(示意圖/圖片來源:Pixabay)

燃煤依然是目前最廣泛被使用的能源,卻對空氣汙染、動植物棲息地及人體健康造成嚴重影響。同時,若將燃煤轉換成其他替代能源,需要進行大規模工業與電網等基礎設施的重新投資。部分歐洲國家及美國已逐漸將固態生質燃料與燃煤進行混燒,以減少對空氣汙染的影響,並朝向轉換成100%生質能專燒系統。本文將探討這些國家如何將燃煤發電轉換至新的混燒系統?他們提供了什麼樣的誘因?尤其混燒是否真得使碳排放量減少?或者只是補貼了老舊的燃煤電廠?

混燒方式

只要是將植物或生質能料源投入,並與燃煤燃燒,都可稱為「混燒」。混燒有許多方式,有些是更改原始投料,有些則是修改投料方式。一般來說,混燒方式有三種:直接混燒(Direct co-firing)、間接混燒(Indirect co-firing)或並行混燒(Parallel co-firing)。

  • 直接混燒(圖1)是指將生質燃料與其他類型燃料一起投入鍋爐中,它可以單獨進料或混合進料,直接混燒的主要挑戰包括灰分較多、較可能產生鍋爐結著、以及加熱溫度控制範圍限制,由於所投入的生質燃料必須與燃煤的燃燒速度相當,對可同時混燒的生質燃料類型限制較多。

(資料來源: Maciejewska et al., 2006)
圖1 直接混燒示意圖

  • 間接混燒(圖2)使用獨立處理單位,一般來說通常是氣化器,在與燃煤混燒前,間接混燒會先將生質燃料轉換成合成氣,該方法減少爐渣量與氣體停留時間,這種方式可增加生質燃料在混合前轉換成另一種燃料的彈性,也就是說,相較於前者,此技術可以使用不同類型的生質燃料。不同於並行混燒的設置方式,間接混燒可在同一間廠房內完全分離生質能鍋爐,雖然這是三種方式中,費用最高的選擇,可是當只有一個燃煤鍋爐須要取代或需要將現有鍋爐進行改裝時,是可以考慮的選擇。(參考文獻1)

(資料來源: Maciejewska et al., 2006)
圖2 間接混燒示意圖

間接混燒有兩種,(a)是透過生質燃料與其他類型燃料各自獨立的燃燒器,生質燃料燃燒後將氣體打入鍋爐中;(b)是透過生質燃料與其他類型燃料各自獨立的氣化器,生質燃料燃燒後將氣體打入鍋爐中,氣化器體積更小,對廠房空間有限的公司來說,比較方便。(本段為譯者補充說明)

  • 並行混燒(圖3) 使用獨立鍋爐,生質燃料與其他類型燃料投入各自鍋爐中,產生的蒸汽熱能再進入相同管線中。(本段為譯者補充說明)

(資料來源: Maciejewska et al., 2006)
圖3 並行混燒示意圖

生質燃料類型

不同類型的固態生質燃料在混燒時有不同的好處,最常使用的燃料包括:木屑、木顆粒及培燒木。

  • 木屑:可以直接與燃煤混合,但通常只能以低比例進行混燒;

 木屑 (圖片來源:Pixabay)

  • 木顆粒:由於尺寸與表面積均勻,因此有較好的燃燒效率,故市場發展潛力較高;

 木顆粒 (圖片來源:BiomassDesk拍攝)

  • 焙燒木(torreifed wood)則是進一步將木質生物質經過焙燒處理後,使其化學結構改變,具有煤炭特性,增加木材的熱值、耐水性與均勻性,此過程可以降低運輸與儲存成本,並且易於添加至現有燃燒系統中。焙燒木由於需要初級能源投入,因此仍會造成環境影響,因為需要使用更多的能源來製造。

 焙燒木 (圖片來源:臺灣生質能技術發展協會)

一般來說,焙燒木的鹼性金屬、二氧化矽與氯含量較低,但產業界對此仍未有定見。由於焙燒木發展相對較晚,公用事業採用進展較慢,新的標準也尚未建立(參考文獻2)。總體而言,每種類型的生質燃料針對不同市場,均有相對應的價值可提供應用,公用事業在採用前可通盤思考所有可能的選項。

各國混燒的經驗整理

混合燃料由於不須額外鍋爐操作許可,轉換現有設備的成本也低於購買並設置新的設備。故混燒開始吸引公用事業對生質能的興趣,歐洲已廣泛採用混燒,以下我們來探討其發展歷程(參考文獻3)。

(1) 英國混燒經驗(參考文獻4)

