What type of fuel consistently produces the least emissions when burned in a boiler?

Natural gas is known for producing the least emissions when burned in a boiler compared to other fuel types. This is primarily due to its composition; natural gas primarily consists of methane (CH4), which, when combusted, produces carbon dioxide (CO2) and water (H2O) as byproducts. The combustion of natural gas results in lower levels of harmful pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter, which are more prevalent in fuels like coal, heavy oil, and even dung.

Coal, for example, is a carbon-intensive fuel that emits higher levels of CO2 and other pollutants, including mercury, when burned. Heavy oil has similar issues, with additional risks of higher sulfur emissions contributing to acid rain. Dung, while it can be a renewable resource, typically results in higher particulate emissions and variable combustion quality, leading to inconsistent and less efficient burning compared to natural gas.

Thus, the efficiency of combustion and the resulting emissions profile make natural gas the cleanest option among the given fuels for boiler use.