India 48
Annexure G– Water-wall boiler is under construction ... 50
Annexure H -Technical Specification of the converted boiler-paper factory 52
Annexure I -Technical Specifications of the water scrubber for paper factory boiler 53
Annexure J- Water-wall boiler safety & maintenance ... 54
Specifications of water: ... 54
Annexure K –Case study- Brandix, boiler conversion... 55
16 Annexure L – Steam and oil consumption data (Sample) - Brandix Boiler 58
Annexure M – Drawing -Conversion of furnace oil fired boiler to biomass fired boiler 63
Annexure O - Layout of the paper factory boiler with wood storage 65
1 Introduction
Paper and pulp industry in Sri Lanka was introduced in 1950’s by the government of Sri Lanka and was managed as a state property until very recently. As a result, machineries and technology used in this industry are old. Though it has been realized that improvements in energy efficiency and envi- ronmental standards are very desirable, the lack of finance has impeded this development.
At present, this industry is suffering from high energy cost and therefore planning to fuel the mills with biomass.
National paper factories, located in Ambilipitiya (Hambanthota district) and Walachchena (Batti- caloa district), are under government-owned company of the National Paper Cooperation (NPC) and produces paper and paper boards from waste papers.
Batticaloa district is Sri Lanka’s highest rice growing area. Rice mills generate about 78% (weight) rice, broken rice and bran. The remaining 22% is husk, which is the shell around the paddy grain.
Part of this husk is used as fuel in the rice mills to generate steam for the parboiling process. The husk contains about 75% organic volatile matter that is burnt during the firing process and the bal- ance of 25% is converted into rice husk ash (RHA). This RHA in turn contains around 85-90%
amorphous silica and is used as an additive in cement industry.
A survey conducted by the Rice Processing and Development Center has estimated that availability of 4,700 tons of rice husk per year from 30 mills in the Batticaloa district. Rice husk can be used as a fuel by other industries, which is well practiced in India.
The paper company with support of the government is looking for replacing oil fired furnace lo- cated in Ambilipitiya to Gliricidia or rice husk fired furnace. This requires converting the existing 18-ton heavy fuel oil fired steam boiler to run using wood. The company was seeking support for a study of the technology to use Gliricidia or paddy husk as fuel at the factory, information about vendors who may implement the technology and financial support (possibly through the clean de- velopment mechanism).
Brandix is one of the biggest textile manufacturing companies in Sri Lanka. They are using number of furnace oil fired boilers for their dying and ironing purposes. The capacities are in the range of 500 to 3000 kg/hr. There are opportunities to convert these boilers to wood fired boilers using in- ternal furnace method and get the benefit of financial savings. This is also eligible for the said “car- bon credit [1]”.
2 Objective
The paper company has financial problems due to higher energy prices and is therefore looking for ways to bring down the production cost. With the increasing oil prices the company is considering alternative fuels as an option to expensive furnace oils used in boilers. At the same time, it is con- sidering to bring down the environmental pollution, which is caused by fuel oil burning.
Current boiler efficiency (80%) is lower than that it should be (90%). With the increasing trend of oil prices, the energy cost component contributes a major part for the production cost. Hence an efficient conversion for the boiler assists in cutting down the increasing production cost.
As far as the textile factory is concerned, they don’t have so called financial problems. But their ob- jective is to go towards greener production. Also they have won the best greener factory in Sri Lanka for the first time in textile industry. Therefore they are looking for sustainable fuels to gener- ate heat for their process.
The option should therefore be seen as a low cost investment as far as the paper factory is con- cerned while a sustainable development opportunity for both factories.
3 Energ y use
3 . 1 E n e r g y u s a g e i n t h e p a p e r f a c t o r y
Primary energy sources used in the paper factory are thermal energy in the form of steam and me- chanical energy converted from electricity. The thermal energy accounts for 70-80% of the total primary energy and is mainly used for pulping and drying processes.
Oil prices have been increased by 60% over the last ten years (from 2000 to 2010). In 2000, the price of furnace oil was Rs. 20.00 /liter. In fact price was as high as Rs.33.00/liter in the beginning of the year 2010. At present it is Rs.41.00/liter for 1500 redwood seconds furnace oil [2]. Whenever oil prices go up, alternative fuels are considered. But whenever it goes down, the enthusiasm goes down.
Breakdown of process steam
40%
21% 5%
33%
1%
Pulping Hot water making Bleaching Drying Others
Figure 3.1: Breakdown of process steam
Normally, temperature levels of process steam of the paper mill is below 200°C.Steam requirement is about 8-12 tons per ton of paper products. The breakdown of process steam is in the factory is shown in Figure 3.1.