英國Drax電廠為全球最大的生質能混燒電廠,每年需要1,500萬噸生質燃料,以取代10%燃煤,同時每年可以減少200萬噸碳排放。Fiddlers Ferry為另一個大型生質能混燒案例,擁有2座500MWe機組,其中20%使用生質燃料,Fiddlers Ferry先初步將生質燃料共磨後再進行混燒,機組使用多種生質燃料,包括木顆粒、棕櫚殼及其他來自棕櫚樹之廢棄物,而且並未發現有鍋爐結渣(slagging)或積灰(fouling)問題。其他電廠甚至使用了更多生質燃料,但設備須要一些調整。

英國主要的燃煤電廠均已進行混燒,混燒比例平均為3%。這些案例多使用林業剩餘資材、農業廢棄物或特定能源作物,大多數先將生質燃料共磨後,再直接投入至鍋爐中。混燒在2002年以前尚未商業化,2002年電廠生質燃料混燒之發電量僅286 GWh,占能源比率2.57%。英國再生能源憑證制度促使2005年混燒達到2倍,發電量達到2,533GWh,占能源比率14.57%。2011年混燒產生發電量達到高峰,共計2,964GWh,占能源比率6.54%。由於混燒比率低而失去過去電廠運轉的特權,不少公用事業轉換成100%生質能專燒。值得注意的是,工業部門排放指令(Industrial Emissions Directive,簡稱IED)標準提高,混燒案例未來將可能不會繼續增加。對英國來說,生質燃料混燒是一種過渡方式,但也由於混燒取得相當的成功,英國政府也開始擔心太多的再生能源憑證發放給生質能,而非其他再生能源。

(2) 荷蘭混燒經驗

荷蘭一項研究發現,當生質能混燒比例由7%提高至20%時,發電效率會降低3%,這項研究結果建議最適混燒比例應在20%以下。

(3) 德國混燒經驗

德國約50%電廠使用污泥混燒,混燒比例在3%並不會有影響,當污泥可能會造成額外汙染且其它溫室氣體減量選項之成本更高時,以污泥混燒是個不錯的選擇。

(4) 美國混燒經驗

美國並沒有像歐洲一樣蓬勃發展的趨勢,美國560座燃煤電廠中,其中僅40座燃煤電廠採用生質燃料混燒,這些電廠之生質燃料大多為木顆粒、木屑、木質廢棄物及鐵軌枕木等,美國雖然有再生能源推動措施,但料源成本與運輸成本仍是很大的發展障礙。

(5) 日本混燒經驗

日本有12座電廠混燒2-3%生質燃料,其他工業生產也對生質能需求量大,目前不少新電廠申請案仍在審查中,因此日本可能會像英國過去的經驗一樣,在生質能混燒取得成功經驗。

混燒的其他爭議

(1) 對森林的影響

就如同反對生質能,不少人反對大規模或小規模混燒。若用森林剩餘資材作為生質能混燒燃料,有人質疑混燒會增加森林砍伐面積,他們也質疑森林剩餘資材(疏林邊材)對實際溫室氣體的影響,反對者認為森林剩餘資材是森林生態系統的重要組成,並且已在自然中存在成千上萬年。

森林必須重新成長才能夠充分實現溫室氣體減量效益,有鑑於生質能消費成長,部分森林保護主義者認為木質生質能會排放更多溫室氣體,並非碳中和能源,也有些人認為這些效益是以補貼來支持老舊燃煤電廠,不少推動措施對於碳的認知過於狹隘(參考文獻4)。

(2) 對鍋爐的影響

從混燒本身來看,生質燃料也可能會造成鍋爐損害。部分特定型態生質燃料來自生長速度較快植物,例如:稻稈、桉樹或其他生長速度較快的樹木,較可能讓鍋爐損害較快。不過,若從環境角度來看,這類卻是公用事業應該使用的生質燃料類型,因為這類型生質燃料通常會受到法規規定而須被清除處理。生長緩慢的樹種,無論在經濟面或環境面都伴隨著較高的成本,但以現有鍋爐技術來說,卻是混燒較好的選擇,因此,生質能電廠必須在維護成本與環境影響效益間取得權衡。政府政策,尤其是推動措施方面,則可透過政策來引導電廠使用哪一類型的生質燃料作為混燒燃料。

歐洲公用事業運作經驗發現,一般當混燒比例超過10%,將可能導致沉積增加,其主要挑戰來自生質燃料中的氯易對鍋爐產生高溫腐蝕的問題,當生質燃料比例高,通常會帶來更多的氯,其中又以農業廢棄物作為生質燃料通常比木質生質燃料更容易發生上述情況。

氮氧化物(NOx)是否會隨著混燒比例下降而減少尚不清楚,但從混燒後的灰分中發現,二氧化硫(SO2)會因為鹼的增加而顯著降低。若農業廢棄物或樹木存在汙染時,微量金屬可能會增加,因此找到乾淨的生質能料源相當重要,同時,較大的植物製顆粒較容易收集與處理,儘管部分較大顆粒物也可能會讓PM2.5排放增加。

