Unit 4 | Engineering Chemistry Notes | AKTU Notes

UNIT 4: Water Technology and Fuels & Combustion

PART A: WATER TECHNOLOGY

4.1 Sources and Impurities of Water

Water is one of the most essential resources in industry and daily life. The quality of water varies greatly depending on its source.

Sources of Water:

• Rain water: Purest natural water. Contains dissolved gases (O₂, CO₂, N₂) and dust particles. No dissolved salts (naturally soft).
• Surface water: Rivers, lakes, ponds. Contains silt, clay, organic matter, bacteria, dissolved salts. Turbid (muddy).
• Ground water: Wells, springs, boreholes. Filtered by soil — low turbidity. But contains dissolved minerals (calcium, magnesium salts — hard water). May contain iron, manganese.
• Sea water: Contains very high levels of dissolved salts (NaCl, MgCl₂, etc.) — about 3.5% salinity. Not suitable for drinking or most industrial uses without treatment.

Impurities in Water:

Type of Impurity	Examples	Effect
Dissolved salts (inorganic)	CaCO₃, MgCl₂, NaCl, FeSO₄	Hardness, scale formation, corrosion
Dissolved gases	CO₂, O₂, H₂S	Corrosion, taste problems
Suspended impurities	Silt, clay, organic matter	Turbidity, blocks equipment
Colloidal impurities	Colloidal silica, clay particles	Turbidity, hard to remove
Biological impurities	Bacteria, algae, viruses	Disease, biological fouling
Dissolved organic matter	Humic acids, pesticides	Color, taste, toxicity

4.2 Hardness of Water

Hardness of water is its inability to produce lather (foam) easily with soap.

• Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts.
• These ions react with soap (sodium stearate) to form insoluble curdy precipitates (scum) instead of lather.
• Reaction: Ca²⁺ + 2(C₁₇H₃₅COO⁻Na⁺) → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺

Types of Hardness:

1. Temporary Hardness:

• Caused by dissolved bicarbonates: Ca(HCO₃)₂ and Mg(HCO₃)₂.
• Called temporary because it can be removed simply by boiling.
• On boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂

2. Permanent Hardness:

• Caused by dissolved chlorides and sulfates: CaCl₂, MgCl₂, CaSO₄, MgSO₄.
• Cannot be removed by boiling — needs chemical treatment.

Total Hardness = Temporary Hardness + Permanent Hardness

Units of Hardness:

• Hardness is expressed as equivalent amount of CaCO₃ (molecular weight 100).
• Units: ppm (mg/L) or degrees (°French, °German, °Clark).
• 1 ppm hardness = 1 mg of CaCO₃ per liter of water.
• Soft water: 0–75 ppm, Moderately hard: 75–150 ppm, Hard: 150–300 ppm, Very hard: >300 ppm.

4.3 Boiler Troubles

When hard water is used in boilers (industrial steam generators), it causes several serious problems:

1. Scale and Sludge Formation:

• Scale: Hard, adherent deposits on the inner walls of the boiler. Formed by calcium and magnesium salts (CaCO₃, CaSO₄, Mg(OH)₂) that precipitate on heating.
• Sludge: Soft, loose, non-adherent precipitate at the bottom of the boiler. Less harmful than scale but still a problem.
• Harmful effects of scale: Poor heat transfer (scale is a poor conductor) → fuel waste → overheating of boiler walls → boiler can explode.

2. Priming and Foaming:

• Priming: Carrying over of water droplets along with steam. Caused by high dissolved salts, high steam velocity.
• Foaming: Formation of persistent foam/bubbles on the surface of boiler water. Caused by dissolved organic matter, oils, alkalis.
• Both cause wet steam → less efficient → damages turbine blades.

3. Caustic Embrittlement:

• Boiler steel becomes brittle in regions exposed to highly concentrated alkali (NaOH).
• Occurs at riveted joints, crevices where water evaporates and NaOH concentrates.
• Dangerous — can cause sudden failure of boiler.
• Prevention: Add sodium sulfate or tannin to boiler water to prevent concentration of NaOH.

4. Corrosion in Boilers:

• Dissolved O₂ causes pitting corrosion.
• CO₂ dissolves in condensed steam → carbonic acid → corrosion of pipelines.
• Prevention: Remove dissolved gases by deaeration (heating and degassing of feed water).

4.4 Techniques for Water Softening

Water softening is the process of removing Ca²⁺ and Mg²⁺ ions from hard water.

