(示意圖/圖片來源:Pixabay)

全球有不少農耕土地,農業不但提供糧食,也可轉換成再生能源。雖然將農業廢棄物轉換成能源的技術已經存在許久,但將這類廢棄物轉換成能源,並非是我們期望的簡單解決方式得以解決,本文將分析農業廢棄物在北美地區轉換為生質能的挑戰。

不同於化石燃料或其他再生能源,使用農業廢棄物作為能源料源的其中一個好處是全球各個角落幾乎都會產生廢棄物,例如:稻米、玉米及小麥的外殼、莖桿或作物的其他部分。雖然各地區之農作物不同,但全球仍有大量農業廢棄物產生(請見表1),農業在美國使用了最多的土地,有超過一半以上土地被用於農業。部分農業廢棄物被做成動物飼料,但並沒有足夠的時間被土壤所分解,目前32萬噸料源中,僅6萬被用來轉換成生質能,按照這個邏輯推論,生質能工廠之料源應該可以仰賴農業廢棄物。

表1  可作為生質酒精料源之農業廢棄物數量(百萬噸)
(資料來源:Sarkar et al., 2012)

農業廢棄物轉換成能源的第一個挑戰為技術。農業廢棄物必須先經過幾次處理,包括生物、物理及化學處理,這些技術會將原料分離成可轉換成能源與無法轉換成能源的不同成分,這些前處理流程可以改變廢棄物特性,將可作為能源之糖分提取出來,而這些提取出來的成分則進一步被轉換成能源,這些轉換技術最大的障礙在於成本高、商業化困難。

農業廢棄物轉換成能源第二個挑戰為永續問題。去除後續生長所需重要營養成分,永續農業(sustainable agriculture)仰賴未來作物成長上的營養循環,移除農業廢棄物將會顯著減少土壤養分,清除農業廢棄物可能帶來問題,例如:水土流失、地表逕流及養分流失,為避免這些問題並使用生質能作為再生能源,必須在營養循環(nutrient cycling)與能源消費間取得平衡。

農業廢棄物轉換成能源第三個挑戰為灰分問題。透過將生質能燃燒產生的灰分讓營養物質回到土地,可能是平衡的方式,灰分可分為三類:底灰、旋風飛灰及集塵飛灰,其中只有底灰被認為是安全且可使用於農業上,旋風飛灰則包含了絕大多數的營養物質,但同時也包含了不少有毒物重金屬成分,這些成分不但對未來農作物成長會有不好的影響,同時也會對人類與動物的健康造成不好的影響,目前之解決方式在於混合飛灰與底灰,以產生能夠被接受且能夠安全地作為肥料使用,然而由於這些毒素仍可能帶來風險,儘管這種方式所產生之灰分屬於可被接受水準,但應該僅被視為是暫時的解決方式。

對於農業廢棄物作為生質能料源所面臨的挑戰,長期的解決辦法是開發新技術,目前使用的諸多技術在財務成本與時間上都相當昂貴,在環境層面也非完全無毒安全,希望透過新技術研發才是讓農業廢棄物用作生質能料源可長可久的方式。

(責任翻譯:吳周燕)

 


Waste to Energy: How agriculture residues can produce environmental benign energy

Around the word, a significant portion of land is set aside for agriculture. Agricultural practices provide us with food, but they also have the potential to provide us with energy through their “wasted” parts using biomass technology. While converting the waste into energy is not the easy solution we hope for, it is ideal to create and improve the technologies needed for this conversion to take place, as agriculture already used a vast amount of land, creates huge quantities of potential energy sources, and is a significantly cleaner green energy source.

One of the advantages of using agriculture waste as an energy source is that, unlike fossil fuels or other renewable resources, these waste products are spread to most areas of the world. Products such as rice, corn, and wheat, all provide waste in the form of husks, stalk, or other parts, which could be converted into sugars to be used for energy production (Sarkar et al. 2012). While the products grown may differ between regions, there is a huge quantity of agrowaste available worldwide (seen in Table 1). Agriculture is the biggest land use, taking up more than half of the land available in the United States (Pimentel et al. 1984). Some of the waste is used as animal feed, but not have time to break down back into the soil. Currently only 0.6 billion of 3.2 billion tons of available resources are being used in biomass energy conversion (Pimentel et al. 1984). Following this logic, it makes sense for biomass energy plants to rely mainly on the wasted potential in products from agricultural fields.

Table 1  Quantities of agricultural waste (million tons) reportedly available for bioethanol production
(Source: Sarkar et al., 2012)

In order for agricultural waste to be converted into available energy source, the waste must first undergo several treatments. These include biological, physiochemical, and physical treatments, which will separate the materials which can be utilized for energy production from those which cannot (Sarkar et al. 2012). These pretreatment modify the waste to extract the sugars which could be used for energy (Sarkar et al. 2012). These extracted materials are later used to produce energy. The biggest issues with these conversion techniques is the costs and technologies necessary to make it happen (Sarkar et al. 2012).

However, there is another important issue regarding the use of agrowaste for energy, which is the removal of vital nutrients necessary for later growth. Sustainable agriculture is reliant on the recycling of nutrients for future crop development, and the removal of the nutrients found in waste products would weaken the soil significantly. The removal of waste could lead to problems such as soil erosion, water runoff, and nutrient removal (Pimentel et al. 1984). In order to prevent these problems and use biomass as a sustainable energy source, a balance must be reached between what goes to nutrient cycling and wat to energy consumption.

There is the potential to balance this delicate scale by using bioenergy residues in the form of ash to return nutrients to the land. However, this too comes with its problems. The ash is broken up into three categories: bottom ash, cyclone fly ash, and filter fly ash. Only the bottom ash is considered safe to use on agricultural fields, but it is the cyclone fly ash that contains most of the nutrients, but also a heavy concentration of toxic heavy metals (Narodoslawsky & Obernberger 1996). These metals would not only be detrimental to the growth of future agricultural products, but also the health of humans and animals who encounter those plants. Solutions to this issue so far have been to mix the fly ash with the bottom ash in order to create an acceptable and safe product to use as a fertilizer (Narodoslawsky & Obernberger 1996). However, due to the risks posed by the presence of these toxins, despite being at an acceptable level, this solution should only be considered temporary (Narodoslawsky & Obernberger 1996).

The only long term solution to the problems facing biomass regarding agricultural waste as fuel is the development of new technologies. Many of the technologies currently being used are either expensive- both in financial cost and time- or inefficient for removing the environmental hazards which renewable energy sources hope to rectify. It is hoped that technological advancements will create a solution to make the use of agricultural waste viable as a biomass energy source.

 

Reference

  1. Narodoslawsky, M. & I. Obernberger (1996). From Waste to Raw Material – the Route from Biomass to Wood Ash for Cadmium and Other Heavy Metals. Journal of Hazardous Materials, 50 (2-3), 157-168.
  2. Primental, D. C. Fried, L. Olson, S. Schmidt, K. Wagner-Johnson, A. Westman, A. Whelan, K. Foglia, P. Poole, T. Klein, R. Sobin and A. Bochner (1984). Environmental and Social Costs of Biomass Energy. BioScience, 34 (2), 89-94.
  3. Sarkar, N., S. K. Ghosh, S. Bannerjee, & K. Aikat (2012). Bioethanol Production from Agricultural Wastes: An Overview. Renewable Energy, 37 (1), 19-27.
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