Life Cycle Energy Analysis and Evaluation of Retreaded Engineering Tires

In this paper, energy consumption models of retreaded engineering tires were constructed based on life cycle analysis, theoretical calculation model, and energy consumption method during the four stages of retreaded engineering tires, i.e., production, transportation, usage, and recycling stage. The energy substitute model and energy evaluation index during the recycling stage, which involves one of five classical retreaded engineering tire recycling methods, i.e., secondary retreading, mechanical smash, low-temperature smash, combustion decomposition, and combustion power generation, were presented. Life cycle energy analysis of retreaded engineering tires was conducted, and the energy consumption during the different life cycle stages was quantitatively analyzed, thus obtaining the energy restoration rate of the five classical recycling stages of retreaded engineering tires. Energy consumption analysis and energy evaluation at different stages were performed. Main conclusions indicate that the energy consumption during the production stage is the highest, and energy consumption during the transportation stage is the lowest. The energy recycling result of the secondary retreading or combustion decomposition of retreaded engineering tires is obtained.


Introduction
The output of Chinese tires can reach about two or three times the global average level and will account for 50% of the global output, and yearly productive capacity will be over 1 billion. In the future, China will become a major tire-producing country and the tire trading center of the world [1][2][3]. At present, about two hundred million old and useless tires are generated every year in China; this number will progressively increase at a surprising speed, thus resulting in the increasingly serious problem of "black pollution" [4][5][6]. At present, the yearly production capacity of engineering tires is about 10 million, which takes up about 1% of total tire production and accounts for about 8% of sale figures. Engineering tires are relatively larger than normal tires and have a weight that is about equal to that of several truck tires or tens of passenger tires. Moreover, the rubber resources consumed by engineering tires during production account for about 15% of total rubber; thus, the corresponding added value is about 30%-50% more than that of other tires [7][8]. Therefore, improving the operation rate of junk engineering tires, especially improving the retreading rate, will promote the development of the retreaded engineering tire industry. Studies on the effect of retreaded engineering tires on enterprises, society, and the environment lack a systematic approach, direction, quantitative analysis, and evaluation. For this reason, this paper constructs a consumption model of retreaded engineering tires considering the index of 26.5R25 retreaded engineering tire life cycle as the evaluation standard. It also qualitatively and quantitatively describes and evaluates the energy recovery rate (ERR) of five processing methods during the recycling stage of retreaded engineering tires. Lastly, it provides theoretic guidance to promote the application of retreaded engineering tires and policymaking of the retreaded tire industry[9-10].

Life cycle energy consumption analysis of retreaded engineering tires
The life cycle of retreaded engineering tires consists of four stages: tire production, transportation of raw materials and finished goods, tire usage, and recycling. Natural resources and energy are consumed in various degrees at every stage, and the recycle function of new resource and energy occurs at the recycling stage [9][10]. The recycling stage of retreaded engineering tires has been mainly studied through energy consumption analysis of five classical recycling techniques: secondary retreading, mechanical smash, low-temperature smash, combustion decomposition, and combustion power generation.

Energy consumption model
An energy consumption model of the retreaded engineering tires' life cycle is built based on the first law of thermodynamics. The energy relationship at four stages is different. The corresponding energy function matches each stage. The energy consumption models of the retreaded engineering tires' life cycle constructed through superposition principle are shown in formula (1) .
where TE is the total energy consumed; 1 TE is the energy consumed during the production stage; 2 TE is the energy consumed during the transportation stage; 3 TE is the energy consumed during the usage stage; and 4 TE is the energy consumed when old and useless retreaded engineering tires are recycled.

Energy consumption during production stage
The energy consumption of retreaded engineering tires during the production stage is influenced by consumed materials and energy, and it is calculated according to formula (2).
where 1 TE is the energy consumption during the production stage; i PM is the raw material i consumption during the production stage; i PM  is the raw material energy density during the production stage; j PE is the energy consumption of energy j during the production stage; and j PE  is the energy density of energy j during the production stage.