(3) 對投資人的影響

從歐洲絕大多數案例來看,補貼費率是影響是否混燒的關鍵因素。為鼓勵燃煤電廠改採生質能混燒,政府需要承諾對生質燃料的補貼,但這並非是每個國家都容易做到的事情,政府政策不一致也會為投資人與公用事業造成營運風險,若政府不願承諾長期補貼,投資人或公用事業或許會因為擔心未來成本上漲,而不願意進行生質能混燒(參考文獻5)。

結論

當規劃正確並有相關法規支持下,混燒可以做為由燃煤轉換成生質能較具成本效益的選項,混燒確實有減少硫化物排放的好處,對空氣汙染與溫室氣體排放則有較大爭議。生質能種類很多,混燒方式與生質燃料類型的影響也因地而異,有人認為混燒是一種補貼老舊燃煤電廠的方式,德國及部分國家主要目的為解決廢棄物去化問題;作為最大的混燒用戶,英國經驗顯示混燒將不會是最終解決方案,而是替換燃煤到其他生質燃料的一個過渡階段。

(責任翻譯:吳周燕)

 


Coal to Biomass Conversions

Coal energy remains the most widely used energy source, yet causes terrible impacts on air pollution, habitats, and human health. At the same time, transitioning to other energy requires large scale industrial and grid level modifications. Several countries in Europe and the United States have mixed biomass with coal, called co-firing, to mitigate these pollution effects and transition towards full biomass systems. How do countries transition to these new systems? What benefits do they provide? And of special concern, does co-firing lead to decreased emissions or does it just subsidize older coal power plants?

 Co-firing Methods

Anytime you burn plant or biological material with coal you are “co-firing”. Within this exists many different methodologies, some modify the original inputs and others modify the process in various ways. Generally there are three different methods: direct, indirect, or parallel cofiring. Direct firing means feeding the biomass into the furnace with other types of base fuel. It can be fed in separately or mixed in. The main challenge with direct firing comes with higher ash deposition, fouling, and limited heating range. This also limits the flexibility of the biomass type as it must match the firing rate for the coal.

Indirect co-firing uses a separate processing unit, often times a gasifier, which first converts the biomass into syngas before firing it with the coal mixture. This method reduces slag, and residence time of the gas. This method also increases flexibility as the biomass ends up converting into another fuel before mixing meaning multiple types of biomass can be used based on availability. This differs from a parallel setup. With this method, there are completely separate biomass fired boilers that exist within the same facility. While this is the most expensive option it can make sense when only one coal boiler needs to be replaced or when existing retro-fits are required. [1]

 Biomass Types

Different types of biomass create different benefits and draw backs to co-firing. The most commonly used materials are: wood-chips, wood pellets, and torrefied wood. Wood-chips can be mixed directly with the coal and should only be used at low-firing ratios. Wood pellets have a well developed market and higher efficiency due to their uniform size and surface area. Torrefied wood is when standard lumber undergoes a type of pyrolysis that changes it’s chemical structure to have coal characteristics. This increases the calorific value, water resistance, and homogeneity. This process also reduces transport and storage costs and is easier to add into existing systems. Torrefication does require initial energy inputs so some questions about environmental impact remain.  In general torrefied wood has lower alkali, silica, and chlorine but not always. As torrefied wood is relatively new, utilities have been slower to adopt it and new standards are not as developed. [2] In general each type of biomass has different benefits for different markets and utilities should consider all available options before committing.

 Benefits of Co-firing

While each type of co-firing and biomass provides different benefits some consistent trends remain. Mixing fuels lowers investment costs for new machines. Changes to an existing facilities cost less then setting up an entire new facility as it does not require additional permitting or investment. Further, these retro-fits can be part of a larger transition away from coal into full biomass operations. New boilers for co-firing generally have higher efficiencies as well. They also have increased flexibility to use multiple biomass fuel types based on price fluctuations. Co-firing can also serve as an incentive to further develop utilities interest in biomass. Europe has widely adopted co-firing, and it’s worth exploring their development process. [3]

 Cofiring in Europe [2]

The DRAX plant is the largest co-firing power station in the world, processing  1.5 million tones per year, co-firing at 10% heat input. Through this they offset 2 million tons of CO2 per year. Another large site, Fiddlers Ferry has two 500 MWe units using 20% biomass. They have dedicated co-firing after initial co-milling. They use a multi-fuel mixture including wood-pellets palm kernels, and waste from olive trees. They report no difficulties with slagging or fouling. Other plants use even more biomass but require some modifications.

