Simple and accurate determination method for isobaric combustion heat of substances with unknown composition or complex components

Abstract

The invention discloses a simple and accurate determination method for isobaric combustion heat of substances with unknown compositions or complex components, and belongs to determination of isobaric combustion heat. The method comprises the following steps: measuring isometric combustion heat by adopting an oxygen bomb calorimetric method, and obtaining an isobaric heat effect by measuring pressure in an oxygen bomb before and after reaction; the method is suitable for measuring the isobaric thermal effect of pure substances, unknown component substances and mixtures, the content of each component in the mixtures does not need to be measured in advance, and an ideal gas assumption is not introduced in the derivation process. The isobaric high calorific value of the standard coal sample (GBW11107) measured by the method is 30.42 MJ / Kg, and the error between the isobaric high calorific value and the standard value 30.79 MJ / Kg is only 1.2%; the isobaric high calorific value of a certain actual coal sample measured by the method is 25.097 MJ / Kg, and the error between the isobaric high calorific value measured by the method and 25.067 MJ / Kg measured according to the national standard GB / T213-2008 (coal calorific value measuring method) is only 0.11%. More importantly, the content of elements such as sulfur, nitrogen and hydrogen in the coal does not need to be measured according to a national standard method, and the isobaric high calorific value can be directly obtained.

Description

Technical Field

[0001] The present invention relates to the determination of isobaric combustion heat, and in particular to a simple and accurate determination method for the isobaric combustion heat of a substance of unknown composition or complex components. Background Art

[0002] Oxygen bomb calorimetry is a widely used method for determining the heat of combustion. This method has relatively high measurement accuracy and can be used to determine the heat of combustion of pure substances or mixtures of known components. Oxygen bomb calorimetry is carried out under constant volume conditions, so the isochoric heat effect (Q V ). However, whether in industrial applications or daily life, combustion reactions are carried out under constant pressure conditions. Therefore, determining the isobaric heat of combustion is more practical. In order to obtain the isobaric heat of combustion (Q P ), it is usually necessary to convert the isochoric heat effect of oxygen bomb calorimetry into isobaric heat effect.

[0003] According to the first law of thermodynamics, the combustion process does not do non-expansion work, and at this time Q V Equal to its internal energy change ΔU,Q P Equal to the enthalpy change ΔH. According to the definition of enthalpy H = U + pV, ΔU and ΔH have the following relationship:

[0004] ΔH=ΔU+Δ(pV) (1)

[0005] Since internal energy and enthalpy are both state functions, their changes can be calculated through the design process. The reaction enthalpy change Δ of the reactants converted into products under isobaric conditions r H, can be designed as the reactant first undergoes an isochoric process to transform into the product, and then isothermally changes p and V to make the pressure the same as the initial pressure, then Δ r H is the sum of the enthalpy changes of the two processes:

[0006] Δ r H=Δ r H1+ΔH2=Δ r U1+Δ(pV)+ΔH2 (2)

[0007] In the second process, the enthalpy change of the condensed phase (solid or liquid) and the gas isothermal change p, V process is 0, that is, ΔH2=0.

[0008] So

[0009] Δ r H=Δ r H1=Δ r U+Δ(pV) (3)

[0010] In traditional combustion heat determination experiments, the gases in the reactants and products are regarded as ideal gases. The ideal gas state equation pV = nRT can be used to obtain:

[0011] Δ r H=Δ r U+Δ(pV)=Δ r U+Δ(n)RT (4)

[0012] Right now

[0013] Q P =Q V +Δ(n)RT (5)

[0014] Where Δn is the difference between the amount of gas in the product and the reactant; gas constant R = 8.314 J mol -1 K -1 ; T is the thermodynamic temperature of the reaction.

[0015] Therefore, in the traditional oxygen bomb calorimetry, for substances with known combustion reaction equations and molecular weights, Q can be measured under constant volume conditions. V , and then obtain Q through the above conversion relationship P However, it is impossible to measure substances with unknown composition or complex ingredients, or complex pre-tests are required to determine the specific amount of each element and component in the mixture.

