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目前废旧风机叶片的主要回收再利用技术可分为机械回收法[7]、热回收法和化学回收法[10],将叶片材料转化为热能或产品再利用[11-13],如图1所示。
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机械回收法指将叶片机械粉碎(或撕碎、磨碎、研磨等类似过程分解)或直接切割后的复合材料作为原料进行二次利用。
1)机械粉碎包括两类:(1)粉碎为小碎片;(2)机械切割为较大碎片后加工利用。
将粉碎后的风机叶片作为原材料二次使用,此过程产生的材料主要可分为树脂和纤维产品(GF和CF)两大类。粉碎加工法回收成本低,是目前国内外主流的处理方法。
风机叶片粉碎加工法回收的再生玻璃纤维rGF产品易在机械回收过程中受损,纤维回收率低,一般只能降级利用,且碎片化的GF在用于混凝土骨料等典型应用场景时机械性能更容易受损,强度和刚度进一步降低。因此,由于原生玻璃纤维vGF生产成本不高,rGF的回收再利用附加值较低,rGF的回收再利用需要在相关政策的推动下才能得到更广泛的应用。对于风机叶片中的再生碳纤维rCF产品,由于原生碳纤维vCF价格高昂,再生碳纤维rCF具有更可观的经济价值。因此,rCF的回收再利用具有更好的吸引力。
将风机叶片切割为较大碎片能保持树脂和纤维组分的热固性复合材料的结构相对完整性,而且切割产生更小的表面积,切割过程耗能更低。
2)直接(切割)后加工再利用。风机叶片中部多为平面状态,且材料强度高、质量轻、防腐蚀,切割后壳体可进行加工拼接后再利用,制作为围栏、公园步道、躺椅等。叶尖切割后可制作为公园景观、道路指示牌等。
机械回收法不包含任何化学工艺过程,能耗低(为常规化学法的0.5%~5%、或热解法的0.4%~16%),温室气体减排效果显著,回收成本低,广泛用于纤维增强聚合物复合材料(FRPs)的回收。
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风机叶片的热固性复合材料主要为树脂和纤维(GF或CF)。热解法是在无氧或缺氧条件下,将风机叶片加热至350~700℃,将树脂转化为热解气和热解油(热解液)作为燃料回收;不溶不熔的纤维组分转化为固体残渣(惰性),经筛分后回收纤维和炭。热解法能耗在机械法和化学法之间。
热解法热解气主要成分为H2、CH4、CO、CO2(热解气中的碳氧化物主要为树脂中富氧组分在高温下分解生成),热解油是C5-C20的复杂混合物(主要是芳香族化合物例如BTEX_苯、甲苯、乙苯、对二甲苯、苯乙烯等,以及含氧化合物),热解固体残渣筛分生成纤维回收。风机叶片不同反应条件下的热解产物产率情况如表1所示[14]。
表 1 风机叶片不同反应条件下的热解产物
Table 1. Pyrolysis products of wind turbine blades under different reaction conditions
热解产物 热解气
(质量分数)热解液/热解油
(质量分数)固体残渣
(质量分数)GFRPs+邻苯二甲酸聚酯 6%~12% 9%~13% 72%~82% GFRPs+热固性聚酯/苯乙烯共聚物(450 ℃) H2、CH4、CO、CO2 高浓度C2-C4碳氢化合物 — GFRPs聚酯玻璃纤维(550 ℃,3 h) 8% 24% 68% 慢速热解反应 GFRPs(550~700 ℃) H2、N2、CH4、CO、CO2、C2H6 — — CFRPs,固定床反应器(350~700 ℃,60 min) 0.7%~3.8%
H2、CH4、CO、CO214%~24.6%
苯、甲苯、乙苯、苯酚70%~83.6% CFRPs+环氧树脂(400 ℃) — — 再生纤维平均长度、直径分别为5.6 mm、6.5 mm;与原生CFRPs比,纤维的模量和拉伸强度相同 CFRPs热解+气化(中试)
I. 热解(500~700 ℃)
II. 气化(550 ℃,空气量12 L/h,30~180 min)H2、N2、CH4、CO、CO2、C2H6 — — 热解法再生纤维结构蓬松、不整齐且纤维不连续,劈裂抗拉强度降低5%~10%、弯拉强度降低约10%,市场应用潜力较焚烧再生纤维高。在不同温度下的CFRPs预浸剂热解回收的CF与环氧树脂分离均良好,然而在较高热解温度下再生的纤维表面覆盖着一层薄热解炭,该热解炭由非氧化环境下聚合物降解残渣组成;在有氧环境下,该热解炭薄层变得碎片化[15],如图2所示。
图 2 不同温度下热解回收碳纤维的扫描电子显微镜图像
Figure 2. SEM images of carbon fibers recycled from pyrolysis at different temperatures
在能量和纤维材料回收方面,与当前市面上其他主流技术相比,通过热解法回收的纤维清洁、质量高。反应器缺氧环境下的热解反应有效避免了废弃叶片直接焚烧潜在的二噁英的生成与排放等问题,解决了废弃叶片二次污染的问题[16]。但热解法的成本和能耗高于机械法。
典型的热解法主要有流化床法和微波热解法。流化床法是由英国诺丁汉大学开发[17],具体是在流化床反应器中的低温(450 °C)热解回收CFRPs和GFRPs的过程。该过程使用空气作为流化气体,分解复合基质得到了纤维材料,并产生了清洁燃料气用于能量回收。值得注意的是,该方法会导致纤维性能降低。在450 °C时,再生rGF强度损失50%[18],在550 °C时,强度损失25%[19],回收过程中难以获得连续纤维。微波热解法是指通过微波辐射分解复合材料中的树脂基质。树脂通过吸收来自CF的微波能量进行内部加热,可以更快地分解树脂。与其他热分解技术相比,该方法所需设备更少,时间更短。作为一种新兴技术,微波热解具有清洁环保的优点,是一种易于控制、高效的CF回收技术。
