3.2. Determination of Biomass in Scrap Tires, period 2010–2016
To measure the biomass, biannual surveys of scrap tires were taken in concordance with three sampling surveys. There are 12 samples for each type of vehicle and for each survey.
Table 2 shows the outcomes of the undefined tires, which are the result of having carried out four thermogravimetric tests of each sample to obtain the percentages of NR, SBR, and black carbon for each indefinite tire of each survey.
Natural rubber was analyzed first in light vehicles. Its average value increases 8.75% from survey 1 until 3 and decreases 0.76% from survey 3 to 4. The maximum value is seen in survey 3 with a difference of 2.13% with respect to survey 4. The minimum value is identified in survey 1, which, with respect to survey 3, increases 9.73% and decreases 1.05% in survey 4.
In the analysis of SBR, the average value decrease 8.6% from survey 2 to 3 and increases 1.46% from survey 3 to 4. The maximum value decreases 8.02% from survey 1 to 2, and from survey 2 until 4 this value remains practically unchanged. The minimum value can be found in survey 3, it decreases 10.83% from survey 1 until 3, and increases 2.15% in survey 4 with respect to survey 3.
The highest average value for the analysis of black carbon is in survey 2, with a previous increase of 5.51% from survey 1, and with a decrease of the same percentage from survey 2 until 4. The maximum value in survey 2 decreases around 2.00% until survey 4. The minimum value increases 1.99% from survey 1 to 2, decreases 1.24% from survey 2 to 3.
The highest average value of natural rubber in heavy vehicles corresponds to survey 1 followed by survey 4, with 7.81% difference between both. In survey 2, there is a decrease of 21.95% with respect to survey 1, and in survey 3 it increases by 11.72% with respect to survey 2. The maximum value can be seen in survey 1 with a difference of 18.86% with respect to the following maximum value in survey 3, this value remains practically unchanged from survey 3 to 4. The minimum value is observed in survey 1, which increases until survey 4 by 12.81%.
For the analysis of natural rubber in heavy vehicles: When it comes to the SBR analysis, the average value increases by 26.97% from survey 1 to 2, decreases around 17.08% from survey 2 to 3, and then decreases 2.34% from survey 3 to 4. The maximum value is appreciated in survey 2 with a difference of 0.16% with survey 1, and this value decreases by 11.30% until survey 4. The minimum value is observed in survey 1, which has a difference of 28.78% with respect to survey 2, it decreases by 13.91% in survey 3, and an increase of 1.78% is observed in survey 4.
The analysis of black carbon shows that the highest average value is observed in survey 2 and it has a decrease of 9.18% until survey 4. The maximum value is in survey 2, decreases 3.25% until survey 3, and increases 0.51% from survey 3 to 4. In survey 1, the minimum value increases by 3.49% from survey 1 to 2 and decreases 1.74% from survey 2 until 4.
Additionally, mono-brand samples were taken to see the evolution of biomass content by isolating one of the variables involved,
Table 3. Both samples are from light vehicles. In the study carried out for the two mono-brand samples specifically, the maximum SBR value is 39.37%, the minimum value is 22.62% in natural rubber, and the highest SBR average value is 39.20%. According to the standard deviation, the highest value is 2%.
3.3. Calibration Line
3.3.1. Patterns, Data, and Composition
To obtain the calibration line (NR/(NR + SBR)), four pattern samples, composed by the known mixture of NR and SBR in different proportions, were prepared, with the proportion of black carbon fixed in the four pattern samples. Three new concentration patterns have also been obtained close to the real samples. The results are the seven patterns shown in
Table 4.
3.3.2. Determination of the Calibration Line
The laboratory has used one calibration line made with pattern samples and known NBR/(NR + SBR) ratios vulcanized solely for these analyses. With the calibration line, for both light and heavy vehicles, the surveys were analyzed to find out the most and least favourable sample, and the calibration line was also used to find out the evolution of the surveys. In the following graphs, we considered supplier 1 from survey 1 but supplier 2 has been discarded.
Figure 5 shows the result of the different surveys and tests carried out in this study.
3.3.3. Analysis of Results of the Second Survey, 2011–2012
When talking about light vehicles, the percentage differences are 2 and 1.7 in absolute values with the calibration line, and in deviation terms all samples obtain expected deviations: Less than 1, being the maximum value 0.88. The maximum value from survey 2 increased by 6.6% with respect to survey 1, the minimum value rose almost 12.7% and the average value increased 10%.
Regarding heavy vehicles, the percentage differences are 1.43 and 1.64 in absolute values with the calibration line. The maximum value from survey 2 decreased by 47% with respect to survey 1, the minimum value rose almost 4.7% and the average value decreased by 21%.
3.3.4. Analysis of Results of the Third Survey, 2013–2014
The percentage differences of light vehicles are 3.27 and 5.8 in absolute values with the calibration line. The biomass value has a range between 37.37% and 45.7%. Compared to survey 1, the biomass value in tires of light vehicles has a range between 22% and 31% and with an average value of 26.5%, regarding survey 2 its average value is 36.18%. The maximum and minimum values for survey 3 increase around 15% regarding survey 1, and the average increases 15%, and the best biomass value can be seen through the surveys.
The two mono-brand samples are very close to the calibration line, with a percentage difference of 1.46 and 1.82 in absolute values. The biomass value has a range between 36.5% and 39.5%.
Regarding the values obtained for tires of heavy vehicles, the percentage differences are 3.27 and 5.86 in absolute values with the calibration line. Moreover, in deviation terms all samples are between 1.1 and 1.7, being the maximum value 1.63. The maximum value decreases 23% and the minimum value increases 14% with respect to survey 1. The average value is 4 points lower than that of survey 1 and 17.04 points upper than that of survey 2.
3.3.5. Analysis of Results of the Fourth Survey, 2015–2016
For the analysis of light vehicles, the percentage differences are 1.2 and 2 in absolute values with the calibration line, and in deviation terms all samples are below 1. The biomass value has a range between 35.84% and 42.39%, it increases by 12.62% with respect to survey 1. The maximum value has increased by 11.39% and the minimum value has an increase of 13.93%, both values with respect to survey 1.
The percentage differences in heavy vehicles are 11.68 and 14.44 in absolute values with the calibration line, and in deviation terms all samples are between 0.9 and 1.1. The biomass has an average value of 61.35% and this value decreases 3.15% with respect to survey 1. The maximum value has decreased by 25.3% and the minimum value has increased 19%, both values with respect to survey 1.
3.3.6. Undifferentiated Scrap Tire and Renewable Energy Assessment
Considering the renewal of the vehicle fleet, light and heavy vehicles determine the percentage of scrap tires for each survey,
Table 5.
Table 6 shows the biomass content in scrap tires for each survey, the kilograms of biomass in said scrap tires, and the clean energy produced. Biomass content is calculated with Equations (1) and (2), whose results are in
Table 2 and
Table 5, respectively. In Equation (3), the kilograms of biomass included in scrap tires are identified for each survey, this includes the scrap tires used in the 22 Spanish cement plants. Moreover, the measurement of clean energy has been done according to the average calorific value of the scrap tires referenced in the previous sections and the kilograms of biomass included in scrap tires.
Looking at the variation of biomass content in scrap tires from survey 1 to 2 there is a decrement of 6.21%, from survey 2 to 3 an increment of 6.72%, and from survey 3 to 4 an increment of 0.72%. The kilograms of biomass included in scrap tires have a progressive increase from survey 1 to 4, with a total increase of 37,557.84 Kg. Clean energy has a progressive increase from survey 1 to survey 4 of 1,314,524.48 MJ.