4.4.1 Lime-Soda Process

• Adding calculated amounts of lime (Ca(OH)₂) and soda (Na₂CO₃) to hard water.
• Calcium and magnesium salts are precipitated as insoluble CaCO₃ and Mg(OH)₂.

Reactions:

• Temporary hardness (Ca²⁺): Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O
• Temporary hardness (Mg²⁺): Mg(HCO₃)₂ + 2Ca(OH)₂ → 2CaCO₃↓ + Mg(OH)₂↓ + 2H₂O
• Permanent hardness (Ca²⁺): CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄
• Permanent hardness (Mg²⁺): MgSO₄ + Ca(OH)₂ → Mg(OH)₂↓ + CaSO₄ (then CaSO₄ removed by Na₂CO₃)

• Precipitates are removed by sedimentation and filtration.
• Advantage: Cheap, can treat large volumes.
• Disadvantage: Not very precise, produces sludge, slight hardness remains.

4.4.2 Zeolite Process (Permutit Process)

• Zeolites are hydrated sodium aluminosilicates (Na₂O·Al₂O₃·xSiO₂·yH₂O) — natural or synthetic ion exchangers.
• Zeolites exchange their Na⁺ ions for Ca²⁺ and Mg²⁺ ions in hard water.
• Reactions: Na₂Ze + Ca²⁺ → CaZe + 2Na⁺ (Ca²⁺ removed, Na⁺ added)
• Na₂Ze + Mg²⁺ → MgZe + 2Na⁺ (Mg²⁺ removed, Na⁺ added)
• Water coming out is soft (contains NaCl instead of CaCl₂ or MgCl₂).
• Regeneration: When zeolite is exhausted (all Na replaced by Ca/Mg), treat with 10% NaCl solution: CaZe + 2NaCl → Na₂Ze + CaCl₂. Zeolite is regenerated and can be reused.
• Advantages: Gives almost completely soft water, simple operation, automatic.
• Disadvantages: Cannot remove suspended impurities, turbidity, or dissolved salts other than Ca/Mg. Cannot treat acidic water.

4.4.3 Ion Exchange Process

• Most complete method — removes all dissolved salts (not just Ca/Mg) → produces demineralized water (deionized water).
• Two columns are used:
• Cation exchanger (H⁺ form): Contains strong acid resin (R-H). Exchanges H⁺ for all cations (Ca²⁺, Mg²⁺, Na⁺, Fe²⁺): R-H + Ca²⁺ → R-Ca + 2H⁺
• Anion exchanger (OH⁻ form): Contains basic resin (R-OH). Exchanges OH⁻ for all anions (SO₄²⁻, Cl⁻, HCO₃⁻): R-OH + Cl⁻ → R-Cl + OH⁻
• The H⁺ and OH⁻ released combine to form water: H⁺ + OH⁻ → H₂O
• Water coming out is completely demineralized — useful for laboratories, power plants, pharmaceuticals.
• Regeneration: Cation resin regenerated with dilute HCl or H₂SO₄. Anion resin regenerated with dilute NaOH.

4.4.4 Reverse Osmosis (RO) Process

• Osmosis: Natural flow of water from less concentrated solution to more concentrated solution through a semipermeable membrane.
• Reverse Osmosis: Applying pressure greater than osmotic pressure to force water to flow from concentrated (impure) side to dilute (pure) side — opposite to natural direction.
• The semipermeable membrane allows only water molecules to pass — blocks dissolved salts, bacteria, viruses.
• Result: Pure water on one side, concentrated brine on the other side.
• Applications: Drinking water purification, seawater desalination, industrial demineralization.
• Advantages: Removes all types of impurities — salts, bacteria, viruses, heavy metals. No chemicals needed.
• Disadvantages: High energy consumption (high pressure needed), generates concentrated waste brine, membranes can foul.

4.5 Determination of Hardness of Water

EDTA (Complexometric) Titration Method:

• EDTA (ethylenediaminetetraacetic acid) forms stable complexes with Ca²⁺ and Mg²⁺ ions.
• Indicator: Eriochrome Black T (EBT) — wine red with Ca/Mg ions, blue when no Ca/Mg ions remain.
• Buffer: NH₃/NH₄Cl buffer (pH 10) is used.
• Procedure: Take known volume of water → add buffer and EBT indicator → solution turns wine red → titrate with standard EDTA solution until color changes from wine red to blue → note EDTA volume used.
• Calculation: Total hardness (ppm as CaCO₃) = (Volume of EDTA × Molarity of EDTA × 100000) / Volume of water sample.

