Natural gas power generation

The main component of natural gas is methane, and its combustion products are water and carbon dioxide. It does not produce ash, does not emit toxic gases, and does not have heavy metal elements harmful to the human body. The volatile hydrocarbons, sulfides, carbon dioxide and nitrogen oxides produced by it are far lower than coal and petroleum, and it is easy to use, and it is a high-quality, high-efficiency and clean energy.

In 1791, the British Barber described the working process of a gas turbine for the first time. In 1920, the German Watt made the first practical gas turbine. In 1939, a 4MW gas turbine for power generation was made in Switzerland. From then on, the gas turbine entered the practical stage and began to develop rapidly.

The process of natural gas power generation is the process of converting the chemical energy of natural gas into heat energy and then into electric energy through a gas generator set. The basic principles are as follows:

According to the characteristics of thermal cycle, gas turbines can be divided into two categories: simple cycle and gas-steam combined cycle.

The cycle system of the simple cycle gas generator set consists of a gas turbine and a generator. The high-temperature exhaust gas after combustion is directly discharged into the atmosphere without any use. Its working principle is shown in Figure 1.

Natural gas power generation
Figure 1 – Schematic diagram of simple cycle power generation process

The simple cycle gas turbine has a simple thermal cycle and is currently the most commonly used gas turbine. When working, the compressor first compresses the air to a certain pressure, and then adds fuel to the combustion chamber to burn high-temperature gas, and then expands through the turbine to do work. The exhaust gas after combustion is directly discharged into the atmosphere.

Because the simple cycle exhaust temperature is still very high (450 ~ 650 ℃), it is directly discharged into the atmosphere, causing a high heat loss. The gas-steam combined cycle is equipped with a waste heat boiler in the system, The high-temperature exhaust gas produced by the steam turbine is used to release heat in the waste heat boiler, and the feed water in the waste heat boiler is turned into steam, which is then sent to the steam turbine to perform work, thereby forming a cycle that combines the gas cycle and the steam cycle.

Its working principle is shown in Figure 2.

Natural gas power generation
Figure 2 – Schematic diagram of gas-steam combined cycle power generation process

According to whether the high-temperature gas discharged from the gas turbine has post-combustion, the gas-steam cycle can be divided into two categories: pure waste heat boiler type combined cycle and exhaust gas post-combustion type combined cycle.

1. Pure waste heat boiler type combined cycle

In this type of cycle, the exhaust gas from the gas turbine is directly passed to the waste heat boiler without any treatment on the way, and the waste heat is used to heat water into steam.

The combined cycle unit of the pure waste heat boiler type can be equipped with a gas turbine, a waste heat boiler and a steam turbine to form a “one tow one” method, or it can be used for multiple gas turbines, waste heat boilers and a steam turbine, “multiple tow one” approach.

In the “one-to-one” approach, if the generator, steam turbine, and gas turbine are connected to the same shaft, this type of unit is called a single-shaft unit, as shown in Figure 2.

In the “multiple tow one” approach, multi-shaft combined cycle units are often used. Take the “two to one” unit as an example, each of the two gas-fired units is equipped with a generator. After the steam from the two waste heat boiler outlets is merged into the main pipe, it is sent to a common steam turbine to perform work, which drives the other generator to generate electricity, and the gas turbines and steam turbines are in different shafting systems.

2. Exhaust supplementary combustion combined cycle

Since the exhaust gas of the gas turbine contains excess air, supplementary fuel can be added to the pipe or container in front of the waste heat boiler. Supplementary combustion can ensure that the temperature of the gas entering the waste heat boiler remains basically unchanged under variable operating conditions. Adjusting the amount of supplementary combustion can adjust the power of the unit and improve the efficiency of the unit under partial load. But at present, the combined cycle of simple power generation in the world rarely uses exhaust gas supplementary combustion, and supplementary combustion units are mostly used in combined heat and power or other multi-generation units.

Natural gas power generation has the following advantages:

(1) High efficiency. The thermal efficiency of combined cycle power generation technology can reach 55%, while the thermal efficiency of a typical coal-fired power plant is only about 35% on average.

(2) Low cost and short design and construction period. The investment in the construction of combined cycle power plants is US$400-500 per kilowatt of installed capacity, which is only about half of the investment in the construction of conventional coal-fired power plants using flue desulfurization (FGD) technology. Due to the standardization and modularization of the equipment, its installed capacity can be expanded in small increments, making infrastructure investment more economical.

(3) Low operating and maintenance costs. The operating and maintenance costs of a combined cycle power plant are half of that of a conventional coal-fired power plant.

(4) High reliability and high availability. The availability rate of combined cycle power plants reaches 90%. The availability rate of conventional steam power plants is 80% to 85%.

(5) Low pollutant emissions. Germany’s Nossener Brücke natural gas power plant has an installed capacity of 265MW, which is used to replace the previous coal-fired power plant. As a result, SO2 has been reduced by 99%, nitrogen oxides have been reduced by 75%, suspended particulates have been reduced by 97%, and CO2 has also been reduced by 50%.