Ten methods for judging furnace conditions based on coal rod gasification ash and slag

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Ten Indicators of Furnace Condition Based on Coal Rod Gasification Ash and Slag / Author/Source: Lei Xincheng (Gas Generation Technology Department, Junhao Foundry, Pizhou City) Date: 3-6-2009 Ash and slag are minerals resulting from the conversion of raw coal in gas generators; the quality of these ash and slag reflects the efficiency of carbon utilization. It is well known that in the gas generation section, whether gas is produced from lump coal or coal briquettes, low furnace temperatures result in poor gas output, while excessively high furnace temperatures cause scorching on the furnace. Of course, there are other factors as well that lead to furnace scorching. For many years, operating gas stoves by referring to the set temperature has become a* habit. Analyzing the furnace condition based on the ash quality not only provides guidance for actual operations but also holds significant and far-reaching importance for the improvement and development of gas production technology. To date, there is no good method for directly measuring the temperature of the vaporization layer to obtain accurate data. Most manufacturers still use traditional measurement methods to control the temperatures of the gas flowing upward and downward, adjusting the process by observing the ash condition whenever slag removal takes place. The ash and slag produced by coal rod gasification are used to assess the condition of the furnace based on their quality; properly adjusting the process remains one of the important criteria for this purpose. I. Carbon recovery rate of ash and slag: The carbon recovery rate refers to the ratio of the carbon content in the ash and slag to the total amount of slag. A high carbon content in the ash and slag, around 30% or more, is usually caused by low furnace temperature, excessive rotation speed of the furnace rods, improper operation, poor gasification, inadequate adjustment of the proportion of upward and downward air flow as well as the amount of steam used, insufficient wind pressure and volume entering the furnace, upward movement of the flame layer, unreasonable distribution of air through the grate, high resistance in the gas generation system, poor quality control of the gasifying agents, too high moisture content in the coal rods, high foam content, improper formulation of the materials used to make the coal rods, as well as issues related to the length, strength, and reactivity of the coal rods. Generally, it is advisable to keep the carbon return rate in coal rod gasification at around 5%. If this rate is too low, large lumps of carbon are likely to form, which can affect the stability of the furnace operation. Therefore, during normal production, it is essential to prevent incidents involving black rods or unprocessed carbon, in order to maintain an optimal carbon return rate. II. Quantity of ash The amount of ash is determined by the fixed carbon content of the coal rods, their melting point, and the ash content. Generally, the number of times ash needs to be removed in the process of producing gas from coal briquettes should be 4 times per shift. For low-quality coal briquettes, this frequency is 1 time per hour; for φ2.4 type furnaces, 20 tons of coal briquettes are used per shift, and the amount of ash that can be removed is approximately 7–9 tons. If the flame layer moves upward, and the speed of the furnace rods is too high, the ash discharge volume can also increase. The disadvantage of coal rod gasification is that slag tends to accumulate in the lower part of the furnace, thereby reducing the normal slag discharge volume. For gas production using low-quality coal rods from the south, the large-capacity ash hopper can be emptied 4–5 times per shift, with each discharge containing approximately 800–900 kilograms of ash. The ash hoppers on both sides should remain balanced and not tilt; this is considered normal. The grate machine must operate at a constant speed, without variations in speed, and it must not stop and start intermittently. If the carbon layer is thick, increase the number of furnace rods; if it is thin, reduce the number of furnace rods. By extending the operation time, stability of the carbon layer must still be maintained. In the case of uneven ash discharge from the furnace, there are instances where ash is present and other times when none is present. The reasons for this uneven discharge include large particles of coal dust, insufficient length of the coal rods, resulting in high resistance; as well as air flowing along short circuits. Another cause is the irregular shape of the slag removal strips, leading to deformation in the slag channels on both sides. During the gas production process using the same raw materials, the same amount of ash removal must be maintained. Only then is process equilibrium achieved. III. Clumping rate of slag The clumping rate of slag is the goal that every gas furnace operator strives to achieve. It is advisable to keep the lumping rate at 60%-65%. The slag lumps are porous and lightweight, with very little fine ash; their size is mostly between 150-200 millimeters, and those the size of a fist make up a small proportion. Under these conditions, the reactions within the furnace proceed normally, gasification is concentrated, the process conditions are appropriate, operation control is reasonable, and ash discharge is even without any uneven distribution, resulting in a uniform amount of ash. It indicates that from the raw material ratio to coal rod processing, as well as the strength and moisture content of the coal rods, all are within acceptable limits for use as coal rods in furnaces. If there is too much fine ash and white rods account for more than 30%, it indicates