Farklı biyokütle numunelerinden torrefaksiyon işlemi ile kaliteli yakıt peleti üretimi
Producing high quality fuel pellets by torrefaction of different biomass samples
Yazar: YAĞMUR IŞIK
Danışman: PROF. DR. HANZADE AÇMA
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Kömüre alternatif biyokütle yakıtları arayışı günümüzde büyük bir önem arz etmektedir. Biyokütle genellikle depolanması ve taşınması zor, homojen olmayan ve düşük yoğunluklu bir yapıya sahiptir. Bu nedenlerden dolayı peletleme yöntemi ile biyokütlenin yoğunluğu arttırılarak daha kolay ve daha ekonomik depolama, taşıma
ve enerji dönüşümüne olanak sağlanabilir. Odun peletleri yüksek kütle yoğunluğuna ve düşük nem içeriğine sahip olsalar da, pelet enerji yoğunluğu biyokütlenin yüksek oksijen içeriği nedeniyle kömüre kıyasla daha düşüktür. Ayrıca odun peletleri kömüre kıyasla yüksek oranda fiber yapılı ve hidrofiliktir. Bütün bu olumsuz durumların
üstesinden gelmek, peletleri kömürün alternatifi olarak kullanabilmek ve yüksek enerji yoğunluğuna sahip katı bir yakıt üretmek için peletler oksijen yokluğunda termal olarak ön işleme tabi tutulabilir. Torrefaksiyon olarak adlandırılan termal ön işlem, biyokütlenin atmosferik basınç altında, inert ortamda 200 ile 300°C sıcaklığa kadar
ısıtıldığı düşük sıcaklıktaki hafif piroliz işlemidir. Biyokütlenin torefaksiyonu ile öğütülebilirlik, tanecik boyutu, tanecik şekli, karbon ve hidrojen içeriği, kalorifik değeri, nem içeriği, peletlenebilirlik gibi fiziksel özellikleri geliştirilir. Torrefaksiyon sırasında, biyokütledeki çoğunlukla oksijenli bileşikler bozunarak torefiye edilmiş biyokütle olarak adlandırılan kırılgan ve yüksek enerji içerikli katı yakıtı oluştururlar.
Torefiye edilmiş biyokütle peletleri kömür gibi fosil yakıtlarının yerini almak için kullanabilecek yüksek kaliteli ve yenilenebilir enerji üretmek adına kullanışlıdırlar. Ayrıca biyokütle peletlerinin torefiye edilmesi piroliz, gazlaştırma ve birlikte yakma uygulamalarında da yüksek kalitede katı biyoyakıt elde edebilmek için kullanışlı bir termal ön-işlem teknolojisidir. Torrefaksiyonun ve yoğunlaştırma teknolojilerinin entegrasyonu, kömürün yerini doldurabilecek bir enerji kaynağı olarak taşıma, kullanım ve depolama ile ilgili maliyetleri düşürdüğünden kritik öneme sahiptir. Biyokütlenin torrefaksiyon ve yoğunlaştırma işlemlerine hangi sıra ile tabi tutulduğu önemlidir. Bu çalışmada dişbudak (orman biyokütlesi), fındık kabuğu (tarımsal biyokütle) ve RDF (atıklardan türetilmiş yakıt) kullanılarak torrefaksiyon işlemi ile beraber kaliteli yakıt peleti üretimi amaçlanmıştır. Torrefaksiyon işlemine numunenin toz halinde torefiye edilmesinin etkisi de bu çalışma kapsamında incelenmiştir. -250 μm tane boyutu altına düşürülmüş dişbudak, fındık kabuğu ve RDF numunelerinden önce pelet
üretilmiş daha sonra bu peletlere farklı sıcaklıklarda (230°C, 260°C ve 290°C) oksijensiz inert ortamda torrefaksiyon işlemi uygulanmıştır. Sonrasında meydana gelen ağırlık kayıpları ölçülerek sırasıyla üst ısıl değer analizi, basma testi, suya dayanım testi, kısa analiz ve elementel analizler uygulanmıştır. Ayrıca peletleme
prosesinden önce toz halde bulunan biyokütle numulerine de torrefaksiyon işlemi uygulanmış ve sonrasında üst ısıl değer analizi, FTIR, tanecik boyutu analizi, numunelerin yanma davranımlarını belirlemek amacıyla da termal analiz (TGA, DTA, DSC, DTG) uygulanmıştır. Torrefaksiyonun etkisini gözlemlemek amacıyla torefiye edilmiş peletlere uygulanan bütün testler orijinal biyokütle peletlerine de uygulanmıştır ve bütün sonuçlar detaylı olarak  orumlanmıştır. Deneysel çalışmalar sonucunda üretilen biyokütle peletlerinin daha hidrofobik yapıya,
daha yüksek ısıl değere sahip olduğu, karbon miktarının daha yüksek, nem içeriğinin daha düşük olduğu gözlemlenmiştir. Toz ve peletlenmiş biyokütle numunelerine uygulanan torrefaksiyon işleminin sonuçları incelenmiştir.