All major coal plants in the UK have adopted co-firing, averaging a 3% mixture. They either use forest residue, agricultural waste, or specific energy crops.  Most started with co-milling and moved on to direct injection. Co-firing was not commercially productive until 2002, in that year power plants only produced 286 GWh totaling 2.57% of energy. Credits for renewable energy lead to co firing double every year until 2005, producing 2533 GWh in 2005, accounting for 14.57% of energy. 2011 was the peak accounting for 2964 GWh yet making up only 6.45%. As co-firing at low ratios lost grandfather privileges, many utilities switched over to full biomass. Co-firing is unlikely to increase in the future due to increasing standards under the Industrial Emissions Directive (IED). For the UK, co-firing acts as a transition pathway, due to it’s initial success the government feared that too many credits would go to biomass instead of other types of renewable energy.

In general, European utilities have found that mixtures above 10% cause a modest increase in deposition. The main challenge arises from the chlorine corrosion, more biomass usually means more chlorine. Biomass from agricultural waste is generally harder to remove than woody materials. It’s unclear whether NOx emissions decrease with cofiring. SO2 however has decreased significantly due to the increase of alkalis in the co-fired ash. Trace metals can actually increase if the agricultural waste or trees had contaminants, therefore finding a clean feedstock remains essential. At the same time, larger particulates from vegetable matter are easier to collect and block. Despite some larger particles, PM 2.5 emissions may increase as well.

 Other International Examples

One Dutch study found that increasing from 7% to 20% biomass cofiring reduced electricity efficiency by 3%. This seems to suggest lower than 20% co-firing achieves optimal results. There are a variety of options for biomass, around 50% of German plants co-fire with sewage sludge, they can be co-fired up to 3% without impacts. This realizes a full benefit as the sewage sludge creates additional pollution and has expensive mitigation options.

This trend did not catch on in the US the same way it did in Europe. Only 40 out of 560 coal plants in the US use coal firing. They mostly use wood pallets, wood chips, wood waste, and rail road ties. Despite renewable energy incentives the cost of material and cost of transportation remain strong barriers.  In Japan, 12 plants use 2-3% mixed biomass, large demand for biomass in other industries, many more new plants are under trial. Japan may therefore follow the UK example as a transition.

Controversy Environmental Impact

Many disagree with the practice of co-firing at macro and micro scales for the same reasons as biomass. Some claim that this increases and supports more logging. Regarding forest residues for co-firing, some doubt the actual greenhouse gas effects noting that these forest residues make up an important part of the forest ecosystem and have naturally degraded for thousands of years. Then to fully realize the GHG benefits the forests must regrow, given the rate of biomass consumption some forest conservation advocates find the energy to be worse than carbon neutral. Lastly, some see these benefits as subsidies to support aging coal plants, as many of the incentives take an overly narrow look at carbon. [4]

For the operation of co-firing itself, the biomass can degrade the boiler. Certain types of biomass from fast growing plants such as straws, eucalyptus, or other fast-growing trees degrade the boilers more quickly. Yet from an environmental perspective these are the types of biomass that utilities should use, often these are invasive trees which have legal mandates for removal. Slower growing trees, which often come at a higher cost either economic or environmental work easier with current boiler technology. Therefore, biomass plants must balance maintenance costs and efficiency with environmental impact. Government policy, especially incentives can guide plants on which type of biomass to co-fire with.

For most cases across Europe subsidies remain a key factor in deciding to co-fire. Reworking existing coal plants for co-firing requires a commitment to biomass which not every country has access to easily. Inconsistent government policies also create risk for investors and utilities. If the government does not commit to subsidies over a larger time scale investors or utilities may not want to make the switch for fear of rising future costs.  [5]

Conclusion

Co-firing has mixed results. When planned for properly with a supporting regulatory framework it can act as a cost-effective transition to biomass or away from coal. While benefits exist for reduced sulfur emissions, air pollution and green-house gas emissions have less clear results. The type of biomass matters greatly. Germany and other countries mix in waste materials, this seems to net other co-benefits. But impacts vary from site to site on methodology and biomass type. Some critics call co-firing a new way to subsidize outdated coal plants. Given that the largest user of co-firing, the UK, intends to shift away from it in the coming years this sends a signal that co-firing is likely not a last solution but a step away from coal in to other biomass fuels.

 

Reference

  1. Roni, M. S., S. Chowdhury, S. Mamun, M. Marufuzzaman, W. Lein, and S. Johnson (2017), “Biomass co-firing technology with policies, challenges, and opportunities: A global review,” Renewable and Sustainable Energy Reviews 78, 1089-1101. Available at: ScienceDirect or ResearchGate.
  2. IEA Clean Coal Centre (2015), “Cofiring of biomass in coal-fired power plants- European experience.” Available at: here.
  3. Available at: here
  4. Paul Shukovsky (2016), “Converting Oregon Coal Plant to Biomass Stokes Controversy.” Available at: here.
  5. Biofuelwatch (2015), “Coal-to-biomass conversions: Supplementing one (climate) disaster with another?” Available at: here.
Print Friendly, PDF & Email