Air pollution has always been a serious environmental concern. The combustion of fuels, primarily in power plant utility boilers, provides the bulk of earth’s energy needs. Despite releasing useful heat to generate energy, there are unwanted byproducts emanating from the process, which are the stack gases. Normally, the unwanted byproducts, also known as pollutants, are either in gaseous or particulate in form. Gaseous pollutants are sulfur dioxide, various oxides of nitrogen (NOx), and carbon monoxide. Particulate matters can be made of unburned hydrocarbons, soot, and particles. Bad air quality is also measured in term of sulfur dioxide, nitrogen oxide, and particulate matters.
From the standpoint of steam boiler operation, the air pollution control is primarily motivated by the legal requirement as covered by ordinances and statutory regulations. For example, the permissible black smoke limits are Ringlemann Chart No. 2 for solid fuel and Ringlemann Chart No. 1 for other fuel boilers, while the particulate matters concentration in flue gas emission is limited to 0.4 g/Nm3 based on the isokinetic stack sampling. For black smoke emission, under the ordinances, the allowable period to exceed both standards is 5 minutes in any one hour or 15 minutes in any 24 hours.
Most sulfur dioxide is formed from sulfur in the fuel, therefore, solid fuel especially coal generally produces higher levels of sulfur dioxide than oil. Natural gas gives the lowest level for comparable furnace condition. Sulfur dioxide is the most damaging air pollutant produced by boiler combustion because it is the leading source of acid rain. Based on legislation, the permitted ambient concentration for sulfur dioxide is 0.03 ppm on annual average. Sulfur dioxide reacts with moisture in the air to form sulfuric acid, which is the source of acid rain. Acid rain can be exceedingly detrimental to flora and fauna. Acid rain affects the aquatic lives due to the acidification of lakes and rivers, harms the ecosystems, harms humans by increasing toxicity in air, and hampers the growth of crops and vegetation by increasing toxicity in soil. Acid rain also decays building materials and paints. Sulfur dioxide aggravates asthma, coughs, bronchitis, and chest illnesses.
There are two main mitigation controls to reduce sulfur dioxide in flue gas emission, which are pre-combustion reduction or removal at source and post-combustion reduction or removal from the flue gas. Pre-combustion reduction includes using low-sulfur fuel such as natural gas, which can reduce sulfur to near zero, and desulfurization. Nowadays, many power plants have desulfurization plant, or De-SOx, to clean the sulfur from coal before combustion. De-SOx plants, also known as scrubbers, use limestone to absorb sulfur gas, reducing the sulfur content of the pre-combusted coal by 80%, which is from 1.78% to 0.4% in high-sulfur coal. Post-combustion approach includes wet or dry scrubbing, which contacts sorbent limestone slurry with sulfur dioxide in flue gas in a reactor, producing synthetic gypsum which is a component of drywall.
Although nitrogen in the combustion air is essentially inert, at high temperature for a finite period of time it combines with oxygen to form NOx (various oxides of nitrogen). The permitted ambient concentration for NOx is 0.053 ppm on annual average. Nitrogen oxide helps in formation of acid rain however it is the principal source of photochemical smog. It also depletes the ozone layer and may contribute to the global warming. For human health, nitrogen oxide can cause coughs, sore throats, and cardiovascular illnesses.
There are many types of controls to minimize NOx, which include adjustment of excess air and flame pattern in furnace, staged combustion to delay combustion and reduce heat flux, flue gas recirculation, reburning, and using low NOx burners. Low NOx burners attempt to decrease both the peak temperatures and the time nitrogen spends at higher temperatures, thereby reducing NOx emissions by 50%. The fluidized-bed coal boiler, characterized by low-temperature combustion has become a popular solution in reducing sulfur dioxide and NOx emission to the atmosphere. It also has in-furnace desulfurization because the limestone used as the bed material can absorb more than 90% of the sulfur released during the combustion process.
The unburned fuel, in the form of soot or carbon dust, or in the form of carbon monoxide, also contributes significant potential pollutants. The unburned fuel products are more prevalent with coal and heavy oils than with light oil and natural gas. Carbon monoxide formation is dependent on temperature and excess air. Fuel rich mixtures increase carbon monoxide level due to insufficient oxygen for complete combustion. Therefore, regulating excess air is crucial to prevent poor combustion and unburned fuel. Normally, the permitted ambient concentration for carbon monoxide is 9 ppm on eight-hour average. Carbon monoxide is highly poisonous gas. Carbon monoxide absorbed by the lungs impairs physical and mental capacities by reducing the absorption of oxygen by hemoglobin affecting the nervous system. To achieve the ambient standard for particulate matters, soot and carbon dust are removed from the exhaust gas by mechanical means such as filter bags, wet scrubbers, and multicyclones.
The emission of sulfur dioxide and nitrogen dioxide shall be an ongoing global issue to be addressed in boiler emission since coal is widely used in power plant utility boilers for now and years to come as long as it is cheap and plentiful.