3.1. Study on the Promotion Mechanism of Nitromethane and Hydrogen Peroxide to Methanol Combustion
The chemical reaction mechanism of methanol combustion was analyzed. The chemical mechanism used here is a detailed chemical reaction mechanism of methanol combustion constructed by Li [
19]. The mechanism that has been experimentally verified from low temperature to high temperature is comprehensive. During the combustion of methanol, methanol undergoes a series of dehydrogenation processes and oxidation processes to generate CO, and then produces the final product CO
2 through reaction with OH radicals. The main reaction pathway was shown in
Figure 5. Among them, OH was the most important free radical in the reaction. In theory, the combustion of methanol can be promoted by increasing the amount of OH as well as other hydroxyl radicals.
The chemical reaction kinetics analysis shows that the strong oxidant hydrogen peroxide will produce a large number of OH through reaction H2O2 (+M) = OH + OH (+M). The Nitromethane pyrolysis reaction products, CH3 and NO2, will consume mainly through CH3 + HO2 = CH3O + OH, NO2 + H = NO + OH and NO + HO2 = NO2 + OH under certain conditions. These reactions above will transform HO2 into a more active free radical OH. Therefore, it was proposed to add nitromethane and hydrogen peroxide to enhance the combustion characteristics in the micro space of methanol fuel.
The combustion kinetics of methanol and hydrogen peroxide mixed fuel was analyzed. The chemical mechanism used here is a detailed chemical reaction mechanism of methanol combustion constructed by Li [
19]. Li constructed a detailed chemical reaction mechanism for methanol combustion based on summarizing the mechanisms that appeared in the past, and Li has verified the mechanism by the way of experiments from low temperature to high temperature. The mechanisms that are comprehensive can predict methanol combustion in a wider range. The combustion process simulation was performed by using a closed homogeneous reactor model provided in the Chemkin-PRO program.
Figure 6 was the effect of different hydrogen peroxide ratios on ignition delay when the equivalent ratio Φ = 1; the initial pressure is 3 MPa. It was obvious that, when the methanol fuel was added with hydrogen peroxide, its ignition delay was shorter than that of the pure methanol; the greater the proportion of the increase, the more favorable for the ignition.
Figure 7 shows combustion analysis sensitivity of methanol and methanol–hydrogen peroxide hybrid fuel by using the closed homogenous reactor model provided in the Chemkin-PRO program (Version 4.5, Ansys Inc., Canonsburg, PA, USA). Here, the equivalent ratio is 1, initial temperature and pressure are respectively 800 K and 3 MPa, and the mixture contains 10% volume of hydrogen peroxide. The
Figure 7 shows the eight elementary reactions with the largest temperature coefficients. As shown in the Figure, for pure methanol, sensitivity analysis shows that the elementary reaction with the maximum positive temperature sensitivity coefficient was CH
3OH + O
2 = CH
2OH + HO
2, and the second one is CH
3OH + HO
2 = CH
2OH + H
2O
2. Here, positive temperature sensitivity coefficient means that the elementary reaction promotes the temperature increase, and promotes the combustion. After the addition of hydrogen peroxide, the elementary reaction with the maximum positive temperature sensitivity coefficient was H
2O
2 (+M) = OH + OH (+M). The reaction of CH
3OH + O
2 = CH
2OH + HO
2 has no significant effect on temperature. It could be seen that H
2O
2 produces OH through reaction H
2O
2 (+M) = OH + OH (+M). The addition of hydrogen peroxide directly added the active radical OH into the whole reaction, shortening the chemical reaction chain and accelerating the combustion.
In order to analyze the effect of nitromethane on methanol combustion, the chemical reaction mechanism of methanol and nitromethane mixed fuel combustion was constructed [
17,
18,
19,
20,
21,
22,
23], including the sub mechanism of methanol and nitromethane combustion and the oxidation reaction sub-mechanism of methanol and nitrogen oxide, with a total of 52 components and 240 elementary reactions. (The mechanism can be seen in
Table S1.) The mechanism has been verified by the engine combustion test.
Figure 8 was the ignition delay of different nitromethane adding ratios when the equivalence ratio was 1, and the initial pressure is 3 MPa. Obviously, the addition of nitromethane will reduce the ignition delay of methanol, which was favorable for ignition.
