Problems associated with low water-to-carbon ratio operation in a furnace

Operating with a low water-to-carbon ratio in the furnace is an emerging trend; however, the lower the water-to-carbon ratio, the more likely carbon deposition occurs on the catalyst. As a result, some catalyst manufacturers are modifying the conversion catalysts by adding trace amounts of metal elements such as potassium. Currently, the water-to-carbon ratio can be as low as around 2.5.

The energy-saving effects of operating at a low water-to-carbon ratio are quite significant, but the requirements for catalysts are high, both for those used under normal conditions and those used under extreme conditions. Fortunately, domestic catalysts currently pose no problems. In some factories, temporary drops in the water-to-carbon ratio below 2.5 due to steam fluctuations do not cause any major issues. Although the progress toward using domestic catalysts in large-scale ammonia synthesis plants in China is slow, these domestic catalysts have basically reached international standards, which is encouraging!

Abstract: By analyzing the constraints on low water-carbon operation of Unit 1 furnace, corresponding solutions are proposed, and the energy-saving and consumption-reduction effects of such low water-carbon operation are summarized. Keywords: single-stage furnace; water-carbon ratio; catalyst; fuel gas; medium-pressure steam; energy saving and consumption reduction. 1. Overview: The ammonia synthesis unit at Ningxia Petrochemical Company’s second fertilizer plant is a second-hand unit that was imported as a whole from Canada in June 1992. Its process flow is basically the same as the Kellogg-type process introduced in China in the 1970s. To achieve high production, energy savings, and improved efficiency, the plant was upgraded by 50% in September 2005; after the upgrade, its designed daily output of synthetic ammonia was 1,500 tons. The plant uses natural gas as a raw material and is a typical ammonia synthesis plant using the hydrocarbon steam reforming method. The principle involves a catalytic conversion reaction using natural gas and water vapor to produce H2, which is required for the synthesis of ammonia; simultaneously, CO2 and CO are generated. After carbon removal, CO2 is sent as a raw material for urea production. The catalytic conversion of natural gas steam mainly takes place within the tubes of the first-stage converter; the main reactions that occur are① and②. The ratio of the number of steam molecules participating in these reactions to the total number of carbon atoms in the feed natural gas is known as the water-to-carbon ratio. This ratio has a significant impact on the conversion rate and equilibrium concentration of methane. Additionally, in order to prevent carbon deposition on the catalysts in the first-stage converter, the normal operating range for this ratio is higher than the designed value, typically ranging between 3.2 and 3.5. However, an excessively high water-to-carbon ratio does not significantly improve the methane conversion rate; it only increases the consumption of excess steam, resulting in a high heat load on the first-stage furnace and an increased consumption of fuel gas. If the water-to-carbon ratio is controlled too low, it hinders the forward progression of the methane steam conversion reaction, and carbon deposition reactions are likely to occur, posing a certain risk to the safe operation of the furnace. Therefore, how to maintain the water-to-carbon ratio within an appropriate range – so as to reduce unnecessary steam consumption and the amount of fuel gas used in the first-stage furnace, thereby achieving energy savings and cost reduction, while simultaneously ensuring the safe operation of the first-stage furnace – has become a focus of attention in the industry. CH4 + H2O → CO + 3H2 – Q ① CO + H2O (vapor) → CO2 + H2 + Q ② 2. Constraints on operating at a low water-to-carbon ratio in the first stage of the furnace 2.1 Impact of operating at a low water-to-carbon ratio on the conversion catalyst in the first stage For the steam reforming of hydrocarbons, under fixed temperature and pressure conditions, according to chemical reaction equilibrium, there is a minimum water-to-carbon ratio for raw gas with certain compositions. Theoretically, it is sufficient for this value to be greater than 1, but this is only the result under equilibrium conditions and does not reflect the non-equilibrium state in actual production processes. If the water-to-carbon ratio is below this value, the methane conversion process will favor reactions ③ and ④, resulting in the formation of elemental carbon, namely what is commonly referred to as carbon deposition or carbon buildup. If carbon deposition occurs inside a furnace tube, it will lead to uneven gas flow, increased resistance, reduced gas volume, and overheating of the furnace tube. Severe carbon deposition can sometimes even completely block the furnace tubes, leading to dry burning; this reduces the lifespan of the furnace tubes, results in the complete loss of catalyst activity, and forces the system to shut down. Therefore, to prevent carbon deposition, the normal control requirement for the water-to-carbon ratio is that it must be no less than 2.5. Moreover, most