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The Impact of the Water Wall Effect on Coal Consumption and Solutions Author/Source: Lin Qingfu (Shandong Linyi Haoyu Thermal Equipment Co., Ltd.) Date: 2008-11-10 Keywords: water wall effect, slag formation on the furnace, steam decomposition rate, water wall, oil wall, gasification intensity, temperature difference, heat transfer, medium. Abstract: It introduces the impact of water jackets on coal consumption and the fate of the thermal energy in the raw coal, proposes an appropriate temperature for the inner wall of jacketed boilers, and outlines the actual energy-saving effects achieved by raising the temperature of the inner wall of the jackets. The water jacket of fixed-bed batch gas generators has undergone more than a dozen improvements, all of which involved changes in shape; these improvements were aimed at addressing issues related to lifespan, gas production volume, and safety. The heat transfer medium remained unchanged, so the temperature of the inner surface of the jacket did not change or changed only slightly. The various drawbacks associated with the water-cooled wall effect have yet to be completely resolved. For example: ① The water jacket has a high heat absorption capacity. The heat absorbed per square meter is around 23,100 kcal/hour. Taking a φ2600 gas generation furnace as an example, with an inner surface area of the jacket of 20㎡, such a furnace can produce approximately 18 tons of steam per day; it consumes more than 2 tons of raw coal per day, resulting in an annual consumption of over 700 tons per furnace. ②The gasification strength is low. Since the average temperature of the water in the jacket is only around 110°C, 30% of the area in the outer ring region of the furnace will be affected to varying degrees; 15% of this area lies in the low-temperature zone, resulting in a 10–14% decrease in gasification intensity. ③The ash contains a high amount of combustible material. The low-temperature zone in the outer ring of the furnace is the main area where residue carbon is formed. Based on data provided by various manufacturers as well as information presented in articles, the amount of residue carbon in the ash and slag is generally between 15–20%, which corresponds to 3–5% of the total coal fed into the furnace; this results in an annual loss of over 800 tons per furnace. ④The steam decomposition rate is low. Since 15% of the area around the furnace chamber is in a low-temperature zone, as the gas production time increases, the rate of steam decomposition gradually decreases; (the average decomposition rate for downward gas generation is only 35%), resulting in an increase in the amount of undecomposed steam and, consequently, an increase in the heat carried away. The effective components of semi-water gas also decrease accordingly. For every 1% increase in CO2 in the gas, the consumption of ammonia per ton will rise by 17 kg. ⑤The water wall effect also leads to an increase in the oxygen content in the gas; for every 0.1% increase in oxygen content, 236 kg more steam is required per ton of ammonia produced, resulting in annual losses of over 300,000 yuan per furnace. The main purpose of installing a water jacket is to prevent scarring inside the furnace, which could prevent ash from being discharged properly and thus affect production stability. Steam generation is a secondary function; the water used in the jacket serves as a cooling medium to lower the temperature of the furnace walls. This is one method, though not the only one, to prevent scarring and issues with ash discharge. Using water as a heat transfer medium has many advantages, including operational flexibility and simplicity. The disadvantages are the five aspects mentioned above. So, what exactly is the temperature of the inner surface of the jacket that will ensure the furnace wall does not develop scale or deposits, while simultaneously minimizing the side effects caused by the cold wall effect? A brief analysis here: In actual production, the temperature inside the furnace undergoes continuous changes as a result of alternating processes of heating by blowing air and cooling during gas production. When air is blown in to heat the furnace, the temperature rises faster in the central area of the furnace where the base temperature is high, while it rises more slowly in the outer regions where the base temperature is lower. The temperature changes during the gas production and cooling process are the opposite of these. The drop is rapid in the outer ring area, while it is slower at the center of the furnace. There is currently no accurate information regarding the detailed temperature distribution; only a rough estimate can be made based on several basic parameters such as the temperature of the jacket fluid, the ignition point of the raw coal, the ash fusion point, and the amount of coking present in the ash. The temperature of the inner surface of the jacket is determined by the temperature of the medium in the jacket and the thermal resistance of the inner wall steel plate; it is approximately 200°C when water is used as the medium, and about 430°C when heat transfer oil is used. The value of the steel plate’s