Katı yakıtların yanma reaktivitesinin farklı yaklaşımlarla incelenmesi
Investigation of combustion reactivity of solid fuels with different approaches
Yazar: BUSE BİLKİÇ
Danışman: PROF. DR. SERDAR YAMAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Kimya Mühendisliği = Chemical Engineering
Dizin: Biyokütle enerjisi = Biomass energy ; Diferensiyel termal analiz = Differential thermal analysis ; Kömür yanması = Coal combustion

Özet:
Günümüzde teknolojinin gelişmesi, sanayileşmenin ve nüfusun artması ile enerjiye duyulan ihtiyaç gün geçtikçe artmaktadır. Isı ve elektrik enerjisi en çok ihtiyaç duyulan enerji türlerindendir. Bu enerji ihtiyacını karşılamada kullanılan başlıca yöntem katı yakıtların yakılmasıdır. Dünyanın her yerinde kömür, biyokütle, atık, arıtma çamuru gibi farklı katı yakıt türleri yakma sistemlerinde ayrı ayrı ya da birbirleriyle karıştırılarak kullanılmaktadır.
Yakıtların fiziksel ve kimyasal özellikleri çok farklılıklar gösterir. Nem içeriği, karbon içeriği, yapısındaki inorganik maddeler, kül oluşturma miktarları gibi özellikleri birbirlerinden farklıdır ve bu farklılıklar yanma reaktivitesini etkiler. Bu yüzden katı yakıtların yanma reaktiviteleri de çeşitlilik gösterir.
Reaktivite, yakma sistemlerinde yakıtın yakılması sırasında dikkat edilmesi gereken parametrelerin başında yer alır. Çünkü bir yakıtın reaktivitesi ısı salınımını, yanma süresini dolayısıyla kazanda kalma süresini ve yapılması gereken yakıt takviyesi gibi parametreleri etkiler.
Literatürde yakıtların reaktivitelerini inceleyen birbirinden farklı yaklaşımlar mevcuttur ve belirli bir katı yakıt türünün reaktivitesi hesaplanmak istenildiğinde hangi yaklaşımın seçilmesi gerektiğini belirten çalışmalara rastlanmamaktadır.  
Bu çalışmada çeşitli katı yakıt türlerinin hangi reaktivite hesaplama yöntemine daha iyi cevap verdiğinin belirlenmesi amaçlanmıştır. Farklı katı yakıtların yanma özelliklerini inceleyen 9 farklı deneysel çalışmanın değerleri kullanılmıştır. Çalışmaların TGA, DTA, DTG ve DSC grafiklerinden (dw/dt)%5, T%5, t%5, (dw/dt)w-ort,  Tw-ort, tw-ort, (dw/dt)x-%5, Tx-%5, tx-%5,  TH-max, Hmax, T∆T- max, ∆Tmax, Ti, Tb, (dw/dt)max, Tmax, EA, değerleri ve Di, Db, Dw, S, R, Rm, Rc, X-Tb hesaplamaları için gerekli diğer parametreler de bulunup hesaplamalar yapılmıştır. Bunların yanı sıra, her numune için reaktivite ve dönüşüm grafikleri oluşturulmuştur. 
Öncelikle her deney prosesinin kendi içinde reaktivite karşılaştırılmaları yapılmıştır. Bu deney setlerinde farklı yaklaşımlarla hesaplanan reaktivitelere göre en fazla yaklaşımda en büyük ve en küçük değerleri veren numuneler, reaktifliği en yüksek ve reaktifliği en düşük olarak belirlenmiştir. Sonrasında bu değerler diğer deney setleriyle karşılaştırılmış ve genel sonuçlara varılmıştır.