[0016] Furthermore, the ideal gas hypothesis assumes that gas molecules are treated as point masses with no size and no intermolecular forces other than elastic collisions. In practical applications, gases are generally considered ideal gases only under low pressure conditions. However, during measurement, the system pressure is between 1.2 and 2.5 MPa, which is not ideal. Therefore, the ideal gas approximation under these conditions can lead to errors in the measurement results. Summary of the Invention

[0017] The present invention aims to address the drawbacks of conventional oxygen bomb calorimetry, which is unable to determine the isobaric heat of combustion of materials of unknown or complex composition, or requires complex preliminary testing to determine the specific amounts of each element or component in the mixture. This method provides a simple and accurate method for determining the isobaric heat of combustion of materials of unknown or complex composition. This method uses manometric oxygen bomb calorimetry to rapidly and accurately determine the isobaric heat of combustion of materials of unknown or complex composition, including coal, straw biomass, kerosene, and others.

[0018] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0019] A simple and accurate method for determining the isobaric heat of combustion of a substance of unknown or complex composition is provided. The method comprises the following steps: the isochoric heat of combustion is measured using an oxygen bomb calorimeter, and the isobaric heat effect is obtained by measuring the pressure inside the oxygen bomb before and after the reaction. The specific principle is as follows: the oxygen bomb calorimeter is a constant volume instrument, and the process is an isochoric process. According to the following formula:

[0020] Δr H=Δ r H1=Δ r U+Δ(pV) (3)

[0021] The derivation is:

[0022] Δ r H=Δ r U+VΔp=Δ r U+V(p 反应后 -p 反应前 ) (6)

[0023] Right now

[0024] Q p =Q v +V(p 反应后 -p 反应前 ) (7)

[0025] Based on formula (7), the pressure change Δp before and after combustion is measured and multiplied by the volume V of the combustion system to achieve the constant volume heat effect Q V Transformed into constant pressure thermal effect Q P .

[0026] Furthermore, the method is applicable to the determination of the isobaric thermal effect of pure substances, substances of unknown components, and mixtures, and does not require the prior determination of the content of each component in the mixture. The ideal gas assumption is not introduced in the derivation process, and the method has the characteristics of simple operation and accurate results.

[0027] The present invention has the following advantages over existing technologies: The isobaric higher calorific value of a standard coal sample (GBW11107) measured by the present invention is 30.42 MJ / Kg, with a deviation of only 1.2% from the standard value of 30.79 MJ / Kg. The isobaric higher calorific value of an actual coal sample measured by the present invention is 25.097 MJ / Kg, with a deviation of only 0.11% from the result of 25.067 MJ / Kg measured according to the national standard GB / T213-2008 (Method for Determination of Calorific Value of Coal). More importantly, the present invention eliminates the need to determine the content of elements such as sulfur, nitrogen, and hydrogen in the coal according to the national standard method, and can directly obtain the isobaric higher calorific value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a physical picture of the pressure measuring device used in this experiment;

[0029] Figure 2 This is the connection diagram between the pressure measuring device and the oxygen bomb calorimeter used in this experiment;

[0030] Figure 3 This is a schematic diagram of the ignition wire connection method;

[0031] Figure 4This is a schematic diagram of the connection method of the straw ignition wire;

[0032] Figure 5 Schematic diagram of the sleeve structure;

[0033] Figure 6 Schematic diagram of the plug rod structure;

[0034] Figure 7 This is a schematic diagram of the sleeve and plug rod in use. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0036] The specific structure of the pressure measuring device used in the present invention is as follows Figures 5-7 As shown, the pressure measuring device includes: a sleeve 1 and a plug rod 2, the plug rod 2 is used in conjunction with the sleeve 1, the sleeve 1 is provided with a sleeve upper hole 1-1, the sleeve 1 is provided with a sleeve lower hole 1-2 below the sleeve 1, the sleeve 1 side is provided with a sleeve side hole 1-3, the plug rod 2 is provided with an external thread 2-1, the plug rod 2 is provided with a sealing groove 2-2, and the lower end of the plug rod 2 is provided with a rod needle 2-3.