that the furnace temperature is too low, causing the two types of gasifying agents to take a short circuit. There is a special situation with gas production using wet coal rods: either an excessive amount of steam or too little steam can result in black rods being produced as slag from the gas furnace. How to distinguish this situation and use single-furnace analysis to determine the level of carbon dioxide in the gas in order to take appropriate action. It is advisable to set the carbon dioxide content in the top-blown gas at 5.0–6.0%, while 4.5–5.6% for bottom-blown gas is within the normal range; otherwise, the aforementioned problems will occur. The lumping rate of ash depends mainly on the temperature of the gasification layer and the thickness of the gasification layer. An excessively high temperature in the gasification zone improves the lumping rate, but it also causes large lumps to form inside the furnace, thereby increasing the difficulty of ash removal and reducing operational flexibility. Therefore, it is necessary to control the formation of large scar areas in a moderate manner to ensure the long-term stable operation of the gas stove. IV. Color of the slag: The normal color for the slag resulting from the gasification of humic acid-coated coal rods is yellowish-white or brown. If black appears, it indicates that the furnace temperature is too high in that area; a white rod suggests that the side wind is too strong, and scarring can be observed around the area. A gasification layer temperature higher than the ash fusion temperature can also cause the ash to turn a dark black color. The ash takes on a red-white color due to low furnace temperature and excessive steam usage, which results in uneven distribution of the two gases and the absence of a gasification layer. A short residence time of the ash in the furnace, as well as an excessively high speed of the grate drive, can also lead to the same consequences. The color changes of the ash are also related to the temperature of the steam, the moisture content in the steam, and whether the steam pressure is too high or too low. Only by strictly adhering to all process parameters during operation can an ideal normal color for the ash be ensured. V. Uniformity of slag: The uniformity of the slag is closely related to the temperature stability of the gasification layer. It has a strong interrelationship with the length of the coal rods, their strength, as well as the dimensions of the grates and slag breaking bars. A uniform gasification layer inside the furnace is a prerequisite for uniform ash distribution. Under normal conditions, the minerals remaining after the gasification reaction in the furnace are removed thanks to the function of the furnace grates. If the length of the coal rod is insufficient, the amount of powder is too large, resulting in higher resistance; if the strength of the coal rod is poor, there will be gas deviation, and the uniformity of the ash and slag will be poor. The assembly of the slag-breaking strips is not proper; the air distribution using the selected grate fabric is unreasonable, resulting in uneven air distribution. Both slag breaking and slag removal are related to the uniformity of the ash and slag. To maintain a high degree of uniformity in the ash, it is necessary not only to choose special grates for coal rods that possess excellent overall performance, such as the Pizhou Junhao energy-saving grates, but also to have proper operational conditions that ensure stability in the position and temperature of the gasification layer, thereby improving the uniformity of the ash. VI. Shape of the ash The shape of the ash is closely related to the quality of the coal rods, the properties of the coal, and the temperature of the furnace. The shape of slag can be divided into elliptical, flaky, and strip-shaped types. The elliptical type is formed under high furnace temperatures; the resulting slag lumps are large and take a long time to circulate within the furnace. If the slag outlet is too small, it becomes difficult to remove the slag, but this type of slag has better quality, higher gas volume, and lower coal consumption. Large slag lumps result from the low melting point and high reactivity of the raw coal; the fire layer moves downward, with localized melting occurring. Soft spots form near the grate, and as the temperature drops, the material enters the slag zone, resulting in slag that is typically large in size. They are strip-shaped; in most cases, they result from excessive airflow at the grates of gas stoves, causing slag masses that become red and shiny around the stove to enter the slag area. Once the temperature drops, most of these masses turn into elongated slag pieces. Based on the composition of these three types of scabs, it can also be said that none of them are normal ash residues resulting from gasification. To obtain normal ash, the equipment needs energy-efficient grates, and the process must be optimized so that the ash can form into proper lumps. VII. Hardness of slag: The hardness of slag is high; this is mainly due to operations at high furnace temperatures, as higher furnace temperatures result in slag with greater hardness. Especially in Gansu and Ningxia, where coal is used to make coal rods, the ash produced has high hardness; factors such as an excessively long downward blowing time or an overly large rotation of the downward blowing wheel can both lead to hard ash. High gangue and iron content, as well as cement-coated coal rods, can all lead to high hardness of the ash residues. VIII. Amount of coal sticks entrained in the slag: The presence of coal sticks within the slag refers to the situation where slag lumps containing these sticks are formed. This phenomenon occurs due to the downward movement of the combustion layer, high moisture content in the coal sticks, incomplete