Two steam boilers are the major energy consumer units at NPC, Ambilipitiya. Capacities of these boilers are 18 tons/hr each. One of these boilers is subjected to this study. Even though the boilers have been designed for a capacity of 18 tons/hr at 14 kg/cm2 (~14 bars), they operate at a capacity of 10 tons/hr at 10 kg/cm2 (~10 bar). The boilers are therefore over designed and that must be corrected when converting them to biomass boilers.
This boiler consists of two firing chambers with two rotary cup burners. Past records show that, the fuel oil consumption of a boiler as 7000 liters per day. The Operating hours of the boiler are 20 hours/day.
3 . 2 E n e r g y u s a g e i n t h e T e x t i l e F a c t o r y
The primary energy use in the factory is electricity from the grid and thermal energy from 2 ton/hr steam boilers. There were two similar capacity furnace oil fired, three pass boilers in the factory.
The daily furnace oil consumption of one boiler was identified to be at 432 liters/day with 12 hours working per day. The steam demand is 600 kg/hr at 9-10 kg/cm2(~9-10 bar). This factory is also use overcapacity boilers. Therefore, there is a possibility to convert one boiler to a lower capacity wood fired boiler.
4 Methodolog y
4 . 1 E x t e r n a l f u r n a c e m e t h o d - p a p e r f a c t o r y
First, a technical and environmental feasibility study was carried out to see the viability of the pro- ject. Then, a survey was conducted to find out the average daily steam demand, required steam quality and fuel oil consumption of the existing furnace oil fired boilers. Moreover, all technical in- formation of the furnace, shell, blowers, other accessories of the boiler, battery limit, water con- sumption, electricity consumption was studied. Flue gas analysis was carried out for exhaust at the chimney in a typical working day. (Note that the boiler was not running at full capacity). This is used to find the efficiency of the boiler and the GHG emission from the boiler. The space availabil- ity, continuous availability, quality (moisture content) and quantity of Gliricidia and availability other alternative fuel such as (sawdust, paddy husk, etc) were found. Based on the average steam demand and considering the future steam demand, the design of the boiler conversion was done.
Specially, the design of the water-wall membrane for the boiler is the main concerned. The required modifications should be done for the existing shell. A detailed study of the shell and relocating the shell was needed in order to size the water wall membrane. After designing the water-wall mem- brane, the insulation of the membrane should be done. After having being sized the water-wall membrane, the connecting the water-wall membrane and the modified shell was done. Other acces- sories were sized to suit the demand. (ID Fan, Chimney placement, etc.). Cyclone system (MDC) and water scrubber were designed. Finally, Carbon trading under clean development mechanism (CDM) is proposed.
4 . 2 I n t e r n a l f u r n a c e m e t h o d - t e x t i l e f a c t o r y
Referring to past data records of the factory, the daily steam demand and the furnace oil consump- tion were investigated. At the same time, it was observed that the present steam demand is in the range of 400-600 kg/hr at 9 -10 bar. This boiler conversion could be done based on 800 kg/hr of steam requirement by considering future increase of steam demand. The existing boiler capacity is 2000 kg/hr and even by the internal furnace conversion method, the boiler can deliver over 800 kg/hr capacity. Therefore it was decided to do the conversion by internal furnace method.
The following procedure was followed.
• Removed the burner of the existing boiler (please refer the annexure)
• Designed a fuel feeding door
• Designed an ash removal and primary air intake door
• Laying fire bars
• Constructed a fire brick wall
• Refractory laying for the front door
• Sizing and installation of the Induced draft fan
• Fabrication and installation of necessary ducting for the flue gas path
• Installation of a damper control system and temperature controller and indicator in the flue path
• Re-wiring the electrical control system of the boiler unit
• Repairing the fittings, and safety system of the boiler
• Commissioning the boiler
• Study of the financial savings
5 Technical details of steam boilers
5 . 1 T e c h n i c a l d e t a i l s o f b o i l e r a t t h e p a p e r f a c t o r y
This boiler is a fire tube; three pass system with 18 tons/hr capacity. The designed pressure is 14 kg/cm2 (~14 bar). The steam demand is 4-6 tons/hr with 10 kg/cm2 (~10 bar)working pressure.
As the boiler with large water space, it is suited for rough operation and may be subjected to pulsat- ing and varying load application. The boiler automatically adapts to its load by the fully-automatic firing control. The total heating surface area of the boiler is 350 m2. The following items are also in- cluded in the steam generating system.
