A gas turbine compresses air and mixes it with heated fuel. The hot air-fuel mixture moves through the gas turbine blades, making them spin. The fast-spinning turbine drives a generator that converts the spinning energy into electricity. A Heat Recovery Steam Generator (HRSG) captures exhaust heat from the gas turbine, creates steam and delivers it to the steam turbine. The steam turbine sends its energy to the generator drive shaft, where it is converted into additional electricity.
Sounds straight forward, but the fuel used to operate these turbines can impact the safety of these turbines and the protection of the environment.
One concern involves the levels of sulfur contained in the fuel gas supply and its impact on corrosion in the low-temperature section of the waste-heat recovery unit (HRSG) and the downstream stack. This is caused by the formation and condensation of sulfur acid. The acid is formed in the combustion process from the sulfur introduced into the system with the fuel.
The hot exhaust gases pass through several steam drums in the HRSG and release heat to produce superheated steam. If the flue gas at the latter stages, notably the evaporator, the economizer and the stack, are below its acid dew point, the gaseous acid will start to condensate and cause serious corrosion problems for the equipment.
There are also environmental concerns due to the discharge of acid gas. Sulfur combusts producing mainly sulfur dioxide emission to the atmosphere, which react in the presence of moisture resulting in acid rain.
The US EPA’s national ambient air quality standards for SO2 are designed to protect against exposure to the entire group of sulfur oxides (SOx). According to the EPA website:
Sulfur dioxide (SO2) is one of a group of highly reactive gases known as “oxides of sulfur, and are emitted into the air as result of fossil fuel combustion and other industrial processes …
Short-term exposures to SO2 can harm the human respiratory system and make breathing difficult. People with asthma, particularly children, are sensitive to these effects of SO2….
SO2 emissions that lead to high concentrations of SO2 in the air generally also lead to the formation of other sulfur oxides (SOx). SOx can react with other compounds in the atmosphere to form small particles. These particles contribute to particulate matter (PM) pollution. Small particles may penetrate deeply into the lungs and in sufficient quantity can contribute to health problems.
At high concentrations, gaseous SOx can harm trees and plants by damaging foliage and decreasing growth.
SO2 and other sulfur oxides can contribute to acid rain which can harm sensitive ecosystems.
As a result of these concerns, gas turbine operators provide specifications covering the fuel quality permitted for use in a gas turbine. However, compositions of gaseous fuels (i.e. natural gas, LNG, LPG etc.) can vary quite widely depending on their sources.
To address these issues, on-line sulfur analyzers are used extensively in many oil and gas related industries for determination of total sulfur in many types of liquid and gas hydrocarbon samples.
These sulfur analyzers utilize Pulsed UV Fluorescence technology to provide a continuous measurement of total sulfur with very fast response times to changing sulfur concentrations and excellent linearity and precision specifications. All sulfur species are converted into SO2 by combustion in a process that is consistent with ASTM and ISO standards for total sulfur determination in the lab and online.
With an on-line total sulfur analyzer, the plant users can determine the actual fuel sulfur level received. With the verified gas fuel sulfur concentrations, the operators can further calculate the acid dew points of the fuel gases and specify appropriate design for the equipment. For example, the stack exit temperatures on the waste-heat recovery unit controlled to stay higher, and boiler feedwater temperatures elevated to prevent acid gas condensation within the chimney or the economizer, so as to prevent metal corrosion and reduce maintenance and repair costs.
The understanding of the contaminants in the supplied fuels used in a gas turbine is critical to achieve the high energy efficiency of the overall system, minimize maintenance requirements and prolong component lifetime.
And it could be critical to our environment.
For more detailed information, read On-line monitoring of total sulfur at combined cycle gas power plants