thermal resistance used here is the one applicable for design calculations, and it should be accurate. Since the temperature inside the furnace is always higher than that of the jacket fluid, the jacket always conducts heat outward. The ignition temperature of raw coal is generally around 400°C, while its ash fusion point is 1250°C. In plants where the quality of the coal fed into the furnace is good, with little gangue and coal dust, and under conditions of reasonable processes and stable operation, the amount of coking material in the ash is not significant; there is no formation of raw coke either. This shows that even though the temperature of the inner surface of the water jacket is only 200°C, the coal in direct contact with the inner wall of the jacket can still be almost completely burned; its temperature should be above 400°C. In abnormal situations such as excessive blowing time or insufficient steam, scorching and deposit formation on the walls of the water jacket can occur, which proves that the temperature of the inner surface of the jacket can sometimes exceed 1250°C as well. From this, the temperature in the outer ring area of the furnace can be inferred: it should be around 900°C after blowing air, and around 450°C after gas production. This is also evident from the steam output of the water jacket; in a blowing and gas production cycle, the time required for heating through blowing accounts for only about 27%, yet the steam produced by the water jacket makes up at least 60% of the total ； And the time required for gas production and cooling accounts for 73%, while steam production accounts for at most 40% ； A large temperature difference between the inner and outer surfaces of the water jacket leads to more steam production (the temperature difference is the only condition for heat transfer; without it, no heat transfer occurs) ； A small temperature difference results in less steam production ； The longer the blowing time, the more steam is produced by the jacket; the shorter the blowing time, the less steam is produced by the jacket ； The larger the internal surface area of the water jacket, the more steam will be produced; the smaller the area, the less steam will be produced ； Higher steam production leads to higher consumption of raw coal, while lower steam production results in lower consumption of raw coal ； It’s a very obvious truth. The gasification reaction inside the furnace is a complex process that is difficult to describe clearly. What is mentioned here refers only to the oxidation layer; the conditions in the drying layer, carbonization layer, reduction layer, and ash layer are different again. The temperature difference across the plane in the drying layer is much smaller, while this difference increases as one goes deeper. In the ash layer and the upper part of the furnace, there is practically no temperature difference at all. It can be seen from this that the temperature of the inner surface of the jacket should not be set too high, otherwise scarring and fouling of the furnace are likely to occur, affecting production stability. In my personal opinion, 400°C to 600°C would be appropriate. That is, it ensures no scarring or sticking of the furnace under high-load production conditions, while also minimizing the side effects caused by the cold wall effect. The gasification process of coal is a process of thermal energy conversion and balance; our goal is to convert thermal energy into the desired substances as effectively as possible. How does heat conversion take place in fixed-bed gasification? The raw coal reacts with oxygen in the furnace, releasing heat; as a result, the temperature inside the furnace rises. A large amount of blast air carries this heat to the waste heat boiler, where it is used to generate steam (or is released). This conversion of thermal energy is something we do not want to occur. There are two ways to address this issue: one is to use pure oxygen or oxygen-enriched gas for combustion, thereby reducing the volume of blast air and thus minimizing the transfer of heat outside. Second is to effectively recover and utilize the heat from blowing air. The heat removed by the blast air accounts for a large proportion of the total energy of the coal fed into the furnace. The second application of the thermal energy from raw coal is for generating steam through gasification, which is the process we desire and that is the main one. The third destination of thermal energy is the heat absorbed by the furnace jacket; the heat absorbed per square meter per hour by the water jacket is approximately 23,100 kcal. For a ￠2650 gas generation furnace with a water jacket that is 500 mm higher, the daily steam production is around 18 tons, which corresponds to 2.3 tons of raw coal – accounting for 4% of the total coal fed into the furnace. The fourth destination of thermal energy is the sensible heat carried away by the upward and downward flowing gas and undecomposed steam; this portion of heat also accounts for a significant proportion, and it can be roughly estimated from the amount of steam produced by the waste heat boiler. The fifth destination of thermal energy is the thermal radiation loss from equipment