Determination of Alkalinity:

• Alkalinity is due to OH⁻, CO₃²⁻, and HCO₃⁻ ions in water.
• Determined by double indicator titration method using phenolphthalein and methyl orange indicators with standard H₂SO₄.

PART B: FUELS AND COMBUSTION

4.6 Definition and Classification of Fuels

Fuel is a substance that undergoes combustion (burning) in air/oxygen to produce heat and light energy.

Classification of Fuels:

Based on physical state:

• Solid fuels: Coal, wood, charcoal, coke, bagasse.
• Liquid fuels: Petrol (gasoline), diesel, kerosene, fuel oil, alcohol.
• Gaseous fuels: Natural gas (CNG), LPG, biogas, hydrogen, producer gas, water gas.

Based on origin:

• Natural fuels: Coal, petroleum, natural gas, wood.
• Artificial (manufactured) fuels: Coke, charcoal, petrol, kerosene, LPG, biogas.

Based on calorific value:

• High calorific value fuels: Natural gas, petrol (~47 MJ/kg).
• Low calorific value fuels: Wood, straw (~15 MJ/kg).

4.7 Characteristics of a Good Fuel

• High calorific value: Should produce maximum heat per unit mass or volume.
• Low cost: Should be economically cheap and abundantly available.
• Easy to transport and store: Should be safe and convenient to handle.
• Moderate ignition temperature: Should not ignite too easily (fire hazard) or too with difficulty.
• Low moisture content: Moisture reduces calorific value.
• Low ash content: Ash is waste — blocks equipment, causes pollution.
• Low sulfur content: Sulfur combustion produces SO₂ — causes acid rain and air pollution.
• Clean burning: Should produce minimum smoke, soot, and harmful gases.
• Controllable burning rate: Rate of combustion should be easily controllable.

4.8 Calorific Values — Gross and Net

Calorific Value (CV) is the amount of heat energy produced when unit mass (or volume) of fuel is completely burned in oxygen.

Gross Calorific Value (GCV) or Higher Heating Value (HHV):

• Total heat produced when unit mass of fuel is burned completely AND the products are cooled back to the original temperature (25°C).
• The water produced during combustion condenses to liquid → latent heat of condensation is also included.
• GCV is always higher than NCV.

Net Calorific Value (NCV) or Lower Heating Value (LHV):

• Heat produced when fuel burns completely but the water in the products remains as steam (not condensed).
• NCV = GCV − Latent heat of steam produced during combustion.
• Formula: NCV = GCV − 9H × 587 (kcal/kg)
• Where H = mass fraction of hydrogen in fuel, 9H = mass of water produced from H per kg of fuel, 587 kcal/kg = latent heat of steam at room temperature.
• NCV is practically more useful because in most engines, exhaust gases leave hot (steam does not condense).

4.9 Determination of Calorific Value by Bomb Calorimeter

The Bomb Calorimeter is used to measure the Gross Calorific Value (GCV) of solid and liquid fuels.

Construction:

• Bomb: A thick-walled stainless steel vessel (can withstand high pressure). Has an oxygen inlet valve and electrodes for ignition wire.
• Calorimeter vessel: Contains a known mass of water in which the bomb is placed.
• Jacket: Outer water jacket to prevent heat exchange with surroundings (adiabatic condition).
• Thermometer: Precise Beckmann thermometer to measure temperature rise.
• Stirrer: For uniform temperature in calorimeter water.

Procedure:

• Known mass of fuel (about 1g) is placed in the crucible inside the bomb.
• Bomb is filled with oxygen at high pressure (25–30 atm).
• Bomb is placed in calorimeter vessel with known mass of water.
• Fuel is ignited electrically (through the ignition wire).
• Fuel burns completely in oxygen → heat is released → raises temperature of water in calorimeter.
• Maximum temperature rise (ΔT) is noted.

Calculation:

• GCV = (W + w) × ΔT / m
• Where W = water equivalent of calorimeter (in kcal/°C), w = mass of water (g), ΔT = temperature rise (°C), m = mass of fuel (g).