Energy consumption during the transportation stage
The transportation stage of retreaded engineering tires consists of three parts: from raw material to producing site, from producing site to sale site, and from the collection site of old and useless tires to the recycling disposal site. Energy consumption during the transportation stage is influenced by transportation method, transportation distance, and fuel used by transportation tools, and it is calculated using formula (3).
where 2 TE is the energy consumption during the transportation stage; TD is the average running mile during the transportation stage; TE is the energy consumption amount during the transportation stage; and  TE is the energy density during the transportation stage.

Energy consumption during the usage stage
The energy consumption of retreaded engineering tires during the usage stage is influenced by average running mile and used fuel, and it is computed using formula (4).
In the formula, 3 TE is the energy consumption during transportation; UD is the average running mile during the usage stage; UE is the energy consumption amount during the usage stage; and  UE is the energy density of the energy during the usage stage.

Energy consumption during the recycling stage
Energy consumption and energy recycling occur simultaneously during the recycling stage of retreaded engineering tires. The energy consumption of the retreaded engineering tires during the recycling stage is computed according to formula (5).

Alternative energy model
New production and energy occur in the five classical recycling methods during the recycling stage of retreaded engineering tires, which are considered alternative energy during the research process and whose value is equal to the needed energy consumption during production. Its value during the recycling stage is calculated by formula (6).
where AE is the alternative energy; i RPP is the output of production i ; i RPE  is the energy density of production i ; j RE  is the output of energy production j ; and j RE  is the energy density of energy production j .

Energy evaluation index
NES expresses the relation between the production and consumption of alternative energy during the recycling stage of retreaded engineering tires, as shown in formula (7). 4

TE AE NES
  (7) ERR expresses the recovery degree of input energy caused by five recycling techniques during the recycling stage of retreaded engineering tires. The ERR's value is the percentage ratio of how much output energy accounts for input energy during the recycling stage (mainly including energy consumption during the production and recycling stages), as shown in formula (8 Table 1.shows that the energy consumption of one ton of retreaded engineering tires throughout its life cycle is about 144607 MJ. The maximum energy consumption of a retreaded engineering tire during the production stage is about 132913 MJ, which is about 91.91% of the energy consumption throughout its life cycle. The usage stage consumes about 10591 MJ, which is 7.32% of the energy consumption of a retreaded engineering tire throughout its life cycle. The last transportation stage consumes about 1103 MJ, which is 0.77% of the energy consumption of a retreaded engineering tire throughout its life cycle.

Energy evaluation
Figures1 and 2 show the maximum NES (91062 MJ) and the maximum ERR (74.88%), respectively, of five retreaded engineering tire recycling methods. The energy recycling results of the five retreaded engineering tire recycling methods are sorted in descending order, i.e., secondary retreading>combustion decomposition> mechanical smash>low-temperature smash> combustion power generation. Thus, secondary retreading is the most effective method of recycling retreaded engineering tires.

Conclusion
Energy consumption models and calculation method of life cycle of production stage, transportation stage, usage stage, and recycling stage of engineering retreaded tire were built based on life cycle principle. Energy consumption of engineering retreaded tire during production stage is max, which accounts for 91.91% of energy consumption of engineering retreaded tire life cycle, usage stage is next, which accounts for 7.32% of energy consumption of engineering retreaded tire life cycle, and last transportation stage accounts for 0.77% of energy consumption of engineering retreaded tire life cycle. Sort-order of energy recycling results of five kinds of recycling methods of engineering retreaded tire is descending, i.e. tire secondary retreading, combustion decomposition, mechanical smash, smash at low temperature, and combustion power generation. This shows that secondary retreading or combustion decomposition of engineering tire can be the most effective way for recycling of engineering retreaded tire. NES of five kinds of retreaded engineering tire recycling methods

Fig 2.
ERR of five retreaded engineering tire recycling methods