[0037] The upper sleeve hole 1-1 and the lower sleeve hole 1-2 are coaxially arranged. The upper sleeve hole 1-1, the lower sleeve hole 1-2, and the sleeve side hole 1-3 are all connected and provided with internal threads. A sealing gasket is provided in the sealing groove 2-2. The sealing gasket slides and seals with the wall of the upper sleeve hole 1-1.

[0038] The working principle of the present invention is as follows: the oxygen filling nozzle of the oxygen bomb calorimeter is screwed and connected with the lower hole 1-2 of the sleeve, the precision digital display digital electronic pressure gauge is screwed and connected with the side hole 1-3 of the sleeve, the plug rod 2 is inserted into the upper hole 1-1 of the sleeve, and is screwed and connected with the internal thread of the upper hole 1-1 of the sleeve through the external thread 2-1. When the upper end of the lower hole 1-2 of the sleeve is flush with the lower end of the rod needle 2-3, the upper hole 1-1 of the sleeve is sealed by the screw connection of the external thread 2-1 and the internal thread and the sealing gasket, thereby completing the assembly of the sleeve 1 and the plug rod 2; the plug rod 2 is rotated to move the rod needle 2-3 downward and insert the oxygen filling nozzle of the oxygen bomb calorimeter. At this time, the oxygen filling nozzle of the oxygen bomb calorimeter is pushed open by the rod needle 2-3, and the internal gas of the oxygen bomb calorimeter enters through the lower hole 1-2 of the sleeve, diffuses to the side hole 1-3 of the sleeve, and is directly connected to the precision digital display digital electronic pressure gauge and displays the reading.

[0039] The pressure changes during the combustion process in the oxygen bomb calorimeter can be displayed by a precision digital electronic pressure gauge, enabling real-time measurement. The precision digital electronic pressure gauge can also be used to detect the tightness of the oxygen bomb calorimeter.

[0040] Example 1:

[0041] A 0.6182-gram actual coal sample was placed in a 300-mL oxygen bomb calorimeter with an ignition filament length of 10.7 cm and a calorific value of 3.8 J / cm. The oxygen bomb calorimeter was sealed and filled with pure oxygen. The initial pressure measured by a pressure measuring device was 1.984 MPa. After ignition, the temperature rise was 1.123 degrees Celsius, and the water equivalent was 13854.1 J / °C. The remaining ignition filament length after the reaction was 4.0 cm, and the measured pressure was 1.936 MPa. The calculated high-pressure isobaric calorific value was 25.097 MJ / Kg. The result determined according to the national standard GB / T213-2008 (Method for Determination of Calorific Value of Coal) was 25.067 MJ / Kg, with an error of 0.11%.

[0042] Example 2:

[0043] A 0.6015-gram standard coal sample was placed in a 300-mL oxygen bomb calorimeter. An ignition filament length of 11.7 cm was connected, and the calorific value of the filament was 3.8 J / cm. The oxygen bomb calorimeter was sealed and filled with pure oxygen. The initial pressure was measured by a pressure measuring device at 2.429 MPa. After ignition, the temperature rise was 1.30 degrees Celsius, and the water equivalent was 14052.4 J / °C. The remaining ignition filament length after the reaction was 5.1 cm, and the measured pressure was 2.332 MPa. The high-pressure isobaric calorific value was calculated to be 30.42 MJ / Kg, which is comparable to the standard value of 30.79 MJ / Kg, with an error of only 1.2%.

[0044] Example 3:

[0045] 1.0692 g of straw biomass solid fuel was placed in a 300 mL oxygen bomb calorimeter with an ignition filament length of 11.7 cm and a calorific value of 3.8 J / cm. The bomb calorimeter was sealed and filled with pure oxygen. The initial pressure measured by a pressure measuring device was 1.385 MPa. After ignition, the temperature rise was 1.462°C, and the water equivalent was 14212.0 J / °C. The residual ignition filament length after the reaction was 3.9 cm, and the measured pressure was 1.326 MPa. The calculated isobaric calorific value was 19.391 MJ / kg.

