Review on Technologies to Improve the Thermal Efficiency of Oil and Gas Furnaces and Achieve Energy Savings

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Review on Techniques for Improving the Thermal Efficiency of Oil and Gas Furnaces and Energy Saving Zhang Yongzhao (Xi’an Jiaotong University, Xi’an 710049) Abstract: This paper introduces the situation of oil and gas resources as well as energy utilization efficiency in China, and discusses practical energy-saving techniques for improving the thermal efficiency of oil and gas furnaces. Keywords: Energy resources ； Energy efficiency ； Oil and gas furnace ； thermal efficiency ； Energy conservation 1: China’s oil and gas resources and production situation. There are many different opinions regarding China’s oil and gas resources, with significant variations among them. According to a report in U.S. magazine International Oil Economy in March 1999, the situation regarding oil and gas resources in our country is as follows: Oil – the remaining proven reserves amount to 3.27 billion tons, accounting for 2.3% of the world’s total. In 1998, production was 160 million tons, while in 1999 net imports were 43.81 million tons ； Natural gas: Remaining proven reserves amount to 1.37 trillion m3, accounting for 0.95% of the world’s total; production in 1998 was 21.7 billion m3. Based on the above data, it can be seen that China is relatively poor in oil and gas. In recent years, research experts in the fields of petrochemistry and geology in our country have not agreed with such resource data. The authors of the study conducted a thorough analysis and research of recent exploration data, concluding that China’s economically viable recoverable oil and gas resources (based on international standards, rather than geological resource volumes) amount to 11.44 billion tons of oil and 93,000 million cubic meters of natural gas. By 1998, the proven recoverable reserves nationwide were only 5.76 billion tons of oil and 1.56 trillion cubic meters of natural gas; the exploration ratio was 49.7% for oil and 16.8% for natural gas. According to data from various studies, China ranks 9th in the world in terms of recoverable oil reserves, and 6th in terms of natural gas reserves. With further exploration, by the early to middle 21st century, China’s recoverable oil reserves are expected to double, while those of natural gas are set to increase by 3 to 4 times. Clearly, China possesses huge potential and promising prospects for its oil and gas resources. However, given China’s large population and territory, when the average abundance of oil and gas is calculated based on land area (in t/km2 and 103 m3/km2), the world’s average values are 1.75 times and 2.22 times those of China respectively; in other words, China accounts for only 57% and 45% of the world’s levels. On a per capita basis, China has a lower average resource abundance value (see Table 1). As can be seen from Table 1, on a per capita basis, the world’s oil and gas abundance is 5.84 times and 7.57 times that of China respectively; in other words, China accounts for only 17.1% and 13.2% of the world’s total. Therefore, on an average basis considering land area and population, China is undoubtedly a country with relatively low oil and gas reserves. Table 1: Oil and gas abundance values per capita. Population: 106 people. Oil resources: 108 tons; Gas resources: 109 m3. Per capita oil amount: tons per person; Oil abundance ratio. Per capita gas amount: 103 m3 per person; Gas abundance ratio. World averages: 5630, 3111.77, 327694; Ratios: 55.27, 5.84, 58.2, 7.57. China’s figures: 1208.8, 114.4, 9300; Ratios: 9.46, 1.0, 7.69, 1.0. Significant progress has been made in oil and gas exploration and production in recent years. Meanwhile, the remaining recoverable oil reserves and oil production are increasing slowly; to meet the needs of the country’s economic development, oil imports have surged ； The recoverable reserves of natural gas are increasing rapidly, but production growth lags behind. 2 Energy utilization – Energy efficiency: Given the lack of a strict definition for energy efficiency in the country, this paper adopts internationally recognized evaluation and calculation methods. Energy efficiency includes extraction efficiency, processing and conversion efficiency, storage and transportation efficiency, and end-use efficiency. The internationally accepted definition is: Energy efficiency = Processing and conversion efficiency × Storage and transportation efficiency × End-use efficiency ； Total efficiency of the energy system = Energy efficiency × Extraction efficiency. According to statistical calculations from literature, in 1997 China’s energy extraction efficiency was only 33%, the efficiency in processing, conversion, storage, and transportation was 68.8%, and the efficiency at the point of final use was 45.3% (of which 30.5% was for agriculture, 46.3% for industry, 28.9% for transportation, and 54.8% for domestic and commercial use). The thermal efficiency of industrial boilers (including coal-fired as well as oil and gas-fired boilers) affects the industrial