• Steam line
• Feed water supply line
• Fuel oil lines
• Mud lines and condensate lines
• Ignition gas lines
• Exhaust gas discharge
5 . 1 . 1 B u r n e r s y s t e m
When converting the boiler, the existing burner arrangement should be removed. The existing Ro- tary cup burner system is shown in Figure 5.1.
Figure 5.1: Rotary Cup Burners
The rotary cup oil burner contains a cone shaped cup that rotates around a central tube where fuel oil is supplied.
5 . 1 . 2 B u r n e r c o n t r o l s y s t e m
It uses a modulating control system. A modulating burner control will alter the firing rate to match the boiler load over the whole turndown ratio. Every time the burner shuts down and re-starts, the system must be purged by blowing cold air through the boiler passages. This wastes energy and re- duces efficiency. Full modulation, however, means that the boiler keeps firing over the whole range to maximize thermal efficiency and minimize thermal stresses.
5 . 1 . 3 B o i l e r s h e l l
Figure 5.2: Shell with two firing chambers
A shell that similar to the one fabricated is shown in Figure 5.2. The two rotary cup burners have been installed in front of the two cylindrical spaces (firing chambers). The shell subjected to this re- search is similar to Figure 5.2 after disconnecting the burners and insulation. But the flue flow path is somewhat different. The second pass tubes are arranged with two sets (please refer the annexure 03) while the third pass tubes are remained same. There are 296 tubes with 80 mm diameter and 5 m length is connected to the shell. The diameter and length of the firing chamber is 1 m and 5.7 m, respectively. Heating surface area of the shell is 352 m2. The diameter and thickness of a shell tube is 63.5 mm and 3.66 mm, respectively.
5 . 1 . 4 F u e l a n d e l e c t r i c i t y c o n s u m p t i o n
The boiler consumes furnace oil 1500 Red wood seconds and the average oil consumption is 7000 litres per day (20 hours per day). The monthly fuel cost is approximately Rs. 7,175,000.00. The connected total electricity load of the boiler is 45 HP (33.6kW). The monthly electricity cost is around Rs. 70,000.00.
5 . 1 . 5 E f f i c i e n c y o f t h e b o i l e r
Flue gas analysis was carried out in order to find the combustion efficiency of the existing boiler.
The following table shows the measurement data. The equipment used for this analysis was Micro- processor control CANE flue gas analyzer.
Table 5.1: Flue gas analysis for Paper factory boiler (before conversion)
Parameter Boiler 01
Measurement time 17.45 hrs on 17th April 2010
Capacity (kg/hr) 18,000
Fuel type Furnace Oil - 1500 Red
Wood Seconds Oil pre heat temperature (°C) 100
Low Fire High Fire
Combustion efficiency (%) 85.7 81.9
O2 (%) 7.7 9.2
CO (ppm) 02 01
CO2 (%) 9.9 8.8
NO (ppm) 189 195
NO2 (ppm) 00 00
NOX 189 195
SO2 (ppm) 409 129
Flue gas temperature (°C) 186.1 229.0
Ambient temperature (°C) 34.2 33.8
5 . 1 . 6 C a l c u l a t i o n o f b o i l e r e f f i c i e n c y The boiler efficiency is calculated as follows.
Quantity of steam generated (Q) 7000 kg/hr
Steam pressure 10 kg/cm2
Quantity of fuel oil consumed 567 kg/hr
Feed water temperature 80°C
GCV of F.O 10200 kcal/kg
Enthalpy of steam at 10 kg/cm2 pressure 665 kcal/kg
Enthalpy of feed water 80 kcal/kg
Boiler Efficiency Q kg/hr x (hs-hw)/m x GCV
Boiler Efficiency 7x1000x(665-80)/567x10200} x 100 % = 70.8%
5 . 2 T e c h n i c a l d e t a i l s o f t h e b o i l e r a t t e x t i l e f a c t o r y
The boiler is a 2000 kg/hr fire tube Jane Marshal furnace oil fired steam boiler. According to the flue gas analysis performed before the conversion, the following figures were obtained. It shows that the existing efficiency of the boiler is 86.8 at low fire and 85.3 at high fire condition.
• No heavy smoke visible in the stack
Tested By : Mr. Ajantha Sisira Kumara – Technical officer Supervised By : Eng. Gaya Siriwardhana – Consultant/Engineering
6 Pre-requisite of the boiler conversion
Since this boiler is to be converted into wood fired, the availability of good quality Gliricidia is very important. Continuous supply of Gliricidia is a must. To be feasible of this project, it was found that the boundary of the fuel supply distance should be less than 30 km radius circle. Gliricidia growers around the factory can be easily found. However, the total area of Gliricidia cultivation should be increased in order to supply for the required demand. “Conserve Shakthi”, one of the leading Gliricidia growers in Sri Lanka, has the ability to supply the daily demand of Gliricidia at a reasonable price (Rs.7.00/kg transport to the factory).