units are equipped with a low water-to-carbon ratio interlock, set at 2.0; when this ratio falls below 2.0, the unit shuts down via interlock, thereby protecting the catalyst. CH4 + 2H2 + Q ③ 2CO + CO2 + C – Q ④ Therefore, operating at a low water-to-carbon ratio imposes higher demands on the conversion catalyst; ordinary catalysts cannot meet the production requirements, resulting in greater risks. The first-stage conversion catalysts used in the ammonia synthesis plants for fertilizer production are the Z107 and Z111 series developed by the Southwest Chemical Research Institute. It features strong resistance to carbon deposition, good activity at low temperatures, and a wide range of operational conditions; it is an energy-saving catalyst with a low carbon-to-water ratio, developed specifically for the Kellogg process. According to the catalyst usage documentation provided by the manufacturer, this catalyst fully meets the requirements for operation at a low water-to-carbon ratio; as long as the water-to-carbon ratio is kept within a reasonable range, issues such as excessive methane content at the outlet of the reactor section or carbon deposition will not occur. 2.2 Impact of low water-to-carbon ratio operation on system conditions: In addition to having a significant effect on the safe operation of the furnace in the first stage, low water-to-carbon ratio operation also affects the overall conditions of the system. A decrease in the water-to-carbon ratio directly affects the equilibrium of the steam reforming reaction, resulting in an excessive level of CH4 in the system. At the same time, as the water-to-carbon ratio decreases, the water vapor ratio in the conversion process also drops, which affects the CO conversion reaction; as a result, the CO concentration rises, leading to an increase in the methanation temperature. During operation at a low water-to-carbon ratio, depending on the actual operating conditions, it is possible to actively adjust the hydrogen-to-nitrogen ratio by increasing the temperature at the outlet of the first furnace section, thereby ensuring that the key parameters remain within the specified ranges. 3. Specific implementation methods for reducing the water-carbon ratio and analysis of energy-saving and consumption-reduction effects: 3.1 Specific steps for reducing the water-carbon ratio. The reduction of this ratio is carried out in two stages: the first stage involves reducing it from 3.2 to 3.0, and the second stage involves further reducing it from 3.0 to 2.8. Data is collected and analyzed in comparison with the data obtained when the water-carbon ratio was not reduced, in order to summarize the operational conditions as well as the energy-saving and consumption-reduction effects. When operating with a water-to-carbon ratio of 2.8, care should be taken to minimize pressure fluctuations in the steam pipeline network, as these can lead to either too high or too low water-to-carbon ratios. This not only disrupts the normal operation of the system but may also result in an automatic shutdown due to an excessively low water-to-carbon ratio. 3.2 The impact of low water-to-carbon ratio operation in the first-stage furnace on medium-pressure steam consumption: The effect of operating the first-stage furnace at a low water-to-carbon ratio on medium-pressure steam consumption is evident; with the load remaining constant, operating at a lower water-to-carbon ratio means a reduction in the amount of steam entering the first-stage furnace. The lower the water-to-carbon ratio, the less medium-pressure steam is consumed. Table 1. Water-carbon ratio and medium-pressure steam consumption along with the amount of reduction. Water-carbon ratio: 3.2, 3.0, 2.8. MS consumption (t/h): 85, 79, 74. MS reduction amount (t/h): 0, 6, 11. The impact of operating at a low water-carbon ratio on the fuel gas consumption in the first furnace: When the first furnace operates at a low water-carbon ratio, the amount of excess process steam entering it decreases, the thermal load of the furnace is reduced, its thermal efficiency improves, and as a result, fuel gas consumption drops. Analysis of the collected data showed that, while the furnace exit temperature remained 5°C higher than before reducing the water-to-carbon ratio, the fuel gas consumption still decreased significantly. Table 2. Water-carbon ratio, fuel gas consumption, and the amount of reduction in fuel gas consumption. Water-carbon ratio: 3.2, 3.0, 2.8; FG consumption (Nm3/t): 345, 324, 304; Reduction in FG consumption (Nm3/t): 0, 21, 41. 4. Conclusion: Based on observations of the actual operation of the furnace at a low water-carbon ratio, it was found that no carbon deposition occurred when the water-carbon ratio was reduced to 2.8, and there were no significant changes in the system’s operating conditions. Analysis of the collected data shows that the greatest advantage of operating a furnace at a low water-to-carbon ratio is the significant savings in medium-pressure steam, which in turn reduces the fuel gas consumption of the furnace. The energy-saving and cost-reduction effects are quite evident, and the economic benefits are substantial. References: Ammonia Synthesis Production Process, compiled by Dalian University of Technology