In order to further analyze the effect mechanism of the nitromethane on the combustion of methanol, the reaction path of the main components of methanol and nitromethane mixed fuel combustion was analyzed, which was shown as
Figure 9.
Figure 9 shows that the main reactions of methanol are as follows: CH
3OH is consumed primarily by dehydrogenation reactions with hydrogen–oxygen free radicals and oxygen, as well as NO
X. Among them, the most methanol is consumed in the reaction with OH. There is about 42% methanol consumed in the reaction of CH
3OH + OH = CH
2OH + H
2O, 29% methanol was consumed in the reaction of CH
3OH + OH = CH
3O + H
2O, and 19% methanol is consumed in the reaction of CH
3OH + HO
2 = CH
2OH + H
2O
2. The above three reactions are the most important consumption routes for methanol. The rate of low temperature starting reaction of methanol (CH
3OH + O
2 = CH
2OH + HO
2) is very low, which consumed about 3% methanol. The dehydrogenation reaction between methanol and NO
X consumes less than 10% of methanol.
CH2OH is consumed by dehydrogenation reaction, and the vast majority of CH2OH is consumed by reaction CH2OH + O2 = CH2O + HO2. In addition to being generated by the dehydrogenation reaction of CH3OH, CH3O is also generated by reaction CH3 + NO2 = CH3O + NO and CH3 + HO2 = CH3O + OH. The vast majority of CH3O is consumed by high temperature decomposition reaction of CH3O + M = M + CH2O + H + M and dehydrogenation reaction of CH3O + O2 = CH2O + HO2. The main consumption reaction of CH2O is CH2O + OH = HCO + H2O. HCO generates CO mainly by way of the dehydrogenation reaction of HCO + O2 = CO + HO2. Most CO is oxidized by OH into CO2.
It also can be seen that the addition of nitromethane has little effect on the main reaction pathway of methanol, but it adds many important branched chain reactions to make the reaction more complicated. The main reaction pathway of nitromethane was its decomposition reaction of CH3NO2 (+M) = CH3 + NO2 (+M), which was also the main source of CH3 and NO2, while the majority of CH3 was consumed by the reaction of CH3 + HO2 = CH3O + OH, resulting in OH during the reaction; NO2 was mainly consumed by the reaction of NO2 + H = NO + OH and CH3 + NO2 = CH3O + NO, which was also the main production reaction of NO; the generated NO was consumed through the reaction of NO + HO2 = NO2 + OH and NO + HCO = HNO + CO, during which OH was produced; until then, the nitromethane was finally integrated into the main reaction pathway of methanol.
Thus, nitromethane generated CH3 and NO2 through its own decomposition reaction, and generated a lot more active OH through the above reaction. In addition, at the same time, in the process, the generated NO and NO2 with stronger activity will be involved in the dehydrogenation reaction of methanol and its dehydrogenation intermediate products, making the whole chain reaction various, thereby enhancing the combustion of methanol.
3.2. Test of Methanol Combustion Enhanced by Nitromethane and Hydrogen Peroxide
In order to test the effect of nitromethane and hydrogen peroxide on the combustion characteristics of a micro piston internal combustion engine of methanol with the glow ignition of platinum wire, two kinds of mixed fuels of methanol/nitromethane and methanol/hydrogen peroxide were prepared for combustion diagnosis, respectively. Among them, the volumetric ratio of nitromethane, hydrogen peroxide, and mixed fuel was 10%, the purity of nitromethane was 99.99%, and hydrogen peroxide was analytically pure; its mass fraction was 30%.
After adding nitromethane and hydrogen peroxide, the measured cylinder pressure and instantaneous heat release rate of the micro-piston internal combustion engine are shown in
Figure 10. As it can be seen from the Figure, after adding nitromethane, the cylinder pressure significantly increased, the heat release rate increased significantly too, and the combustion duration was short. It was calculated from the indicator diagram that, after adding nitromethane, the mean indicated pressure increased significantly. Its
Pmi value increased from 0.137 MPa at pure methanol to 0.182 MPa at present, with an increase of 30%. However, after adding hydrogen peroxide solution, the cylinder combustion conditions worsen, and the cylinder pressure reduced, the combustion heat release rate slowed, and the combustion duration extended.