Before converting the existing boiler, the space requirement should be investigated. The total area requirement for the boiler is about 200 m2. Another 50 m2 area is required for one week Gliricidia storage. This required space is currently available at the factory.
Getting the management consent and the investment for conversion are the key issues. The man- agement realized the important of the conversion and they have given fullest cooperation towards the success of the project.
In case of sudden unavailability of Gliricidia, there should be another fuel option. Plenty of saw dust, paddy husk and wood chips are available in factory located area.
Following table shows the availability of each source within the area of 30 km radius circle.
Table 6.1: Biomass fuel availability
No Fuel Cultivated area / Tons of source 1 Gliricidia in tea plantations 2500 hectare 350 tons/week
2 Saw dust 100 tons/week
3 Paddy husk 3000 hectare 10 tons/week
4 Wood chip 350 tons/week
Gliricidia is an important forage crop in cut-and-carry systems in Sri Lanka. Gliricidia sepium leaves have a high feeding value, with crude protein comprising 20-30% of the dry matter, a crude fiber content of only about 15%, and in vitro dry matter digestibility of 60-65%. It was found that in Sri Lanka, gliricidia leaves had higher crude protein content in the wet season than in the dry season.
The characteristics features of Gliricidia are shown in table 6.2 [3].
Table 6.2: Characteristics of Gliricidia fuel Carbon (%) 47
Ash (%) Up to 2.6 Hydrogen (%) 6.4
Moisture (%) Less than 5 Other (%)(O2+ N2) 45
Sulpor (%) 0 GCV (kcal/kg) 4900
7 Retrofitting of the existing boilers
7 . 1 T e c h n o l o g i e s a v a i l a b l e
The present steam demand of the paper factory is around 8,000 kg/hr. It was estimated that the maximum steam demand would be 10,000 kg/hr with future expansions.
The present steam demand of the textile factory is around 600 kg/hr. The estimated maximum de- mand would be around 800 kg/hr.
The conversion technologies are selected considering the above steam demands.
There are several types of conversion technologies. A simple conversion is an internal grate, which requires only removal of its existing burner-blower assembly, adding a grate to burn the biomass in same combustion chamber and installation of induced draft (ID fan). The other one is the external grate firing in which an external furnace is used.
7 . 1 . 1 I n t e r n a l g r a t e w i t h I n d u c e d d r a f t ( I D F a n )
Partially loaded boilers (say up to 40%) and with uniform load pattern can be converted by internal firing method. Modification of boiler involves removal of burner, blower and providing supports for cast iron grate bars inside the combustion chamber (first pass). The advantages of this method are the cheap and the quickest retrofit, reversal can be carried out in about 4-6 hours and no extra space or ducting is required. The disadvantage of this method is the reduction of the existing capac- ity of the boiler.
Case study: Boiler conversion at Brandix group [Annexure 12]
7 . 1 . 2 E x t e r n a l g r a t e f i r i n g
The advantage of using this modification is the retaining over 75 % of the existing capacity of the boiler. An external furnace is erected with sufficient grate area. Hot combustion chamber of the ex- ternal furnace, multi-window air supply enables a firing and cleaning to be carried out simultane- ously. This helps to maintain the steam generation at a constant level.
Here we can expect a clean combustion and reversal to oil firing is possible in 6-8 hours. The only disadvantage of this method is need of an extra space.
Table 7.1: Boiler conversion methods No. Capacity Modifications
1* Up to 30% Internal Grate in same combustion chamber with removal of burner assembly 2 Up to 66
%
External fuel fired furnace and coupling its exhaust to existing boiler at burner assembly location.
3* Beyond 75%
Raising the height of boiler with water tubing arrangement and addition of Ex- ternal fuel fired furnace
4 Beyond 75%
New set up boilers in large numbers to be installed at separate location with large thermal mass furnace. Steam piping to be connected to the main header Superscripts (*) are used to indicate the methods used in this study.
Since our steam requirement is 12 tons/hr converting existing 18 ton/hr boiler, method 3 (Table 7.3) should be followed for the paper factory boiler conversion.
As far as the textile factory is concerned, the required capacity is 800 kg/hr and the 2,000 kg/hr boiler is used to deliver it. The method 1 is sufficient to achieve that capacity.