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化学回收法是通过化学改性或分解将废弃物制成其他可回收材料的方法。与机械回收相比,这种回收热固性复合材料的方法难度大、成本高,但回收效果更好。化学回收方法主要包括焚烧法、超临界流体法和溶剂分解法。
焚烧法是将化学能转化为热能回收使用。然而,焚烧过程中会产生温室气体和二噁英等有毒物质,且焚烧产生的飞灰若处理不当会对环境造成二次污染。焚烧法只能回收纤维增强材料中有机组分的热值。当前焚烧工艺的热能回收效率较低,且叶片回收再利用减少的碳排放不能完全抵消GFRPs/CFRPs材料燃烧过程释放的碳[20],因此,将风机叶片材料热值最大化是提高焚烧回收法能量输出的有效方式。表2概述了主流风机叶片材料的热值[9]。
表 2 不同叶片材料的热值和能量强度
Table 2. Energy intensity and calorific value of different blade materials
主流风机叶片材料 能量强度/MJ·kg−1 平均热值/MJ·kg−1 聚合物 聚酯纤维PE 72 43 环氧树脂EP 80 30 聚氯乙烯PVC 80 17 纤维 玻璃纤维GF 32 — 碳纤维CF 286 34a 注:碳纤维CF的热值由含碳量决定:无烟煤热值34 MJ/kg,含碳量98%;木炭30 MJ/kg,含碳量90%;煤炭15~27 MJ/kg,含碳量70%。 超临界流体法是指利用超临界流体优异的溶解性和传质特性来分解或降解聚合物废物,并获得固体、液体和气体产物。超临界流体法的分解介质主要采用水或醇,由于超临界醇的临界点较低,氢气供应能力较好,溶解性能优于超临界水,超临界醇被用作回收CFRPs的优良介质[21-22]。该方法是一种新型的回收方法,具有回收工艺清洁无污染,再生纤维表面清洁、性能优良等优点。但超临界条件要求更加严格,对反应设备要求高、成本高、安全系数低,尚处于实验室阶段,离工业化应用还有一定距离。
溶剂分解法是指在加热条件下利用溶剂的化学性质使聚合物解聚。该方法对纤维和树脂的回收具有更好的效果,但是使用的大多数溶剂都有一定的毒性且价格较高,降低了该方法的适用性。例如,硝酸具有强氧化性和腐蚀性,低温条件下可在短时间内分解环氧树脂复合材料,但对实验设备的耐腐蚀性和抗氧化性有较高的要求,后处理也比较复杂[7]。因此,该方法需要研发更合适的反应介质以减少其对环境的影响。
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综合回收法指将机械、热解、化学等回收技术相结合,用于废旧风机叶片的处理及资源化回收。华中科技大学科研团队发明了一种废旧风机叶片处理回收高品质燃料和玻璃纤维的方法[23],如图3所示,将机械破碎、化学燃烧、热解技术相结合,首先将破碎得到的废旧碎风机叶片分离得到主要成分为树脂的组分a和主要成分为玻璃纤维的组分b;进而将组分a一部分用于燃烧提供高温烟气,一部分经压制生成高品质燃料;组分b利用组分a燃烧供给的高温烟气加热、燃烧得固态残渣,固态残渣经回收筛分后得到玻璃纤维。
Current Status and Prospect of Recycling Technology of Wind Turbine Blades
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摘要:
目的 世界风电行业蓬勃发展,老旧风机叶片迎来退役高潮。如何实现废旧风机叶片的高效回收和资源化利用已经成为影响风电行业绿色循环发展和低碳足迹目标的关键问题。 方法 传统废旧风机叶片的处理方式为填埋或燃烧,资源浪费的同时产生了巨大的环境问题。在此背景下,文章系统梳理了国内外主流风机叶片回收技术(机械回收、热解回收、化学回收、综合回收),分析了不同回收技术的优劣。 结果 基于对当前国内外废旧风机叶片回收工业化、资源化应用场景的总结,对未来的风机叶片回收利用市场及前景进行了建议和展望。 结论 风电产业对于助力节能减排及实现“碳达峰、碳中和”目标有重要作用,而风机叶片回收市场尚处于中小规模的探索性阶段,距离大规模产业化还有一段距离,亟需政策、标准、技术研发方面的支持。文章从多方面梳理论述了国内外废旧风机叶片回收技术和应用场景的现状及发展趋势,以期为风电行业的绿色循环低碳发展提供科学参考。 