Burning agricultural waste: the "black cloud"

• At the end of the harvest (usually October–November) some farmers burn rice straw to clear the fields quickly.
• Piled straw receives too little oxygen, so combustion is mainly incomplete, releasing carbon monoxide, carbon dioxide and soot (fine carbon particles).
• These form dense smoke called the black cloud, which reduces visibility, causes breathing difficulties and increases asthma and chest allergies.
• The alternative is to use the straw for compost, animal feed, paper or biofuel and not burn it.

• Burning petrol or diesel in the engine gives the heat that becomes motion.
• The exhaust contains carbon monoxide, carbon dioxide, soot, nitrogen oxides, and (if the fuel has sulphur) sulphur dioxide.
• It is a main cause of urban air pollution.
• When too little air enters the engine, or the engine is inefficient, burning is incomplete and the harmful emissions rise.
• Reducing exhaust pollution:

1. Regular engine maintenance to keep efficiency high.
2. Cleaner fuels such as natural gas, or electric cars.
3. Public transport that does not harm the environment.

• Metal smelting, ceramics, cement, glass, oil distillation and many power stations burn coal, oil or natural gas.
• Their chimneys can emit large amounts of pollutants.
• Reduction measures:

1. Use natural gas, which causes less pollution.
2. Fit filters to clean the gases before leaving chimneys.
3. Use renewable energy (solar and wind) for electricity.

• Why is natural gas preferred to coal? Burning natural gas gives less carbon dioxide for the same heat, and very little sulphur dioxide and soot.
• Coal gives much more carbon dioxide, more sulphur oxides and more particles.
• Technology: acid-gas scrubber. A scrubber is placed in factory chimneys. The polluted gases pass through a spray of an alkaline solution. Acidic gases such as sulphur dioxide are neutralised (for example SO₂ + Ca(OH)₂ → CaSO₃ + H₂O) and form harmless solid salts, reducing the chance of acid rain and smog.
• Acid rain

• forms when sulphur dioxide and nitrogen oxides dissolve in rain water to give acids.
• It damages buildings, harms plants and kills life in lakes.
• Cross-cutting concept cause and effect. More combustion processes (cause) lead to emissions that disturb the ecological balance (effect).
• Be an environmental engineer.
• Choose one problem (the black cloud, car exhaust, or burning waste) and propose a practical solution: a filter that converts harmful gases, a recycling scheme for straw, a better maintained engine.