[0046] Example 4:

[0047] 0.7876 g of kerosene was placed in a 300 mL oxygen bomb calorimeter with an ignition filament length of 11.4 cm and a calorific value of 3.8 J / cm. The bomb calorimeter was sealed and filled with pure oxygen. The initial pressure measured by a pressure measuring device was 1.33 MPa. After ignition, the temperature rise was 2.55°C, and the water equivalent was 14080.3 J / °C. The remaining filament length after the reaction was 3.9 cm, and the measured pressure was 1.068 MPa. The calculated isobaric calorific value was 45.45 MJ / kg.

[0048] Example 5:

[0049] Frozen pears were thawed, crushed in a juicer, and filtered to obtain frozen pear juice. The sugar colloid in the frozen pears after freeze-drying was collected. 0.9657 g of the sugar was placed in a 300 mL oxygen bomb calorimeter with an ignition filament length of 10.8 cm and a calorific value of 3.8 J / cm. The oxygen bomb calorimeter was sealed and filled with pure oxygen. The initial pressure was measured by a pressure measuring device and was 1.183 MPa. The temperature rise after ignition was 1.034 degrees Celsius, and the water equivalent was 14263.8 J / °C. The residual ignition filament length after the reaction was 3.3 cm, and the measured pressure was 1.181 MPa. The calculated isobaric calorific value was 15.245 MJ / kg.

[0050] The following further introduces the specific operating steps of the oxygen bomb calorimetry:

[0051] 1. Sample preparation and pretreatment

[0052] (1) Benzoic acid, cinnamic acid, and sucrose: Weigh 1.00 g of benzoic acid, 0.80 g of cinnamic acid, and 1.2 g of sucrose, respectively, and press them into tablets using a tablet press. Hold the tablet with tweezers and gently tap it against a clean piece of paper to remove any surface debris. Accurately weigh the tablet to the nearest 0.0001 g, place it in a crucible, and set aside.

[0053] (2) Frozen pears: Prepare three varieties of frozen pears: Wangcun pear, Suizhong white pear, and Hebei crown pear. Mark the variety name, squeeze the frozen pear juice with a juicer, filter it three times with gauze, and collect the filtrate. Take about 20mL of each variety of frozen pear each time and put it into a centrifuge tube, place it in a vacuum freeze drying box and dry it for 48 hours to obtain a maltose sample. Put it into a hot water bath to melt it so that it falls off the tube wall, and finally put it into the refrigerator to freeze to obtain a block of frozen pear candy. Take about 0.7g and weigh it, record the data accurately to 0.0001g, put it into a crucible and set aside.

[0054] (3) Mixed coal, standard coal, straw, and kerosene: These samples do not require pretreatment steps such as tableting. First, peel the crucible and place 1.4-1.5 g of mixed coal (granular), standard coal (granular), or straw, and 0.6-0.7 g of kerosene (liquid) in each crucible. Weigh the samples and record the data accurately to 0.0001 g. Set aside.

[0055] 2. Sample loading and oxygen bomb filling

[0056] Place the oxygen bomb cover on the stand and put the sample into the crucible. After measuring the length of the ignition wire, connect its two ends to the two electrodes, such as Figure 3 As shown, bend the ignition wire into a U shape, close to the sample, and do not touch the crucible. When measuring powder and liquid samples, the wire needs to be buried or immersed in the sample; when measuring straw, the ignition wire needs to be wrapped around it and fixed, as shown in the figure. Figure 4 The above method has a 100% ignition success rate. After installing the oxygen bomb, use a multimeter to measure the resistance between the two electrodes; it should be less than 20Ω. If the resistance is too high or there is a short circuit between the inner walls of the bomb, open the bomb and check the ignition wire connection. Use an oxygenator to inject oxygen. For mixed coal and standard coal, the pressure should be approximately 2.5MPa, for straw, 1.3-1.4MPa, and for other samples, approximately 1.2MPa.