7 . 2 D e s i g n i n g t h e w a t e r - w a l l m e m b r a n e f o r p a p e r f a c t o r y b o i l e r
7 . 2 . 1 I n t r o d u c t i o n t o w a t e r - w a l l t e c h n o l o g y
This is an advanced, well proven and well established technology in large industries. The water-wall boiler is designed with the radiant section, using water wall tubes, capable of withstanding a gas temperature up to 2750˚C. Sufficient furnace volume is created in order to achieve the residence time required to complete the conversion reactions. The entire furnace expands and contracts uni- formly as a unit. This eliminates the expansion problems at the interface of water-cooled and re- fractory casing. For a given volume, the water-wall boiler has a low furnace area, heat release rate and heat flux.
Heat absorption by water-wall in a boiler is mainly by the radiant heat transfer from the high tem- perature gaseous products of combustion to water-wall tubes. The intensity of heat exchange by ra- diation is determined by the incident heat flux from flame onto the water-wall in the boiler furnace as front and rear water cooled surfaces provide additional surface area.
A cross section of a water-wall is as shown in the figure 7.1. [4]
Figure 7.1: Cross section of the water-wall 7 . 2 . 2 D e s i g n i n g t h e w a t e r - w a l l m e m b r a n e
Following equation gives the incident heat flux. According to the Stefan-Boltzmann’s law, the inci- dent heat flux from the flame core can be expressed in the following form.
2 3
4 4 9 1
1 10 /
100 ] 273 10 [
7 .
56 kW m
qinc f − f + × −
×
×
×
= θ
ε Eq.1
6 .
1 =0
εf [Flame emissivity, [5] ] ) (
20000
1 C assumption
f =
θ
2 3
4 4
9 10 /
100 ] 273 2000 10 [
7 . 56 6 .
0 kW m
qinc − + × −
×
×
×
= Eq.2
= 560 kW/m2
2 3
4 4 9
int 10 [1 ] /
100 ] 273 10 [
7 .
56 q xkW m
q r dep dep + × × − dep × inc×
×
×
×
= − θ − ε
ε Eq.3
2 3
4 4
9 10 [1 ] (1 ) /
100 ] 273 10 [
7 .
56 q x kW m
qrefl refl refl + × × − dep × inc× −
×
×
×
= − θ − ε
ε Eq.4
Where x is the fraction of total radiation which falls on the water-wall surfaces
refl r
eff q q
q = +
int Eq.5
eff inc
rad q q
q = − Eq.6
Coefficient of thermal efficiency of water-wall
inc rad
eff q
= q
Φ Eq.7
qrad = 0.45 x 560 kW/m2 = 252 kW/m2
Since the type of fuel is solid, the coefficient of thermal efficiency of water-wall is 0.45 [6]
Higher value of Øeff, is selected to get the higher the operating efficiency of the water-wall boiler.
qeff = 560 – 252 kW/m2 = 308 kW/m2
The boiler is designed as the furnace to achieve the proper combustion with the minimum pressure drop by adjusting the boiler height, tube spacing, and tube counts.
Figure 7.2: Heat transfer across water-wall
The rate of radiant heat absorbed by the water wall is Qrad.
−
=
4 2 4 1
100 100
T AF T
Qrad ε σ W Eq.8
- Emissivity of the flame
A – Area of cross-section of radiant heat absorption surface,m2 F – View factor
- 5.67 X 10-8 W/m2K4
T1 – absolute temperature of flame, K
T2 – absolute temperature of radiant heat absorbing surface / 2
252kW m Qrad =
7 . 2 . 2 . 1 C a l c u l a t i o n o f t h e c a p a c i t y o f t h e b o i l e r All calculations are based on the F and A @ 100oC rating.
Expected steam capacity =10,000 kg/hr at 10.5 kg/cm2 saturated Feed water inlet temperature =600C
Heat load = 10000 (664-60) kcal/hr
=6.34x106kcal/hr where Sat steam enthalpy =664kcal/kg
Inlet water enthalpy =60kcal/kg Steam enthalpy at 100˚C and 1 atm pressure =540kcal/kg Therefore, steam capacity F and A 1000C =6.34x106/540 =11,741 kg/hr
7 . 2 . 2 . 2 C a l c u l a t i o n o f t h e h e a t t r a n s f e r a r e a o f t h e w a t e r - w a l l s e c t i o n
Shell and tube heat transfer area = 352 m2
= 3789 ft2
BHP = 223 BHP
= 6,240 lb/hr
Capacity from the shell and tubes only = 9,472.5 kg/hr
Even though the existing firing chambers and the tubes are in good operating condition, we have to consider the radiation losses from tubes and walls, flue gas losses, blow down losses, etc.
