Minimum wall thickness or the maximum allowable working pressure of ferrous boiler tubing, up to and including 127 mm O.D., the following formulae, as given in PG-27.2.1, are used:

The symbols in the formulae to be used in this module are found in Paragraph PG-27.3 and are defined as follows. It is extremely important that the correct units be applied when performing the calculations:

t = minimum required thickness (millimeters, mm). (Also see PG-27.4, Note 7)

P = maximum allowable working pressure (megapascals, MPa). (Note - this refers to gauge pressure)

D = outside diameter of cylinder (millimeters, mm)

R = inside radius of cylinder (millimeters, mm)

E = efficiency of longitudinal welded joints or of ligaments between openings, whichever is lower. The values allowed for ‘E’ are listed in PG-27.4, Note 1. This is a factor that has no units, (for example, the value of ‘E’ for seamless cylinders is 1.00)

S = maximum allowable stress value, at the operating temperature of the metal, as listed in the Table PG-23.1, (megapascals, MPa). See PG-27.4, Note 2. The tables are located in an Appendix near the back of the 1983 Code Extract. (For example, the max. allow. working stress for SA-192, at 400°C, is 73 MPa)

C = minimum allowance for threading and structural stability, (millimeters, mm). See PG-27.4, Note 3

e = thickness factor for expanded tube ends (millimeters, mm). See PG-27.4, Note 4

y = a temperature coefficient: This factor has no units and has a value between 0.4 and 0.7. The values allowed for y are listed in PG-27.4, Note 6, (for example, for ferritic steel at 550°C, the value of ‘y’ is 0.7)

Basic Calculations for a Power Plant- Calculating the Coal Quantity

Basic Calculations for a Power Plant- Calculating the Coal Quantity

How to make quick estimate of the coal required for running a power plant?

Note: (A detailed calculation required in the context of a contract, tender, performance report or a legal document may require more accurate input data)

We take the example of a 100 MW

Energy Content in Coal

The basic function of the power plant is to convert energy in coal to electricity. Therefore, the first thing we should know is how much energy there is in coal. Energy content of coal is given in terms of KiloJoules (kJ) per Kilogram (kg) of coal as the Gross calorific value (GCV) or the Higher Heating value (HHV) of coal. This value can vary from 10500 kJ/kg to 25000 kJ/kg depending on the quality and type of the coal.

• You should have an idea of the type of coal, or the source or mine from where the the plant gets the coal. Published data about the sources, mines, regions or the procurement data gives an idea about the HHV of coal. For this example we use a HHV of 20,000 kJ/kg.

Efficiency

Energy conversion takes place in two stages.

• The first part of the conversion is efficiency of the boiler and combustion. For this example we take 88 % on an HHV basis that is the normal range for a well-optimized power plant.

• Second part is the steam cycle efficiency. Modern Rankin cycle, adopted in coal fired power plants, have efficiencies that vary from 32 % to 42 %. This depends mainly on the steam parameters. Higher steam pressure and temperatures in the range of 600 ° C and 230 bar have efficiencies around 42 %. We assume a value of 38 % for our case.

• The overall conversion efficiency then is (38% x 88%) 33.44 %.

Heat Rate

• Heat rate is the heat input required to produce one unit of electricity. (1 kw hr)

• One Kw is 3600 kJ/hr. If the energy conversion is 100 % efficient then to produce one unit of electricity we require 3600 kJ.

• After considering the conversion efficiency in a power plant we require an heat input of (3600 / 33.44% ) 10765 kJ/ kw hr.

Coal Quantity

• Since coal has a heat value of 20,000 kJ/kg, for producing one kw.hr we require (10765 / 20000) 0.538 kg of coal. This translates to (0.538 x 100 x 1,000) 53800 kg/hr (53.8 T/hr) of coal for an output of 100 MW.

Coal Cost

• Basic cost of coal depends on the market conditions. Transportation costs, regional influences and government taxes are also part of the cost. Coal trader’s web sites give base prices in the international market.

• We take a coal price of around 65 $ / Ton.

• The cost of coal consumed by 100 MW power plant is (53.8 x 65) 3497 $ /hr

• A 100 MW unit produces 100,000 units of electricity. So the cost of coal per unit of electricity is (3497/100,000) 3.5 cents per unit

Selection criteria of Power Boiler

1.Cost

2.Efficiency

3.Appropriate Load Response

4.High Turndown/full modulation

5.Maintenance

6.Waterside treatment
Efficiency

•Radiation and Convection Losses 2 - 4%

•Heat Loss in Flue Gases, 18%

•Heat Loss in Blow down, up to 4%

•

•Result

Energy Output [Boiler efficiency] @ 75-77%

•The greatest relative source is the exhaust gases from the process of combustion, which can decrease efficiency by 18% or more. The next area of loss pertains to the heat that is radiated from the boiler, whether operating or in standby mode, which can result in losses up to 4%. Too frequent blow down cycles can cause as much as 3% or more in heat loss

Quick Load Response

•this should not be confused with cold start-up when the boiler has been off line and no pressure exists within the vessel. Frequently a boiler is selected based on how quickly it will cycle on from a cold start, meaning no pressure on the boiler and internal water temperature is at ambient. While low water volume boilers may be brought to steam pressure in a very short period of time, rarely are steam loads a batch process where steam boilers cycle off and cool down before the next steam demand. Changing steam loads require that a boiler follow these fast load changes without an excessive change in steam quality or drop in steam pressure

High Turndown

•the boiler must be equipped with a burner that can consistently achieve modulation from minimum to maximum firing rate with consistent fuel/air ratio as the steam demand changes throughout the daily demand. A high turndown burner will minimize on/off cycling [efficiency loss] and maintain steam flow and quality regardless of system demand. For example, a 100 HP boiler with 10:1 turndown can operate with a minimum output of 10 HP, while still maintaining high efficiency. Some “on/off” boiler manufacturers try to approximate modulating high turndown boilers through “modular” configurations but cannot operate efficiently where variable steam loads occur. Consequently, one should ascertain whether the selected boiler has the ability to operate efficiently at minimum or mid-fire input rate without on/off cycling or excessive excess air.