and pipelines, which must be addressed by improving insulation. The sixth destination of thermal energy is the conversion rate of effective gas components. Both latent heat loss. The seventh destination of thermal energy is the sensible heat carried away by the unburned coal along with the ash, as well as that contained in the ash itself. Of these seven pathways for thermal energy, only the second one, which involves gas production, is what we desire; the rest should be restricted or avoided. This is the general principle for the energy-saving renovation of fixed-bed gas generators. By increasing the temperature of the inner wall of the jacket, the heat absorption by the jacket is restricted (or eliminated), and as a result, the temperature difference across the plane inside the furnace inevitably decreases ； The oxide layer will inevitably thicken, and the sensible heat storage in the furnace will inevitably increase ； The gasification intensity increases, the steam decomposition rate rises, the effective components in the gas increase, and the amount of carbon residue in the ash decreases. This is an undeniable and inevitable trend of change. Determining the proportion of heat energy going to each destination is a complex heat energy calculation. The author has not yet found relevant figures; manufacturers can make a rough estimate. For example, it is quite reliable to assume that the heat transferred through the water jacket accounts for 4% of the total heat from the coal fed into the furnace. 4% may seem like a small figure on the surface, but upon closer analysis of the proportions and nature of these seven destinations for thermal energy, 4% has a significant impact on gas production. According to the data cited by Senior Engineer Li Yongheng: \"After blowing, the sensible heat storage in the furnace is considered to be 100%. Of this, 33.5% is absorbed by the gasification reaction, 28.4% is carried away by the gas blown from above (including undecomposed steam), 15.9% is carried away by the gas blown from below (including undecomposed steam), and 22.2% is absorbed by the jacketed boiler.\" ”It can be seen that the jacketed boiler has a significant impact on coal consumption; the solution is to increase the temperature of the jacketing medium as much as possible, while ensuring that scaling does not occur on the boiler. The author upgraded four oil-cooled wall gas generators at an ammonia synthesis plant in Shandong, raising the temperature of the inner wall of the jacket to 430°C. Since they were put into operation in September 2006, they have been running continuously for over six months. Regardless of the load level, coal quality, or fluctuations in wind pressure, it has never experienced issues such as uncontrolled furnace temperature, scorching on the walls, or formation of slag. Its gas production rate, gas composition, and rate of coking due to ash and slag are significantly better than those of gas generators with water jackets. Moreover, it offers a larger operating space, faster temperature rise, and is safe and reliable. The actual analysis and measurement data are as follows: 1. Steam production from the furnace jacket: oil-fired furnaces produce 300 kg/h of steam at 1.2 Mpa, while water-fired furnaces produce 680 kg/h of steam at 0.08 Mpa; thus, there is a reduction of 380 kg in steam production. The annual coal savings per furnace amount to: 0.38 × 24 × 330 ÷ 6 = 502 tons. 2. Ash content: The company operates for 12 hours per shift, with ash being removed 4 times per shift; each removal amounts to 1400 kg. First, the slag and scum pieces were removed from the ash: 473 kg of slag pieces were collected from oil furnaces, and 290 kg from water furnaces. Next, the coking residues were extracted from the ash: 3.4 kg was collected from oil furnaces, and 30.5 kg from water furnaces (these are average values over three measurements). After removing the slag pieces and coking residues, the fine ash remaining was mixed together, and samples were taken for analysis of the combustible content: 14.6% for oil furnaces, and 20.4% for water furnaces. The comprehensive combustible materials are as follows: oil furnaces account for 9.9%, while water furnaces account for 17.9%. Compared to the latter, oil furnaces represent an 8-percentage-point decrease; the annual coal savings per furnace amount to 296 tons. 3. Gas emission volume: (Measurement method: During normal production, the furnace is shut down for three cycles each time, and the degree of drop in the gas tank is measured to calculate the gas emission volume). When the oil furnace is shut down for 6 minutes, the gas holder drops by 1.98 m. Based on calculations, its gas generation rate is 7524 m³/h ； When the water boiler was shut down for 6 minutes, the gas tank dropped by 1.73 m. Its gas production rate was 6574 m³/h; the oil boiler produced 950 m³/h more gas than the water boiler, representing an increase of 14.4%. 