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As the concepts and awareness of energy conservation and environmental protection gain increasing attention, protecting the environment and saving energy have become one of the most effective ways for companies to reduce production costs and enhance the competitiveness of their products in the market. For many power plants, reducing energy consumption and improving combustion efficiency is a matter of great concern, and this article will discuss it. 1 Smoke measurement and combustion efficiency The purpose of all combustion devices is to convert a fuel into thermal energy for use in production. The heat energy generated can be used to produce steam and for heating. In the 1970s, before the fuel crisis, little attention was paid to the efficiency of energy conversion; today, efforts are being made to improve combustion efficiency in order to reduce emissions of NOx, SOx, and unburned fuel. Therefore, emissions meet the standards and the ecological environment is protected. The best way to improve combustion efficiency nowadays is to use instruments for continuous monitoring of flue gases to measure the levels of carbon dioxide, sulfur dioxide, nitrogen, oxygen, and combustion products, or the level of carbon monoxide in those gases. Then, the measured data is analyzed to ensure that it remains within the required limits. 2 Substances involved in combustion – fuel and oxygen The main substances involved in combustion are fuel and oxygen. Generally speaking, the fuel can be gaseous (such as natural gas), liquid (such as various fuels), or solid (such as coal), while oxygen is obtained directly from the air. In other words, the 2 main substances generally become the air that serves as fuel. The air consists of 79% nitrogen, 20.9% oxygen, and trace amounts of other gases. Improving combustion efficiency means ensuring that an appropriate amount of fuel and an appropriate amount of air are combined in the optimal ratio for combustion. Since 79% of air is nitrogen, which does not participate in the combustion process, it is heated during combustion, absorbs energy, and is then released into the atmosphere through the flue gases. That is, in order to involve 20.9% of the oxygen in the air in the combustion process, it is necessary to heat nitrogen, which is nearly four times as abundant as oxygen, and then release it. These energy losses are inevitable, but they can be reduced to a minimum. If the amount of air supplied can be reduced to the greatest extent possible while ensuring complete combustion of the fuel, then this type of loss will be minimized. However, the reduction in air volume must be done while ensuring complete combustion of the fuel; otherwise, the energy loss due to incomplete fuel combustion is also considerable. 3 Current Situation In summary, the key to reducing unnecessary energy losses and improving combustion efficiency lies in achieving the optimal balance between air and fuel, so as to avoid wasting additional fuel or heating up extra air only to release it uselessly. How can this be achieved? Since flue gas contains all this information, the most straightforward approach is to use an analyzer to monitor the energy loss in the flue and adjust the air/fuel ratio accordingly to minimize energy consumption. Before discussing the methods for finding the optimal air/fuel ratio, let’s first take a look at the methods and current status of air/fuel ratio control in China. Since power plants are the largest energy consumers, their boilers can be used as a typical example for discussion. At present, almost all power plants determine the optimal air/fuel ratio for combustion during unit commissioning, and this is indicated by the flue gas oxygen content. Generally, after the oxygen level reaches 3%-5% O2, power plants adjust the air/fuel ratio to keep the oxygen content within this range. However, the problem with this approach is as follows: (1) 3%-5% O2 is a value applicable only during the unit’s commissioning phase; factors such as the type of fuel used, the condition of the unit, the wear and tear of mechanical components like fans and burners, and any leaks that may occur after commissioning are not taken into account. One can imagine what would happen if there was a leak in the flue, with 20.9% of the atmospheric oxygen entering the flue. (2) Due to the aforementioned factors, the energy-saving effect of 3%-5% O2 is reduced; as a result, many manufacturers use low-quality zirconia analyzers and consider the flue gas oxygen content merely as a reference value, relying primarily on experience in operation. This is the general situation regarding the use of flue gas parameters for energy-saving adjustments at present. It is worth noting, however, that power plants have one of the best awareness levels regarding energy conservation among domestic enterprises. There are also many small and medium-sized boilers that are operated based solely on experience, without the use of any flue gas analyzers. 4 Finding the optimal combustion point Determining the optimal combustion point is key to saving energy consumption. Theoretically, as shown in Figure 1, the shaded area representing energy consumption due to excess air indicates the energy loss caused by the surplus air, which is expressed in terms of residual oxygen in the flue gases. If it is blocked, it can be seen that as the oxygen content in the flue increases, the amount of excess air also rises, resulting in more energy being carried away by the heating of nitrogen. To reduce this energy consumption, it is necessary to lower the oxygen content in the flue gas. However, if it is reduced too much, the fuel may not burn completely, which in turn leads to losses; so by how much should the oxygen content be reduced? In Figure 1, the shaded area representing excess fuel energy consumption indicates the energy consumption caused by unburned fuel, as indicated by the combustible gases in the flue. It can be seen that the higher the amount of unburned combustible gas, the greater the energy loss. To reduce energy consumption, it is necessary to lower the level of remaining combustible gas in the flue. However, just like with excess oxygen, if this level is reduced too much, a large amount of air will be needed, which in turn leads to heat losses. So, what level is appropriate? It should be noted here that, regardless of the form of the fuel, the degree of complete combustion can be indicated by the amount of remaining fuel gas. Because whatever form the fuel is in, it is a hydrocarbon (CxHx). When the fuel enters the high-temperature furnace, the hydrocarbons break down and react with oxygen to form CO2+H2O, while a small portion fails to react sufficiently with oxygen and forms CO+H2, which escapes through the flue. From Figure 1, it is clear which point should be used for combustion to achieve maximum efficiency; by overlaying the two shaded areas to form another curve and taking its minimum value, we obtain the optimal point for combustion. At this point of combustion, energy loss is minimized and combustion efficiency is maximized. 5 Proper selection of flue gas analyzers The concentrations of various gas components in flue gas are determined using flue gas analyzers. It uses selective absorption methods to determine the concentrations of various gas components in flue gas. Selective absorption involves bringing a certain chemical absorbent into contact with flue gas in order to selectively absorb a specific gas component from it; by determining the decrease in its volume, it is possible to calculate the percentage concentration of that component. As can be seen from the above analysis, the optimal combustion point can be determined by measuring the residual oxygen and remaining combustible gases in the flue, but as mentioned earlier, most power plants have already managed to find this optimal point by doing so once. The problem is that it may often be necessary to find this optimal point for combustion efficiency, because as the type of fuel changes, as well as the conditions in the combustion chamber and flue, there is always a need to determine the optimal condition for combustion, and adjustments must be made promptly to achieve this optimum state. Since combustion is a dynamic process, it is impossible to keep the combustion process at its optimal point for an extended period of time; it will inevitably follow the curve that has been measured. Therefore, the actual combustion point is not the optimal one, as the optimal point is too close to the steeply sloping line representing energy losses due to incomplete combustion on the left side. As a result, the actual operating point should be shifted slightly to the left, with an increase of 0.25% in O2 being advisable. A flue gas analyzer is used for continuous monitoring of the residual oxygen and combustible gases in the flue; it can detect these two parameters at the same measurement point. By adjusting the air/fuel ratio based on this, we can obtain the two curves shown in Figure 1, and by combining them, we can determine the optimal combustion point.