Fireside/Waterside cleanliness

•Fireside/Waterside cleanliness contributes to effective heat transfer. A design that offers ease of access for inspection and cleaning will contribute to favorable efficiency. Generally, low water volume boilers require intense water treatment, and variables in the treatment program can lead to rapid scaling and a significant decrease in heat transfer efficiency. Likewise, if the fireside is difficult to access, inspect and clean, it will become an area that is too often neglected, which results in a decreased energy transfer.

Maintenance

•proper maintenance includes bottom blow down, surface blow down, water conditioning, control functionality, fuel burning components, water feed equipment and control. Thus, equipment design contributes to this function being easily performed. Is the burner easy to access at a normal work level? Is it easy to open and inspect without having to disassemble part or all of the major components? How frequently must the boiler be subjected to bottom or surface blow down? The greater the frequency of blow down, the greater the heat losses. How easily can a failed/compromised tube be replaced? If a tube replacement cannot be done at a reasonable cost, the entire heat exchanger may have to be replaced instead, which increases the life cycle cost of the boiler.

Water Quality

•While low-water volume, low-mass boilers may produce steam quickly, the water quality must be exceptional to avoid scaling and sludge buildup. For example a 100 Hp boiler with a generous heating surface and water volume is less prone to scaling and corrosion compared to a unit that has half the heating surfaces and a third of the water volume. Each square foot of the heating surface must evaporate 11.5 pounds of steam. If Boiler A has 500 square feet of heating surface and Boiler B has 250 square feet, Boiler B will have a greater occurrence of solids concentration due to its smaller heating surface. A boiler with a small heating surface requires more frequent blow down to maintain proper cycles of concentration and increased chemical make-up.

Project Development steps

1 -Conduct preliminary feasibility study (Fatal Flaw Analysis)

2 -Confirm community support

3 -Assess fuel resource availability

4 -Consider siting and infrastructure issues, including environmental permit review

5 -Complete due diligence Feasibility Study

6 -Secure developer and/or investment banker

7 -Select EPC firm

8-Secure financing

9 -Engineer/construct project

10 -Generate renewable energy

Thermo dynamic stand point for Co-Generation

From the thermo dynamic stand point, energy recovery from the Rankin cycle is more dependent on the temperature than the pressure. Hence, higher the steam temperature , higher the cycle efficiency. However, the limits on temperature are influenced by the metallurgy of the boiler tubing, piping and the turbine components. Carbon steel can be used up to a working metal temperature of 400°C. Beyond this temperature, alloy steels are extensively used. The characteristics of the fuel used in the boiler (e.g. : bagasse) also can pose limitations on the choice of steam temperature. The following are the steam parameters that are widely used in a process industry.

•Pressure (kg/cm2(a)) Temperature (0C)

32 380

42 440

67 485

84 520

110 540

My Profile

Engineer Khurram Masood

Professional Review

More than 4 years’ experience in Conceptual Studies, Basic Engineering/Front End Design Engineering, Designing, Operation and Fabrication of Steam Boilers, Pressure vessels, Storage Tanks, Separators, Design-Erection of Biomass Fired Power Plants

· Design & Detailed Engineering, Greenfield as well as Brownfield Experience

· Solid modeling and product design by using software like PRO-E, SOLID WORKS,3D STUDIO MAX

· Preparation of Mechanical design data sheet for Boiler, Pressure vessels, Heat exchangers, And Storage tanks

· Preparation of Technical Specifications, MR and PR for Boiler, Pressure Vessels, Heat exchangers & Storage tanks

· Design & Detailed Engineering of Boiler & Pressure vessel with “U” and “S” Stamp Requirement

· Performing Strength calculations & Analysis of Pressure vessels Heat exchangers & Storage Tanks both manually and by PV-Elite, Tank3.2 software

· Performing Thermal Design Calculation, Manual Strength Calculation By Using ASME, PD5500, API 650,620

· Reviewing vendor’s Drawing/Documents and preparing Technical Queries

· Preparation of General arrangement drawings and detailed fabrication drawings

· Reviewing and Approval of Pressure vessel & Tank Internals & Externals attachment drawings

· Co-ordinate with Process/Project System/Piping/Civil/Instrumentation/Electrical in Finalizing

· Conceptual Studies of ASME, API, TEMA, BS 5500, EN13445 & client reference standard to fulfill Client & code requirements (U, S-Stamp)

· Field of Experience: Mechanical Design & Detailing of Boiler (ASME SEC I), Pressure Vessel (ASME SEC III) Div-1,2),Tanks(API 650,620), Heat Exchanger (ASME), with Software like PV-Elite, Tank 3.2, KED: Process Plant/Boiler Thermal Design Software