For calculation of the heat transfer area of the water-wall, the efficiency of the shell and tubes is taken as 90%.
Assuming 1ft2 can generate 2.5 kg/hr steam,
Required capacity from the water-wall = 11,741-8,500 kg/hr
= 3241 kg/hr
Heating surface area for the water-wall = 1300 ft2 = 121 m2 Selection of the pipe diameters to suit the above heat transfer area (121 m2) Outer diameter of the down comer = 200 mm
Outer diameter of the riser = 75 mm Inner diameter of the down comer = 188 mm Inner diameter of the riser = 63 mm
Tube thickness = 3.66 mm
The total volume of the water wall = 30 m3
Inside heat transfer coefficient can be calculated using the following correlation:
) )(Pr (Re
023 .
0 inside0.8 inside0.4
inside
Nu = Eq.10
Where
GasCond H Tube
Nuinside = i× ID Eq.11
Outside heat transfer coefficient during boiling is very high and so resistance offered is negligibly small. Ho can be safely assumed to be about 10,000 kcal/hr/m2/C.
Table 7.2: Dimensions and material for water wall tubes
Circulation form Natural
Pressure Low Middle High
Outside diameter(risers) mm 51-60 60 60 Outside diameter(down comers)
mm
51-106 108- 133
159-42 S/d
Bare pipe Membrane type
<2.0 -
1.1-1.2 -
1.05-1 1.3-1.6
Material for risers Low
carbon steel
Low carbon steel
Low carbon steel
7 . 2 . 3 S h e l l m o d i f i c a t i o n , l i f t i n g a n d i n t e r c o n n e c t i n g t h e s h e l l a n d t h e w a t e r - w a l l
As the first step, the existing two rotary burners are removed from the boiler unit. Then the open- ing of two firing chambers can be observed as shown in the figure 7.3. The diameter of one open- ing is 1 meter.
The existing boiler is lifted 2.7 m from ground level as shown in Figure 7.3. Therefore the level of top of the water-wall is equal to the top water level of the shell. The approximate weight of the shell is 20 ton (dry weight). A 30 ton crane is recommended for lifting the shell.
The two columns are made up with reinforced concrete. The dimensions of one column will be 3.7 x 0.6 x 2.7 m. The strength of the existing base concrete is sufficient to bear the total weight of the boiler (wet weight) even after lifting. The distance between the two columns is fixed to 1.8 m.
The interconnecting of the water-wall and the above shell is done using a MS plate ducting. The length, height and width of the ducting are 1.4 m, 1.8 m and 0.75 respectively. The thickness of the MS plate is 4.5 mm. The ducting will be welded to walls of water-wall and the shell. The ducting will be insulated using 64 kg/m3 density Glass wool and 24 G Aluminium cladding.
Figure 7.3: Lifting the boiler
Figure 7.4: Interconnecting the water-wall and boiler 7 . 2 . 4 I n s u l a t i o n o f t h e w a t e r w a l l m e m b r a n e
The riser pipes (diameter 75 mm) arrangement is shown in the following plan view. Asbestos pack- ing is fixed between each pipe. Cold face standard IS 8 insulation brick refractory is laid next to the
2700 mm
Ground level Water level &
top of the wa- ter-wall
Water-wall
Interconnecting duct
Boiler
water tubes. Outer shell lagging is done by 100 mm thick, Wineral wool followed by 24 SWG Alu- minium cladding.
Figure 7.5: Cross section of an Insulated water-wall
7 . 2 . 5 A d v a n t a g e s o f w a t e r - w a l l b o i l e r
There are many advantages of the water-wall boiler compared to other conventional boilers. The furnace front, rear, side walls and floor are completely water-cooled and are of membrane wall con- struction, resulting in a leak proof enclosure for the flame. The entire furnace expands and con- tracts uniformly avoiding casing expansion problems. Water wall boiler will eliminate the other problems such as low temperature corrosion and limits on the startup rates for refractory dry-out.
The water wall boiler will reduce the furnace exit temperature, which helps lowering the radiant heat flux and tube failures. In a natural circulation boiler, tubes are vertical and the gas flows hori- zontally. Natural circulation moves the steam-water mixture through the evaporator tubes, where the gas temperature is low act as down comer tubes, while the rest of the tubes in the radiant and convection section act as risers carrying the steam-water mixture to the steam drum.
