Introduction
Coal is the main fuel for cement production in China and represents one of the most important indicators for raw-fuel consumption in cement production. Coal quality influences not only the yield and quality of cement clinker production, but also its costs, since high-quality coal with a higher calorific value enables higher process temperatures or, at constant process temperature, reduces coal demand, thereby saving fuel costs. Modern testing methods and improvement of coal quality are important factors for ensuring a reduction in coal consumption and an improvement in the quality of cement products.
The combustion behavior of coal in the cement industry is complex. Conventional industrial analytical evaluation methods [1] are no longer applicable. One example is AdiabaticAdiabatic describes a system or measurement mode without any heat exchange with the surroundings. This mode can be realized using a calorimeter device according to the method of accelerating rate calorimetry (ARC®). The main purpose of such a device is to study scenarios and thermal runaway reactions. A short description of the adiabatic mode is “no heat in – no heat out”.adiabatic calorimetry, in which coal is placed in an AdiabaticAdiabatic describes a system or measurement mode without any heat exchange with the surroundings. This mode can be realized using a calorimeter device according to the method of accelerating rate calorimetry (ARC®). The main purpose of such a device is to study scenarios and thermal runaway reactions. A short description of the adiabatic mode is “no heat in – no heat out”.adiabatic vessel and the heat content of the coal is calculated by measuring the temperature change inside the vessel. Although this method requires only simple equipment, the measurement results are relatively inaccurate.
In this work, material characterization of two coal types is carried out by simultaneous thermal analysis using a NETZSCH STA Jupiter® instrument. Based on thermogravimetric (TG) and differential calorimetry (DSC) results, important combustion parameters were determined, including the onset and end temperatures (Tonset, Tend) of the exotherm, the temperature of initial mass loss (Ti), the maximum mass-loss rate (Vmax), the total ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic heat (q), and the average combustion rate (v).
Based on these parameters, the evaluation indices
- combustion intensity (I),
- average heat-release intensity (Q), and
- the combustion characteristic index (S)
were calculated. These indices provide a quantitative basis for evaluating the combustion performance of coal. The requirements for coal use in the cement industry are optimum combustion behavior through low combustion intensity, high average heat-release intensity, and a high combustion characteristic index. This reflects a favorable balance between ignition behavior, heat release, and overall reactivity.
Method and Measurement Parameters
Two different coal samples (coal A and B, mass approximately 10 mg) were heated from room temperature to 1000 °C at a heating rate of 10 K/min in air. Table 1 shows all important parameters of the TG and DSC curves, together with their symbols, units, and definitions.
Combustion indices:
The individual measured values are subsequently used to determine combustion intensity, average heat-release intensity, and the combustion characteristic index [2] (see table 1 for abbreviations).
a) Combustion intensity (I)
The combustion intensity of coal indicates the intensity of instantaneous heat release in this ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic process and is defined as:
b) Average heat-release intensity (Q)
The average ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic intensity of coal indicates the amount of heat released per unit mass of coal and per unit temperature during the period from the beginning to the end of the coal's ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic activity and is defined as:
c) Combustion characteristic index (S)
The combustion characteristics of coal are comprehensive indicators of its quality, including flammability, ignition point, combustion intensity, and combustion duration. From these aspects and taking the relevant literature into account
Table 1: Important parameters of the TG and DSC curves with symbol, unit, and definition.
