The calorific value, often referred to as the energy content, is the amount of energy released during combustion. Coal, depending on its type (anthracite, bituminous, sub-bituminous, lignite), has a calorific value ranging from 15 to 32 MJ/kg. Oil possesses a calorific value of approximately 46 MJ/kg. Natural gas, on the other hand, boasts the highest calorific value of around 55 MJ/kg. This essentially means that for the same mass of fuel burnt, natural gas releases the most energy, followed by oil, with coal releasing the least among the three.

Yes, there are several technologies aimed at reducing the environmental impact of coal combustion. One of the most prominent is "clean coal technology." This encompasses a range of methods designed to reduce the release of harmful pollutants during coal combustion. Techniques include flue gas desulfurisation, which removes sulfur dioxide from the emissions, and fluidised bed combustion, which results in more efficient burning and reduced nitrogen oxide emissions. Moreover, Carbon Capture and Storage (CCS) technology is being developed to capture carbon dioxide emissions at the source and store them underground, reducing greenhouse gas emissions.

Natural gas extraction, especially from conventional reservoirs, bears similarities to oil drilling; however, there are distinctive features. In most scenarios, natural gas is found above oil reserves and is initially released as a by-product of oil extraction. Once the oil is depleted, specialised equipment is deployed to tap into the natural gas. For unconventional sources like shale gas, hydraulic fracturing or fracking is employed. This involves injecting high-pressure fluid to fracture the rocks and release the gas. The infrastructure, like the wellbore design and the wellhead equipment, differs slightly to handle gas at high pressures and to segregate any accompanying liquids.

Coal is indeed formed from ancient plant matter that has undergone millions of years of heat and pressure transformations. However, its designation as a non-renewable resource stems from the timescale required for its formation. Given that coal takes such an extended period to develop, its rate of extraction and consumption far outpaces its natural replenishment rate. In practical terms, once the current coal reserves are depleted, it would take millions more years for new reserves to form, making it essentially non-renewable for human timescales.

Tar sands, also known as oil sands, are a mixture of sand, clay, water, and a dense form of petroleum called bitumen. They are significant because they represent an alternative source of oil. Traditional oil reserves are dwindling, pushing industries to explore unconventional oil sources. Extracting oil from tar sands is more energy-intensive and environmentally damaging than conventional oil drilling. The process involves strip mining the sands, then heating and treating them to separate the bitumen, which is then upgraded to synthetic crude oil.