The membrane wall design acts as a gas tight enclosure for the flue gases and minimizes problems associated with thermal expansion and movement of the various parts of the furnace. In the mem- brane wall design, the entire furnace operates at a uniform temperature, so all the combustible components are reacted and differential expansion is minimal.
The size of the tube and pitch are optimized depending for the generation of low or high-pressure steam. CO formation could be reduced by using water cooled boiler instead of conventional fur- nace.
The water wall boiler will generate the high-pressure steam in the range of 17 to 52 bar. The water wall boiler reduces heat losses compared to conventional reaction furnaces. The design feature in- cludes a radiant section, using water wall tubes, capable of withstanding the 4,000°F inlet gas tem- perature. Sufficient furnace volume (conversion zone) shall be provided to achieve the required residence time/ temperature. The conversion zone may consist of water wall tubes and/ or a re- fractory lined section depending on gas temperature. Refractory is only acceptable below 3,000°F.
Following the conversion zone, a convection section will cool the gas to the desired outlet tempera- ture.
It is a very ideal boiler if the load is constant load because steam fuel ratio can be achieved at 1: 4, if the boiler is operating at 100 % load. In case boiler operates at lesser load i.e. 50 % of the load then the steam fuel ratio drastically gets reduced to 1: 2 because of reasons like more heating surfaces, radiation loss, frequent opening and closing of fire door etc. [7]
7 . 3 C o n v e r s i o n o f t h e t e x t i l e f a c t o r y b o i l e r – I n t e r n a l g r a t e
Internal grating furnace conversion system is a simple method to convert the boilers in to biomass fired boilers as far as the smaller capacity is concerned. As discussed under the methodology, pro- cedure is to remove the existing burner, fixed an internal fire bars and coupled the unit flue gas out
let to an induced draft fan. The following figure shows the piping and instrumentation diagram for the converted boiler.
Figure 7.6: P and I Diagram of the converted boiler (Textile factory) The following figure shows the internal firing chamber arrangement.
Figure 7.7: P and I Diagram – Internal Furnace Arrangement
The ID fan capacity for the converted boiler was selected at 3.5 kW. The existing boiler capacity was 2000 kg/hr three pass fire tube boiler. The heat transfer area of the shell and the fire tube was estimated to be 78 m2. The total length of the shell is 4.5 meters and the diameter of the shell is 1.5 meters. As the designed capacity is expected to be fixed at 800 kg/hr of steam at 100°C Fand A and 9 bar, we have to optimize the heat transfer area of the internal furnace. It is clear that the heat transfer area of the shell is reduced as we use part of the space for the internal firing chamber. But the heat transfer area of the tubes remains same. The only variable is to change the location of the fire brick wall as shown in the Figure 7.7. After several trials we could find the exact place to fix the fire brick wall. Finally the length of the firing chamber was fixed at 1.35 m from the front feeding door to get the required heat transfer area.
The selected ID fan was coupled to the flue gas line and the out let was connected to the existing chimney through a duct.
The flue gas analysis was done after commissioning. The results are as shown in the following table.
Table 7.3: Flue gas analysis-after converting (Textile factory boiler)
No. Parameter Units Values Desirable
Level
1 Net temperature 0C 117.0 -
2 Flue gas temperature °C 149.0 °C
3 O2 % 11.4 %
4 CO2 % 8.6 Around %
5 CO ppm 11.0 -
6 SO2 0
6 Excess air % 121 Around %
7 Combustion efficiency % 89.0 -
8 Ambient temperature 0C 32.1 -
7 . 4 F u e l f e e d i n g s y s t e m a n d f u e l s t o r a g e
Gliricidia is available approximately in 60, 90, 110 mm diameters and 150 mm in length. It can be loaded manually (manual feeding) to the boiler with a shovel. This will add negligible extra cost to the boiler operation; up to a rate of 200-250 Rs. per ton of fuel fired.
Storage of 4-5 days fuel consumption is recommended to cope up with fluctuations and/or trans- portation problems. The storage space is almost one meter square for 2 ton of Gliriciida.
When the Gliricidia is unavailable, the boiler is feed with wood chips, paddy husk, saw dust, etc.
For feeding the said fuels, the following fuel feeding arrangement can be used (Shown in Figure 7.8).
Figure 7.8: Fuel feeding arrangement 1
The wood chips or saw dust, are fed in to the fuel bunker and is passed through a vibratory feeder in to the chute and mixer. Secondary air fan supplies air to convey the fuel from the mixer to the front fuel feeding door at the water-wall.
The following fuel feeding arrangement is also possible (Shown in Figure 7.9). This can be fixed ei- ther to front feeding door of the water-wall or side fuel feeding doors.