In addition, after adding nitromethane, the onset of combustion was significantly earlier, with the corresponding CA05 obtained from the cumulative heat release rate advanced to 8.5 °CA ATDC, as shown in
Table 4, which was about 4 °CA ahead of pure methanol, and the combustion duration (CA05 to CA90) was shortened by about 3 °CA. However, after the addition of hydrogen peroxide solution, the combustion situation in the cylinder became worse, the ignition time was delayed, the combustion heat release rate slowed down, and the combustion duration was prolonged. The crank angle corresponding CA05 obtained from the cumulative release rate was delayed to 24.7 °CA ATDC, and the crank angle corresponding CA90 was delayed to 78.5CA ATDC. The combustion duration (CA05 to CA90) was increased to 54 °CA. The mean indicated pressure
Pmi value measured drop to 0.078 MPa with a large decrease rate.
It could be seen that the effect of hydrogen peroxide solution on the combustion of methanol in micro space was poor, which was different from the theoretical analysis of the previous. The cause was analyzed as follows: it may be because the mass fraction of water in the hydrogen peroxide solution was as high as 70%, the presence of amounts of aqueous solutions counteracts the effect of the increase in the mole fraction of OH. In addition, it is assumed to have the same initial conditions in the theoretical analysis, but, in the actual process, there are differences in the properties of different fuels. When the liquid phase is changed to a gas phase, the temperature of the mixture will change. For hydrogen peroxide with low mass fraction, there is too much water. The latent heat of vaporization of water (2257.2 kJ/kg) is about twice as much as methanol. The injection of hydrogen peroxide solution makes the temperature of the initial mixture lower, resulting in a negative impact on the combustion.
When two kinds of mixed fuels are used, the change in the mean indicated pressure
Pmi of the 120 continuous test cycles was shown in
Figure 11. It can be seen from the Figure that, compared with the no nitromethane fuel, after adding nitromethane, the range of
Pmi value tends to concentrate and the number of misfire cycles decreases.
Among them, the Pmi cycle variation rate of being fueled with methanol/nitromethane decreased from 60% to 16.9% when no nitromethane was used. It could be seen that the effect of the nitromethane additive on the combustion cycle fluctuations in the glow ignition mode of the platinum wire in the micro piston internal combustion engine was obvious. However, after adding the additive of hydrogen peroxide, the number of misfire cycles increased, and the Pmi cycle variation rate increased to 116%.
Figure 12 shows the correlation among the mean indicated pressure
Pmi, the duration of combustion, and the starting point of combustion CA05 when methanol and two mixed fuels are burned. As pure methanol, the change range of CA05 was large, which was more than 40 °CA. The ignition time was relatively delayed, and there are more cycles in which CA05 appears after 25 °CA ATDC, resulting in a serious misfire,
Pmi decreased rapidly. After adding the nitromethane additive, the range of CA05 variation was narrowed and CA05 concentrated between 0 °CA ATDC and 20 °CA ATDC, resulting in a lower misfire rate and enhanced combustion stability. CA05 showed an early trend, with a significant reduction in the number of cycles in which CA05 appears after 25 °CA ATDC. After the hydrogen peroxide solution was added, the ignition time was severely delayed, and the number of cycles in which CA05 appears after 30 °CA ATDC increased rapidly, resulting in a serious misfire.
It can be seen from
Figure 12 that the change of combustion duration had a strong correlation with CA05. With the delay of CA05, the duration of combustion significantly prolonged. Only when CA05 appeared after 25 °CA ATDC was there a superficial phenomenon of a rapid decrease in combustion duration due to misfire. After adding nitromethane, CA05 was ahead of time, and the duration of the combustion had a shortening trend, which was conducive to the reduction in the cyclical changes.
From those above analyses, it could be seen that the combustion-supporting characteristics of nitromethane additive in micro-space were much better than that of hydrogen peroxide. Nitromethane can promote CA05 in advance, and shorten the duration of combustion, lower the cycle change, and enhance the power performance. However, after adding the hydrogen peroxide solution, the combustion rate decreased, the combustion stability deteriorated, the misfire cycle increased, and it did not show the expected combustion-supporting effect.