Abstract:Introduction The growth of wind power industry is accelerating faster than ever worldwide, at the same time, the waste generation from end-of-life wind turbine blades is expected to peak in recent years. Efficient recycling of end-of-life wind turbine blades has become a key issue affecting the sustainable development of the wind power industry and its low-carbon-footprint target. Method Traditionally, majority of the end-of-life blades were either buried or incinerated after being dismantled, causing resources wasting and severe environmental problems. In this context, this paper reviewed the mainstream domestic and international recycling processes of end-of-life wind turbine blades such as mechanical recycling, pyrolysis, chemical recycling and comprehensive recycling, and analyzed their corresponding advantages and disadvantages. Result By summarizing the industrial recycling application scenarios, this paper provides suggestions and prospects for the prospective recycling market of the end-of-life wind turbine blades. Conclusion Wind power industry plays an important role in realizing the "carbon-peaking and carbon-neutrality" goals through assisting in implementation of the energy-conservation and emission-reduction strategy. However, the wind turbine blade recycling market is currently in the exploratory stage with medium and small-size, there is an urgent need for policies, standards and technology development to help with its large scale industrialization. This paper illustrates current status and development trend of domestic and international recycling technology and application scenarios of end-of-life wind turbine blades, and aims to provide the scientific references for the sustainable and low-carbon development of the wind power industry. -
表 1 风机叶片不同反应条件下的热解产物
Tab. 1. Pyrolysis products of wind turbine blades under different reaction conditions
热解产物 热解气
(质量分数)热解液/热解油
(质量分数)固体残渣
(质量分数)GFRPs+邻苯二甲酸聚酯 6%~12% 9%~13% 72%~82% GFRPs+热固性聚酯/苯乙烯共聚物(450 ℃) H2、CH4、CO、CO2 高浓度C2-C4碳氢化合物 — GFRPs聚酯玻璃纤维(550 ℃,3 h) 8% 24% 68% 慢速热解反应 GFRPs(550~700 ℃) H2、N2、CH4、CO、CO2、C2H6 — — CFRPs,固定床反应器(350~700 ℃,60 min) 0.7%~3.8%
H2、CH4、CO、CO214%~24.6%
苯、甲苯、乙苯、苯酚70%~83.6% CFRPs+环氧树脂(400 ℃) — — 再生纤维平均长度、直径分别为5.6 mm、6.5 mm;与原生CFRPs比,纤维的模量和拉伸强度相同 CFRPs热解+气化(中试)
I. 热解(500~700 ℃)
II. 气化(550 ℃,空气量12 L/h,30~180 min)H2、N2、CH4、CO、CO2、C2H6 — — 表 2 不同叶片材料的热值和能量强度
Tab. 2. Energy intensity and calorific value of different blade materials
主流风机叶片材料 能量强度/MJ·kg−1 平均热值/MJ·kg−1 聚合物 聚酯纤维PE 72 43 环氧树脂EP 80 30 聚氯乙烯PVC 80 17 纤维 玻璃纤维GF 32 — 碳纤维CF 286 34a 注:碳纤维CF的热值由含碳量决定:无烟煤热值34 MJ/kg,含碳量98%;木炭30 MJ/kg,含碳量90%;煤炭15~27 MJ/kg,含碳量70%。 -
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