To the Side feeding doors at the water-wall
Figure 7.9: Fuel feeding arrangement 2
7 . 5 S i z i n g t h e i n d u c e d d r a f t f a n ( I D F a n )
Induced draft fan is required to evacuate the exhaust gases from boiler to atmosphere through dust collectors and chimney. Usually ID should take care of draft loss across the boiler from furnace to air heater and then draft loss across multi cyclone duct collector, wet scrubber.etc.
Fuel Consumption = 2300 kg/hr
From combustion and efficiency wet gasses = 14.05 Air = 13.12 kg/kg of Gliricidia Exhausted gasses produced = fuel consumption x unit wet-gas
= 2300 kg/hr x 14.05
Total wet gases,kg/hr =32,315
Combustion air required = 2300 kg/hr x 13.12 = 30,176
Gas density = 1.3265 kg/Nm3
Therefore, gas flow in Nm3/hr =32,315/1.3265
=24,361 Nm3/hr
=6.8Nm3/s Gas flow at 1500C in m3/s = 6.8 x (273+150)/273 = 10.5
ID fan capacity taking 20% margin on flow = 10.5 x 1.2 = 13 m3/s
ID fan static head = draft loss in (boiler+duct + dust
collector) = 150+ 50+50 mm WC
= 250 mm WC Taking 20% margin on head, ID fan head = 250 x 1.2
= 300 mmWC Power requirement of ID fan
Let us assume fan efficiency as 75% and motor efficiency as 90%
Power required for ID fan = Flow x Head/ (efficiency x 75.8)
= 13 x 300/(0.75 x 75.8)
= 69 HP
= 69/0.9 HP
Motor HP required = 76 HP (56.7 kw)
To the Front feeding doors at the water-wall
7 . 6 D e s i g n i n g t h e c y c l o n e s e p a r a t o r
Table 7.4: Cyclone separator parameters 1
Parameter Value
Gas inlet velocity to the cyclone 15 m/s Pressure drop (kPa) 2.0 Cyclone efficiency (%) 90.0
Figure 7.10: Schematic flow diagram of a cyclone
Table 7.5: Cyclone separator parameters 2
Cyclone diameter (mm) 5000
Cyclone gas exit diameter (mm) 1700
Cyclone body cylindrical height (mm) 3000
Cyclone total height (mm) 8000
Cyclone solids exit diameter (mm) 1500
Pressure drop (kPa) 2.0
Table 7.6: Cyclone separator parameters 3
Figure 7.11: Lamda(Friction factor) vs Reynolds number
7 . 7 W a t e r s c r u b b e r a n d m i s t e l i m i n a t o r
The use of the water scrubber and the mist eliminator is to further collect the finer particles emit from the outlet of the cyclone separator. The type of the water scrubber used is a spray tower, which is made up of IS 2062. The material of construction of the mist eliminator is stainless steel.
The capacity of the water scrubber is 18,000 m3/hr and the media is water. The other technical specifications are shown in the annexure. The efficiency would be 95% up to 5 microns at the mist eliminator. A sketch of a water scrubber is as shown in the figure 7.12.
The emission standards for the wood dust particles are as follows.
Particle size : 10 micron
Suspended particle matters : 0.5 mg/m3 for 1 hour CO -58 mg/ m3 for any time
Figure 7.12: Spray Tower
8 Ash and dust collection
8 . 1 A s h a n d d u s t c o l l e c t i o n f r o m w a t e r - w a l l b o i l e r
As far as the converted water-wall boiler is concerned, it consumes around 2900 kg/hr of biomass.
3.5% of the biomass is collected as ash. (I.e. is about 102 kg/hr). Out of this 102 kg/hr, 20% of the ash is collected under the water-wall and remaining 80% goes to the cyclone separator. (i.e. 82 kg/hr) . Since the efficiency of the cyclone separator is 90%, 74 kg/hr is collected bottom. Balance 8 kg/hr goes to spray tower. It again collects 95% of the dust as wet solid (7.6 kg/hr). The remain- ing 400 g/hr finer particle (5 microns) is emitted to the atmosphere.
8 . 2 A s h a n d d u s t c o l l e c t i o n f r o m t e x t i l e f a c t o r y b o i l e r
The biomass consumption of the boiler is 150 kg/hr. Ash content is 5.25 kg/hr. Out of that 4.2 kg/hr duct is entered in to the cyclone dust collector. 90% efficient cyclone separator collects about 3.8 kg/hr dust and remaining goes to the water scrubber. The total emission to the atmosphere is 20 g/hr.