combustion, and insufficient drying processes. Generally, when using wet coal sticks for gas production, a moisture content of within 14% is appropriate; in such cases, 4–5 cycles of gas production can be achieved. An excessively long downward blowing time, along with an overly large opening degree of the downward blowing handwheel, may seem like minor issues such as coal sticks being carried within the ash and slag; yet these problems affect consumption, hinder the increase in the temperature of the gasification layer, and further impact the amount of gas produced per furnace. IX. Solid density of slag: The solid density of slag is also a key factor in assessing the quality of slag. The quality of the gasification conditions in a gas stove is also a prerequisite for ash and slag. The ash lumps should be evenly sized and have a low carbon residue content; this is what gas furnace operators truly desire the most. The excessive compactness of the ash and slag scabs is caused by overly high temperature control in the gasification layer, an imbalance between heat absorption and heat release with heat release exceeding heat absorption, excessive air supply, and insufficient steam. Based on the slag discharge conditions and the carbon dioxide content in the gas, as well as accurate process parameters, blocks with high density are processed. The greater the compactness of the ash, the more detrimental it is to the proper operation of the gas furnace; it increases the load on the ash removal mechanism, and can easily damage the transmission components located below the furnace. In normal operation, great attention must be paid to the bulk density of the ash. X. Slag and red carbon: The appearance of slag and red carbon is clearly due to a longer period during which the fire layer moves downward, and this is closely related to an excessively high speed of the grate mechanism. This is also related to excessive control of the furnace bottom temperature; as a result, the slag gasification layer moves downward, the red carbon gasification layer gets damaged, and the dry distillation layer takes over the gasification layer, severely affecting the operating conditions inside the furnace. It is very difficult to establish five new regional difficulties. The solution involves reducing the speed of the grate mechanism, increasing the amount of steam supplied from above, maintaining the temperatures both below and above the furnace to restore normal operating conditions within the furnace, and establishing a new gasification layer until the ash and slag return to normal. For manufacturers of coal briquette gasification furnaces used for producing gas from briquetted coal, regardless of the volume of gas generated or the level of energy consumption, every operator of such furnaces should strive to ensure that the slag produced is free of solid lumps, that the lumps are loose in structure, that the slag has a low density and high porosity, that it has a honeycomb-like shape, and that it generates large amounts of gas with low energy consumption. These current situations are the direction of our joint efforts. In an ammonia synthesis plant, the production department is responsible for monitoring consumption levels as well as the use of two types of gases (gas and steam). Controlling the use of gas means focusing on the key factors involved in its production, and the most crucial aspect here is the proportion of raw coal used. The first key aspect in obtaining raw materials is to grasp two theoretical aspects. The first of these theoretical aspects involves understanding and mastering the properties of coal, such as the properties of various types of coal powder, its chemical reactivity, the amount of gangue present, its calorific value, the content of fixed carbon, volatile matter, ash content, ash melting point, gasification strength, thermal stability, the content of humic acid, iron content, sulfur content, and so on. The stability of the furnace operation, as well as the amount of gas generated and the level of consumption, are all related to the aforementioned components. There are two strict criteria for coal rods before they are fed into the furnace: one is the strength of the coal rods, and the other is their moisture content. Both of these criteria have a direct impact on the amount of gas produced per furnace, and they are key factors in reducing consumption. The second theoretical aspect relates to gas production technology; when operating a gas furnace, it is necessary to understand that coal gasification is governed by kinetics. During the gasification reaction process, parameters such as pressure, flow rate, fluid properties, temperature, and resistance are the essential theoretical and operational data that every operator must be familiar with. During the production process, especially in coal rod gasification, insufficient pressure, unstable pressure, and pressure fluctuations all make it difficult to maintain stable operating conditions inside the furnace. Insufficient flow rate is a problem; multiple gas generators competing for pressure, flow, and time all pose negative factors. Temperature fluctuations, whether they are changes in the steam temperature entering the furnace or variations in the temperature as it moves upward or downward within the furnace, can be quite large; as a result, it is difficult to maintain a stable position for the gasification zone inside the furnace. Therefore, properly monitoring changes in various temperatures, as well as changes in fluid levels and flow rates, and observing the quality of ash and slag, all provide useful information for adjusting the process. The aforementioned ten indicators of ash and slag are essential practical references for assessing the condition of a furnace; mastering them helps to improve both theoretical and practical knowledge.