4.10 Theoretical Calculation of Calorific Value by Dulong's Method

If the ultimate analysis (elemental composition) of fuel is known, we can calculate calorific value theoretically using Dulong's Formula:

Dulong's Formula:

• GCV = 1/100 × [8080C + 34500(H − O/8) + 2240S] kcal/kg
• Where C = % of carbon in fuel, H = % of hydrogen, O = % of oxygen, S = % of sulfur.
• (H − O/8): The oxygen in the fuel is already combined with H in the form of water → it does not contribute to combustion. So we subtract O/8 from H. This is called available hydrogen.

Heat contributions:

• 1 kg of Carbon produces 8080 kcal.
• 1 kg of Hydrogen produces 34500 kcal.
• 1 kg of Sulfur produces 2240 kcal.

4.11 Ranking of Coal

Coal is classified into different ranks based on its degree of coalification (how much carbon it contains and how much it has been transformed from original plant material).

Rank	Carbon Content	Calorific Value	Moisture	Features
Peat	~60%	Lowest (~3500 kcal/kg)	Very high	Youngest coal, brown, wet
Lignite (Brown coal)	60–70%	~4000–5000 kcal/kg	High	Soft, brown, used in power plants
Sub-bituminous	70–77%	~5000–6500 kcal/kg	Moderate	Black, low sulfur
Bituminous	77–87%	~6500–8000 kcal/kg	Low	Most common, used in power plants and metallurgy
Anthracite	87–98%	Highest (~8500 kcal/kg)	Very low	Hardest, shiniest, cleanest burning coal

4.12 Analysis of Coal

1. Proximate Analysis:

• Determines four parameters: Moisture, Volatile Matter, Ash, and Fixed Carbon.
• Simple and quick method — gives overall quality assessment.

• Moisture content: Coal sample heated at 105–110°C for 1 hour. Loss in weight = moisture. High moisture → low calorific value.
• Volatile matter: Coal heated at 950°C in absence of air for 7 minutes. Weight loss (after subtracting moisture) = volatile matter. High volatile matter → more smoke, easier ignition.
• Ash content: Coal burned completely in air at 700–750°C. Residue remaining = ash. High ash → less calorific value, more waste.
• Fixed carbon: Fixed carbon (%) = 100 − (Moisture% + Volatile matter% + Ash%). Fixed carbon → actual carbon available for burning → most important for calorific value.

2. Ultimate Analysis (Elemental Analysis):

• Determines % of C, H, O, N, S, and Ash in coal.
• More detailed and more useful for theoretical calculations.
• Used in Dulong's formula to calculate calorific value.
• Helps in calculating air required for combustion.

4.13 Chemistry of Biogas Production

Biogas is a mixture of gases (mainly methane and CO₂) produced by the anaerobic digestion (decomposition without oxygen) of organic waste materials.

Composition of Biogas:

• Methane (CH₄): 55–70%
• Carbon dioxide (CO₂): 30–40%
• Small amounts of H₂S, H₂, N₂.
• Calorific value: ~5000–6000 kcal/m³.

Raw Materials for Biogas:

• Cow dung (gobar), buffalo dung.
• Agricultural waste (crop residues, straw).
• Municipal solid waste and sewage sludge.
• Kitchen waste, food scraps.
• Water hyacinth, algae.

Stages of Anaerobic Digestion:

• Stage 1 — Hydrolysis: Complex organic molecules (proteins, carbohydrates, fats) are broken down into simpler soluble molecules (sugars, amino acids, fatty acids) by hydrolytic bacteria.
• Stage 2 — Acidogenesis: Acidogenic bacteria ferment the simple molecules into organic acids (acetic, propionic, butyric acids), alcohols, H₂, and CO₂.
• Stage 3 — Acetogenesis: Organic acids converted to acetic acid (CH₃COOH), H₂, and CO₂.
• Stage 4 — Methanogenesis: Methanogenic bacteria (Methanobacterium) convert acetic acid and H₂+CO₂ into methane.
• CH₃COOH → CH₄ + CO₂
• CO₂ + 4H₂ → CH₄ + 2H₂O

Environmental Impact of Biogas:

• Positive: Renewable energy source, reduces dependence on fossil fuels. Reduces methane emission from open dumps (methane is a potent greenhouse gas — captured and used as fuel instead). Slurry left after digestion is excellent fertilizer (biofertilizer). Reduces open burning of crop residues.
• Negative: If not managed properly, leakage of methane contributes to greenhouse effect. H₂S in biogas is corrosive and toxic.