and commercial sectors within the overall energy end-use efficiency. Thus, China’s energy efficiency in 1997 was 68.8% × 45.3% = 31.2% ； The overall efficiency of the energy system is only 33% × 31.2% = 10.3%. Regarding energy utilization, Table 2 presents a comparison between China and the United Nations Economic Commission for Europe. As can be seen from Table 2, in terms of energy utilization, China’s main issues lie in low extraction efficiency and low efficiency at the industrial end-use stage. Therefore, in addition to improving the recovery rates of energy sources such as coal to enhance mining efficiency, the industrial sector should strengthen technical management, restrict the operation of energy-intensive enterprises and equipment, increase the proportion of oil and gas in the energy mix, and improve the thermal efficiency of industrial boilers, so as to bring the end-use efficiency in the industrial sector close to world standards. 3 Improving the thermal efficiency of oil and gas furnaces and energy-saving technologies Table 2 Comparison of energy utilization China (1997) United Nations Economic Commission for Europe (early 1990s) 1. Extraction efficiency 33 59 2. Processing, conversion, and transportation 68.8 67 3. End-use efficiency – Agriculture Industry Transportation Civil and commercial use 45.3 30.5 46.3 28.9 54.8 51 33 65 25 55 4. Energy efficiency 31.2 34 5. Overall efficiency of the energy system 10.3 20 From the above analysis, it is clear that China has limited oil and gas resources and low energy efficiency; therefore, implementing energy-saving measures and using energy-saving technologies is an urgent priority. Energy conservation involves many aspects; in the case of industrial boilers, coal-fired industrial boilers are crucial. However, given the importance of oil and gas resources to the development of industries such as China’s national economy and defense, this article will provide an introduction to improving the thermal efficiency and energy conservation of oil and gas furnaces. Before introducing it, let’s take a look at the demand for oil and gas resources in our country in recent years: In 2000, the demand for oil was 190–200 million tons, while the production volume was 160–165 million tons ； Natural gas demand: 40 billion m3, production: 30 billion m3 ； In 2010, oil demand was 250–280 million tons, while production was 170–180 million tons ； Natural gas demand: 1000 m3, production: 60 billion m3 ； In 2020, oil demand was 320–350 million tons, while production was 190–200 million tons ； Natural gas demand is 200 billion m3, while production is 1200 m3. 201 From the above information, it is clear that there is a significant mismatch between the demand for oil and gas resources and their production volume in our country, and this situation will persist over the long term. Therefore, efforts to improve energy efficiency in oil and gas boilers will also be a long-term endeavor. The aspects mentioned above are the heat efficiency improvements and energy-saving technologies that can be applied to oil and gas furnaces. (1) A low-temperature heating surface is installed at the furnace tail to recover part or all of the latent heat of vaporization of water vapor in the flue gas. The oil and gas used in oil and gas boilers contain a high amount of H2, which turns entirely into water vapor upon combustion. If no measures are taken, the low-temperature heating surfaces at the rear of such boilers will suffer from acid corrosion (caused by SO2, CO2, and condensed water). Many boilers in China have encountered this problem. To completely eliminate this corrosion, it is necessary to install low-temperature heating surfaces at the rear – such as stainless steel economizers or condensers, either external or internal. This allows for the recovery of some or all of the latent heat of vaporization from the water vapor in the gas, thereby further reducing the exhaust gas temperature. These two measures can significantly improve the thermal efficiency of oil and gas boilers, helping to achieve energy savings. The author performed calculations on a 4 t/h steam boiler; the amount of heat recovered from the latent heat of vaporization and the improvement in thermal efficiency are shown in Table 3. Table 3: Recovery of the latent heat of vaporization of water vapor in flue gas from 4t/h boilers
Fuel type | Name | Unit | City gas | Natural gas | Light diesel
Theoretical water vapor volume H₂O, V | Nm³/Nm³ (gas) | Kg/Nm³ (oil) | 1.2 | 2.08 | 1.667
Theoretical amount of water vapor | Kg/Nm³ (gas) | Kg/kg (oil) | 0.72 | 1.248 | 1.006
Fuel consumption | Nm³/h (gas) | Kg/h (oil) | 6.37 | 302.66 | 247.33
Total amount of water vapor in flue gas* | Kg/h | 458.64 | 377.7 | 248.8
Heat recovered from the latent heat of vaporization of water vapor | kW | 288.5 | 237.57 | 156.5
Effective heat utilization by the boiler | kW | 2606.5 | 2606.5 | 2606.5
Percentage of recovered heat relative to effective heat utilization | % | 11 | 9.1 | 6
Increase in boiler thermal efficiency | % | 11 | 9.1 | 6
*The moisture contained in the excess air is not taken into account.
