Chapter 12 ̶ Environmental Issues

Chapter 12  ̶  Environmental Issues

12.1 Important Environment Issues

i.                    Production of Greenhouse Gases like CO₂, CH₄, chlorofluoro carbon (CFC) and nitrous oxide.

ii.                  Greenhouse gases absorb infrared radiation of sun and prevent the heat reflection into space from earth surface. Thus these gases are responsible for temperature rise of environment.

iii.                Global warming due to green house effect.

iv.                Acid rain

v.                  Pollution

12.2 Components of Environment

12.2.1.  Producers – Plants, which produce the food and liberate oxygen.

12.2.2.  Consumers- Animals

12.2.3.  Decomposers-Micro-organisms (bacteria and fungi) decompose waste material, dead plants and animals and produce CO₂ which are again utilised for making food. Hence play an important role in material re-cycling in eco-system.

12.3 Biogeochemical Cycles

12.3.1 Water cycle

12.3.2 Carbon Cycle

12.3.3 Oxygen Cycle

12.3.4 Nitrogen Cycle

12.4 Carbon Capture Technology

12.4.1 Steps of Carbon Capture and Storage

          Carbon capture and storage (CCS) involves the separation and capture of CO₂ from flue gas, or syngas in the case of  IGCC (integrated gasification combined cycle). CCS is a three-step process that includes:

1. Capture of CO₂ from electric generating units (or other industrial processes);

2. Compression and transport of the captured CO₂ (usually in pipelines);

3. Underground injection and geologic sequestration (also referred to as storage) of the CO₂ into deep underground rock formations. These formations are often a mile or more beneath the surface and consist of porous rock that holds the CO₂. Overlying these formations are impermeable, non-porous layers of rock that trap the CO2 and prevent it from migrating upward.

12.4.2  Methods of CO₂ Capture Technology

In general, CO₂ capture technologies applicable to fossil-fuel fired power generation can be categorized into three approaches:

i.   Post-combustion systems are designed to separate CO₂ from the flue gas produced by fossil-fuel combustion in air.

ii. Pre-combustion systems are designed to separate CO₂ and H₂ in the high-pressure syngas produced at IGCC power plants.

iii.  Oxy-combustion uses high-purity oxygen (O₂), rather than air, to combust coal and therefore produces a highly concentrated CO₂ stream.

The post- and pre-combustion CO₂-capture processes typically use solvents, solid sorbents, and membrane-based technologies for separating and capturing CO₂. Solvents chemically absorb the CO₂ which is separated from the solvent in a regeneration step. Solid sorbents capture CO₂ through chemical adsorption, physical adsorption, or a combination of the two effects. Membrane-based capture uses permeable or semi-permeable materials to produce a highly concentrated CO₂ stream that does not require a separation/capture step.