| Parameter | Symbol | Unit | Definition |
|---|---|---|---|
| Thermogravimetry (TG) | |||
| Onset temperature of mass loss | Ti | °C | The temperature at which the sample begins to lose mass (ignition temperature). |
| Combustion time | t98% | min | Time required to burn 98% of the fuel. |
| Combustion temperature | T98% | °C | Temperature at which 98% of the fuel has been burned. |
| Average combustion rate | V | %/min | Total mass loss divided by combustion time. |
| Differential thermogravimetry (DTG) | |||
| Maximum mass-loss rate | Vmax | %/min | The maximum mass-loss rate. |
| Differential scanning calorimetry (DSC) | |||
| Onset temperature of the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak | Tonset | °C | Temperature at the beginning of the ExothermicA sample transition or a reaction is exothermic if heat is generated. exothermic effect. |
| End temperature of the ExothermicA sample transition or a reaction is exothermic if heat is generated. exothermic peak | Tend | °C | Temperature at the end of the ExothermicA sample transition or a reaction is exothermic if heat is generated. exothermic effect. |
| Area of the ExothermicA sample transition or a reaction is exothermic if heat is generated. exothermic peak | q | J/g | Combustion enthalpy of the sample. |
| Peak height of the exotherm | H | mW/mg | Height of the ExothermicA sample transition or a reaction is exothermic if heat is generated. exothermic peak relative to the DSC baseline. |
| Half-width of the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak | W | min | Time corresponding to the half-width of the exotherm. |
Results and Discussion
The TG and DSC curves of coal A and coal B are shown in figures 1 and 2. Both samples show two mass-loss steps. The first, up to approximately 150 °C, can presumably be attributed to the release of moisture. In comparison, coal A and B show similar moisture contents here (1.8% and 1.6%, respectively). The second, considerably larger mass-loss step corresponds to thermal Decomposition reactionA decomposition reaction is a thermally induced reaction of a chemical compound forming solid and/or gaseous products. decomposition and combustion of the coal samples.
The curves provide the basic parameters of the thermal properties of the two coal types. Table 2 lists the results for the individual samples.
The ignition temperatures of coal sample A (412 °C) and coal sample B (416 °C) show similar values, with the temperature of coal sample B slightly higher. The higher the ignition temperature, the more difficult it is to ignite the coal. The combustion enthalpy of coal B is also higher than that of coal A. The higher the combustion enthalpy, the better the quality of the coal. Another important parameter when using coal is combustion time: the longer the duration, the poorer the combustion performance. A comparison of both samples shows that coal sample A, at almost 28 minutes, is slightly above coal sample B (approximately 26 minutes).
The three evaluation parameters for the two coal types are listed in table 3. For the cement industry, the higher the combustion characteristic index (S), the better the combustion performance of the coal and the better its quality. However, the overall combustion performance of coal cannot be determined from this index alone. Combustion intensity (I) and average heat-release intensity (Q) must also be considered. The coal used should therefore have low combustion intensity (I), high average heat-release intensity (Q), and a high combustion characteristic index (S) to be suitable for use in the cement industry.
Under this consideration and with reference to table 3, it can be seen that coal A has better combustion performance than coal B due to its lower combustion intensity and higher combustion characteristic index.
Table 2: Measurement results of the TG and DSC measurements of coal samples A and B.
| Parameter | Unit | Coal A | Coal B |
|---|---|---|---|
| Ti | °C | 412 | 416 |
| H | mW/mg | 12.24 | 13.16 |
| W | min | 27.80 | 25.80 |
| q | J/g | 20509 | 20520 |
| Tonset | °C | 247 | 248 |
| Tend | °C | 706 | 689 |
| t98% | min | 26.8 | 24.8 |
| T98% | °C | 692 | 676 |
| V | %/min | 3.150 | 3.394 |
Ti: ignition temperature; H: height of the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak; W: half-width of the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak; q: combustion enthalpy; Tonset: onset temperature of the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak; Tend: end temperature of the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak; t98%: combustion time; T98%: combustion temperature; V: average combustion rate; Vmax: maximum mass-loss rate
Table 3: Calculated combustion parameters of coal samples A and B.
| Parameter | Unit | Coal A | Coal B |
|---|---|---|---|
| Combustion intensity I | mW/mg·min | 0.440 | 0.525 |
| Average heat-release intensity Q | J/g·°C | 44.721 | 46.594 |
| Combustion characteristic index S | 5.46 x 10-9 | 5.35 x 10-9 |
Summary
The combustion characteristics of different coal samples can be investigated using the NETZSCH STA Jupiter®. Fundamental parameters of the coal samples during the combustion process can be determined from TG, DTG, and DSC measurement results, for example ignition temperature, heat release, and combustion time.
Based on these measured values, comprehensive indices such as combustion intensity, average heat-release intensity, and the combustion characteristic index can additionally be calculated. These evaluation indices provide an integrated description of the combustion behavior of coal and enable a more reliable comparison between different coal samples, supporting selection of the coal best suited to a particular application.