[0057] 3. Pressure measurement before reaction

[0058] Assemble the pressure measuring device. Connect the electronic pressure gauge to the pressure measuring device, such as Figure 1 As shown, carefully check the airtightness of the pressure measuring device. Thread the sleeve into the oxygen bomb filling nozzle, tighten, and then screw the stopper until the electronic pressure gauge displays a reading. Once the reading stabilizes, record the data to the nearest 0.001 MPa. Remove the pressure measuring device. If the oxygen bomb is slightly leaking, refill it with oxygen to the same pressure. Assuming the oxygen cylinder's filling pressure limit remains unchanged, the pressure error between the two fillings should be within a reasonable range of 0.005 MPa.

[0059] 4. Temperature measurement

[0060] Place the oxygen bomb on the base of the inner drum. Use a graduated cylinder to measure 3000mL of water and pour it into the inner drum, completely immersing the oxygen bomb in the water. Ensure that the amount of water added to the inner drum is consistent each time. Check the oxygen bomb for leaks. Close the lid of the calorimeter and insert the measuring probe into the outer jacket to measure the ambient temperature (point J on the Reynolds calibration curve). Then, carefully insert the measuring probe into the oxygen bomb system through the small hole in the lid. Turn on stirring. Once the system temperature stabilizes, record the inner drum temperature every 5 seconds (automatically recorded by the computer, accurate to 0.001°C). After 10-12 minutes, press the ignition button on the controller. The indicator light will display "off-on-off," followed by a rapid temperature rise, indicating successful ignition. Continue recording for 5 minutes until the cooling rate becomes constant. Turn off all power switches, remove the measuring probe, open the calorimeter, and remove the oxygen bomb.

[0061] 5. Post-reaction pressure measurement and data calculation

[0062] Connect the pressure measuring device to the oxygen bomb and proceed as in "Step 3." Record the pressure inside the oxygen bomb after the reaction. After removing the pressure measuring device, open the vent valve to release any remaining gas. Open the oxygen bomb to check for complete combustion. If there is carbon deposits or unburned sample inside the bomb, re-measure. Measure the remaining length of the ignition wire and calculate the actual length of the ignition wire burned.

[0063] In this experiment, benzoic acid (Q V =-26434 J / g) as the standard sample to measure the water equivalent, and the heat capacity (water equivalent) of the system is calculated by computer. Based on this water equivalent, the combustion heat release Q (J) of the remaining samples is measured and calculated for further data processing.

[0064] In this experiment, all samples burned completely except for a small amount of residual ash after the mixed coal was burned. Ash in mixed coal is normal; accurately weigh the residual ash and subtract it from the remaining ash when calculating.

Claims

1. A simple and accurate method for determining the isobaric heat of combustion of a substance of unknown or complex composition, characterized by: The method is as follows: the oxygen bomb calorimeter is used to measure the isochoric heat of combustion, and the isobaric heat effect is obtained by measuring the pressure inside the oxygen bomb before and after the reaction; the specific principle is as follows: the oxygen bomb calorimeter is a constant volume instrument, and the process is an isochoric process. According to the following formula D r H=D r H1=D r U+Δ(pV) (3) The derivation is: D r H=D r U+VΔp=Δ r U+V(p 反应后 -p 反应前 ) (6) Right now Q p =Q v +V(p 反应后 -p 反应前 ) (7) Based on formula (7), the pressure change Δp before and after combustion is measured and multiplied by the volume V of the combustion system to achieve the constant volume heat effect Q V Transformed into constant pressure thermal effect Q P .

2. A simple and accurate method for determining the isobaric heat of combustion of a substance of unknown composition or complex components according to claim 1, characterized in that: The method is applicable to the determination of the isobaric thermal effect of pure substances, substances with unknown components, and mixtures. It does not require the prior determination of the content of each component in the mixture, and the ideal gas assumption is not introduced in the derivation process.

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