Abstract:
Fossil fuels are the primary fuel sources in the world for producing energy since 1900. With globalization, modernization, lifestyle changes and rapid population growth, energy demand has increased rapidly. Renewable energy sources have gained importance due to depletion of fossil fuels and increased carbon dioxide emissions
from greenhouse gas emissions. In contemporary there is a great effort to find an alternative to coal by enhancing the current bio-fuel technology. Renewable energies offer better options for energy production than fossil fuels because they are more environmentally friendly, carbon neutral and more sustainable. Since the amount of carbon dioxide absorbed by the biomass by photosynthesis is consumed as fuel, it does not add an additional amount of carbon dioxide to the atmosphere. However, bio-fuel has its own problems such as non-homogeneity, low transportability, diffucult for storage and has a low density. Also, untreated biomass absorbs moisture very quickly, resulting in degradation and thermal degradation over long storage times. This feature, together with low energy density, makes it more expensive for transportation. In addition, as biomass has a fibrous state the grinding process needed in order to achieve the desired particle size results in high energy consumption. Since small particle size biomass should be used in fluidized bed combustion systems and gasification processes, dehumidification is of great importance stage in the preparation of biomass. Since biomass is decomposed due its
low density when it is burned with coal ,especially in large scale boilers, it is necessary to produce pellets in order to increase the density of the biomass. Milling wood is costly and requires high capital investment. In order to overcome these difficulties and use biomass efficiently as an energy source, it needs to be pre-treated. Torrefication and pelletization are two important technologies that increase the energy and volumetric density of the biomass raw material. Due to these reasons by increasing its density by pelletization it becomes easier to
transport, store and increase the energy density. Wood pellets have high energy density and low humidity but due to their high oxygen concentration their energy density less than coal. Furthermore, wood pellets are more fibrous and hydrophilic than coal. In order to overcome these non-idealities bio-fuel can be thermally treated in a medium
absent of oxygen. Thermal pretreatment can significantly improve the combustion properties of the biomass. Torrefaction is a low temperature pyrolysis process in which the biomass is heated to atmospheric pressure in an inert environment at temperatures from 200°C to 300°C. The lack of oxygen in the reactor is of great importance in order to prevent oxidation and ignition. During torreaction, moisture and volatile substances in the biomass are released and the biopolymers in the structure are partially degraded. After a torrefaction operation the biomass parameters such as grindability, particle size, particle shape, carbon and hydrogen content, calorific value and humidity are improved. During torrefaction, compounds rich in oxygen experience a degradation and the biomass turns into a brittle and higher energy density fuel. As a result of this process, the torrefied biomass has high calorific value and high energy density. Other important advantages are homogeneous composition, low moisture content and high resistance to moisture absorption and microbial growth. This process improves the physical, chemical and biochemical composition of the biomass, resulting in better performance from co-firing and gasification processes. Torrefraction is also known as slow pyrolysis, mild pyrolysis, carbonization and high
temperature drying. Torrefactions convert raw biomass into a solid with high heating value, high energy density, hydrophobic, compressible, grindable, low (O/C) ratio, suitable for combustion and gasification applications.
Volatiles and torrefied gases are released during torreaction and the remaining solid is called the torrefied biomass. A weight loss occurs in the biomass by the release of moisture, slightly volatile hydrocarbons and a number of gases in the torreaction process. However, the released gases are a small percentage of the energy in the biomass, although the biomass is experiencing weight loss, the energy loss is very little. Torrefied biomass pellets are a promising candidate to replace coal based fuels as they have a close performance to coal plus it is a renewable conservative choice unlike coal. Furthermore, the torrefaction of biomass pellets is also useful for pyrolysis, gasification and other co-firing applications as a pre-treatment method for achieving higher quality fuels. Biomass pellets are solid fuels of homogeneous structure and size obtained by compressing and compacting biomass (pelletization). Increasing the volumetric energy density of biomass enables easier and more economical storage, transportation and energy conversion. The quality of the densified pellets depends on many parameters
such as the biomass used, particle size, moisture content, die temperature and die diameter. Efforts to reduce greenhouse gas emissions and rising oil and natural gas prices have led the wood pellet industry to grow rapidly around the world. Wood pellets are considered as an important source of portable renewable energy. Integrated torreaction and pelletization technology has been developed to produce biofuels. This process is called TOP (Torrefaction and Pelletization). In the TOP process, the biomass is first dried, torrefied, then reduced in size and pelletizing to solid pellets. However, the weakening of the binding forces between the biomass particles and the loss of the natural binding properties of lignin in the biomass after torrefaction are major difficulties to encountered in pelletizing the treated biomass. The combination of these two effects results in more gaps between the particles in the pellet, thereby reducing pellet density and hardness. Therefore, it may be necessary to
add binders, higher mold temperatures or higher compression pressures to produce higher quality solid fuel.
When the TAP (Torrefaction After Pelletization) process is integrated, the existing pelletizing plants have the opportunity to produce both the original raw pellet and the torrefied pellet. The feeding of densified homogeneous raw materials, such as wood pellets, potentially reduces equipment size and hence capital expenditure, and
facilitates uniform heat distribution during torrefacturing. Previous studies of the torrefection of biomass pellets have mostly focused on examining the change in basic compositions and the energy density of the final solid product. The quality of the individual pellets was examined in terms of hardness, strength and water resistance for certain materials. Although various types of biomass have the potential to be used as feedstock, commercial wood pellets are the most promising in increasing the production of the torrefied pellets.
The order of torrefaction and pelletization makes a difference as it is argued. In this study fraxinus wood, nutshell and RDF (Refused Derived Fuel) are torrefied in order to produce high quality fuel pellets. First, reduced to -250 μm particle size fraxinus wood, nutshell and RDF samples are turned into a pellet form, then those pellets are torrefied at different temperatures (230°C, 260°C and 290°C) in an oxygen free environment. The effect of of the sample in powder form is also investigated in this study. After the mass loss due the procedure is measured, several measurements are done in the order: heating value analysis, compression test, water resistance, proximate analysis and elemental analysis. In addition, the powder form of pre-pelletization biomass samples are torrefied and heating value analysis, FTIR, particle size and in order to understand their pyrolysis behavior thermal analyses such as TGA, DTA, DSC and DTG are conducted. To have a comparison all the analyses mentioned are also done to the raw biomass samples and the results are discussed. As a result of experimental studies, it is observed that the biomass pellets are more hydrophobic, with higher thermal value, have higher carbon content and lower moisture content.