As can be seen from Table 3, if the latent heat of vaporization of water vapor in the flue gas is fully recovered, the boiler efficiency can be increased by 6–11% compared to its original level. For example, if the boiler’s efficiency was originally 88%, the adoption of this energy-saving technology can raise its efficiency (calculated on a lower heating value basis) to 94%–99%, thereby significantly improving energy utilization efficiency. With the use of this technology, the flue gas temperature of the boiler can be reduced to 60–80°C; as a result, the boiler’s efficiency can increase by several percentage points. The calculations above do not take into account the moisture present in the excess air, and if it were taken into account, even more heat could be recovered. Of course, it’s not possible to condense 100% of the water vapor in the flue gases and recover all the heat, but given the significant drop in the exhaust gas temperature, it is no problem to increase the boiler efficiency by 10%. With this energy-saving technology, the low-temperature heating surfaces of the boiler must be made of alloy materials resistant to acid corrosion; as a result, the investment cost for the boiler increases slightly ； Furthermore, a lower flue gas temperature hinders the rise of the smoke and its dispersion into the atmosphere. Since both oil and gas furnaces operate under slightly positive pressure without exhaust fans, and the chimney serves only as an exhaust duct, as long as the right burner is selected and the supply fan has sufficient ventilation pressure, it will not affect the dispersion of the flue gases in the atmosphere. (2) The boiler structure should preferably adopt a fully wet-back design. For horizontal oil and gas boilers produced today, their structural design mostly features a rear tube sheet wet-back configuration. If a fully wet-back structure can be used for the boiler, it not only results in a more rational and safe design but also reduces heat loss. By using high-quality insulating materials for the enclosure, it is possible to reduce heat loss from 5 q to less than 0.25%; this measure will also improve the efficiency of the boiler. Of course, the all-wet-back design is complex, increases the number of tube sheets, and raises the boiler cost to some extent. (3) Computers are used to measure and control the salt content in the boiler water or steam; based on the measurement data, the computer controls the amount of slag discharged from the boiler. Thanks to this computer-based control, it is possible to operate with a minimum level of slag discharge, with the usual discharge level being less than 1% pw D. A certain foreign company holds a patent for this technology. Boilers equipped with such technology have lower heat losses and higher efficiency. (4) Use a boiler shell with a larger diameter to improve steam quality, so that the steam humidity w is < 0.5%. (5) The boiler operates in a two-circuit system, with the primary circuit absorbing heat; it has good sealing properties and minimal leakage, resulting in low heat losses. The secondary circuit is used for heating and domestic purposes, thereby achieving a higher overall energy utilization rate. (6) Improve the automation level of combustion technology to achieve fully computer-controlled operation, thereby reducing the levels of NOX and CO in the gas to minimum levels, such as below 25 ppm, in order to enhance the thermal efficiency of the boiler and achieve low-pollution emissions. 4 Conclusions (1) There has been significant progress in the exploration and production of oil and gas resources in our country in recent years, but overall these resources are not abundant. When calculated on a per-unit area and per capita basis, China’s figures are below 50% of the world average, indicating that the country is relatively poor in terms of oil and gas resources. In the current period and over the next few years, aside from a relatively rapid increase in natural gas exploration, oil exploration and production, as well as natural gas production, are growing slowly. Therefore, the import pressure for oil and gas is high, making energy conservation a challenging task. (2) China’s energy efficiency is only 31.2%, primarily due to low end-use efficiency in the industrial sector and low thermal efficiency of industrial boilers, which significantly affect energy utilization efficiency. Therefore, in the industrial sector, in addition to strengthening technical management, restricting the operation of energy-intensive equipment, and changing the energy structure, improving the thermal efficiency of industrial boilers and promoting energy-saving technologies and products for such boilers remain of great importance. (3) Several energy-saving technologies have been provided to improve the thermal efficiency of oil and gas boilers; among them, the installation of low-temperature acid-resistant heating surfaces at the rear, as well as technologies for recovering the latent heat of vaporization from flue gas, are expected to increase the thermal efficiency of boilers by 10% or more. These technologies can also effectively address the issue of low-temperature corrosion in oil and gas boilers, and should be developed and promoted in the production of such boilers in China. References: Zhang Kang. A Preliminary Discussion on the Oil and Gas Situation and Development Strategy in China. Energy Policy Research, 2000, (4). Wang Qingyi et al. Analysis of Energy Efficiency in China and International Comparisons. Energy Policy Research, 2000, (2). Zhang Quangen et al. Design Manual for Fuel and Gas Buildings. Beijing: Machinery Industry Press, 1998. [Author’s Profile] Zhang Yongzhao (1934–), male. He graduated from the Department of Electrical Engineering at Zhejiang University in 1955. In his early career he taught at the Department of Power Engineering at Tsinghua University; currently he is a professor at the School of Energy and Power Engineering at Xi’an Jiaotong University. Has been engaged in teaching and research in fields such as thermal engineering, environmental engineering, gas-solid two-phase flow, and combustion for a long time.