4. Gas production per ton of coal: For oil-fired boilers, 36 buckets of coal are added in 12 hours, while for water-fired boilers, 34 buckets are added in the same time period. The average weight of each bucket is 0.895 tons. Thus, oil-fired boilers consume 2.685 tons of coal per hour, and water-fired boilers consume 2.536 tons per hour. The gas production per ton of coal is: 7524 ÷ 2.685 = 2802 m3 for oil-fired boilers, and 6574 ÷ 2.536 = 2592 m3 for water-fired boilers. Oil-fired boilers produce 210 m3 more gas per ton of coal, which represents an increase of 8.1%. 5. Gas composition (average of 8 analyses) (the unfilled sections have not been analyzed): Oil-cooled furnace (%) / Water-jacketed furnace (%): CO2, COO2, CO2, COO2. Upward blowing: 6.43, 0.42; 7.22, 8.75. Downward blowing: 3.83, 4.9; 5.03, 3. Blowing air: 0.38, 0.4. Raising the inner wall temperature of the jacketed boiler from less than 200°C to 430°C yields significant economic benefits in at least the following four aspects: 1. The heat lost through the jacket is reduced by more than 50%, resulting in a yearly savings of 350 tons of raw coal per boiler ； 2. The sensible heat storage capacity inside the furnace increases significantly, with the gasification efficiency rising by 10–14%. 3. The area of the low-temperature zone in the outer ring of the furnace decreases markedly; the combustible content in the ash and slag drops by 8 percentage points, resulting in a yearly savings of 260 tons of coal per furnace. 4. The furnace temperature is high, allowing for rapid temperature increase; this boosts the steam decomposition rate, reduces the amount of undecomposed steam, thereby decreasing the heat carried away, and increases the effective components in the gas. CO and CO2 increased and decreased by 1.5 percentage points respectively, resulting in annual savings of over 400 tons of raw coal per furnace. There are two issues of concern to everyone: First, safe operation: Safety is ensured through the following seven aspects: ① The furnace jacket is designed with three layers; several reinforcement plates are placed between the first and second layers, and a sealing plate is located on the outermost side. The overall strength and sealing performance of this design are more than three times higher than those of a water jacket. ②The medium inside the jacket is under normal pressure in a sealed state, with an expansion tank at the top that is connected to the atmosphere, thereby eliminating the conditions for overpressure explosions. ③On the inlet and outlet pipes of the jacket, a temperature sensing tube with high reliability is installed in each case, allowing for continuous monitoring of changes in the temperature of the medium. ④The medium circulation pipelines, valves, and pumps are all made of materials that can withstand high temperatures and provide good sealing, ensuring leak-free operation throughout the year. ⑤A level gauge is installed in the high-level expansion tank, allowing for continuous monitoring of liquid level changes. ⑥The high-temperature pumps operate in a round-robin fashion, one in use and the other as a backup, to ensure stable operation throughout the year. ⑦In the event of a power outage, follow the same safety procedures as with a water-jacketed furnace; make sure to seal all air inlets at the bottom of the furnace, so that the medium will not overheat. For manufacturers whose furnace bottom vents are not properly sealed, a manual or steam (diesel) oil pump can be used as a backup (a flow rate of 10 m³/h per furnace is sufficient). Depending on the temperature of the medium, cool oil from a lower oil tank can be used to replace the hot oil inside the jacket. II. Investment and Benefits: Investment for upgrading the furnace involves removing the original water jacket and low-pressure steam drum, replacing them with a new furnace jacket and thermostat, as well as installing the necessary accessories; the investment per furnace is around 260,000 yuan. Newly built gas furnaces: Each one costs about 190,000 yuan more than water-jacketed furnaces, but their gas production capacity is over 10% higher; therefore, the total investment is lower compared to water-jacketed furnaces. Benefits: Taking a φ2650 gas generation furnace as an example, with 330 days of operation per year, the gas production volume increases by over 10﹪, resulting in savings of more than 1,000 tons of raw coal per furnace per year. The entire investment can be recovered in two months. Conclusion: Energy saving and consumption reduction in gas generation furnaces is a systematic task; it is necessary to identify the problems within the system and address them step by step in an effective manner to achieve good results. Today’s gas generators have gone through half a century, with **hundreds of minor improvements made – which one of them was not effective? How effective will it be each time? Therefore, the intrinsic quality of the raw coal, its processing quality, a stable furnace operation, and appropriate operational parameters are always the keys to saving energy and reducing consumption in gas generation furnaces! Any optimization or modification of equipment is aimed at improving operational parameters, stabilizing furnace conditions, and reducing heat losses. The transformation of a device must be based on theoretical foundations and in line with energy-saving principles in order to be effective; it is better not to carry out any transformation without such theoretical basis.