Yapılan hesaplamalar sonucunda katı yakıtların çeşitli yaklaşımlar ile hesaplanan yanma reaktivitelerinin farklılıklar yaratabileceği belirlenmiştir.  Bir katı yakıtın, belirli bir reaktivite hesabına göre yüksek reaktif özellik gösterirken farklı bir hesaplama yönteminde diğer yakıtlara göre daha az reaktif olduğu görülmüştür.  Bu durum, herhangi bir yakıt için her tür reaktivite hesaplama yönteminin güvenle kullanılamayacağı sonucunu çıkarmamıza yardımcı olmuştur. 
Genel olarak (dw/dt)max parametresinin; Ti, Tmax veya Tb gibi parametrelere göre daha güvenilir bir hesaplama yöntemi olduğu belirlenmiştir. Ayrıca (dw/dt)max parametresinden sonra linyit ve biyokütle türlerinde Di, Db, Dw, S, R ve Rc göstergelerinin de (dw/dt)max ile uyum içinde olduğu, ancak Rm göstergesinin hesaplamalarda sapma gösterdiği gözlemlenmiştir

Abstract:
Despite the current technological and scientific developments, about 1.2 billion people suffer from a lack of modern energy supply service that can be attributed to industrialization and population growth. Energy poverty has a serious impact on people's lives and welfare, and the world has to face this problem more rigorously than ever before. Heat and electric energy are the most essential energy types. A great deal of energy is generated based on solid fuels. 
In addition to coal that is the most preferred solid fuel in most countries, the use of biomass, animal or industrial waste from alternative energy sources is increasing day by day.
Coal can be defined as flammable, black or brownish-black sedimentary rock composed of both organic and inorganic materials. Besides, biomass is divided into two main categories: woody and herbaceous biomass. Woody biomass is primarily consisted of the wastes of wood processing industry, forestry products or urban wood waste remains, while the herbaceous biomass is mainly comprised of agricultural products and agricultural wastes.
Nowadays, the reuse and recovery of waste sewage sludge instead of waste storage also becomes important due to environmental problems.
Generally, these wastes are burned to volume reduce, balance, remove pathogenic microorganisms and obtain energy. The waste can be used as fuel when it has a high chemical energy content based on its (organic) flammable ratio and its moisture content.
Refused-derived fuel (RDF) stems from municipal solid waste or industrial solid waste. RDF has a high calorific value associated with its organic fraction that contains plastic, paper, cardboard and as well as various biomass types. 
The variety as well as the amounts of solid fuels is increasing day by day. The increase in the variety of solid fuels used and the combined use of different fuels in combustion systems such as co-combustors create various difficulties.
Combustion is a phenomenon that can be defined as the chemical reaction of the flammable substance with oxygen. As a result of the burning event heat releases, and based on this fact combustion is the most common method to convert the chemical potential energy of fuel into useful energy forms.
However, burning of solid fuels  is a complex event. The most acklowledged approach related to the burning mechanism includes initially heating and drying granules. The dried particles are pyrolyzed and the volatile material is removed from the solid matrix. Then, volatile matter combustion takes place where combustible volatiles burn through homogeneous burning in gas phase. Finally, the char that is the solid residue forming after removal of the volatiles is oxidixed through the surface oxidation, and this process is called as heterogeneous combustion stage. The sub-groups of these reactions also include the catalytic or inhibiting effects of inorganic components including the main inorganics (e.g., silica, alumina, iron, calcium) and trace metals. Homogeneous combustion occurs at lower temperatures compared to those of the heterogeneous combustion and accordingly it is a faster combustion. On the other hand, the heterogeneous combustion that is controlled by diffusion is quite slow, and consequently it determines the burning rate of the solid fuel.
Solid fuels differ in terms of fixed carbon and moisture content, ash yield and ash properties, thermal properties, particle size and physical properties, sulfur and nitrogen ratios, and the volatile matter amounts. These structural differences affect the reactivity of the fuel combustion rate. 
Combustion reactivity is a result of the fuel structure. In general, reactivity measurements are made with the ratio of fixed carbon in the fuel to volatile matter, the maximum volatile efficiency, the devolatilization of the fuel, or the activation energy required for the pyrolysis and the oxidation of the char. 
The reactivity measured by the maximum volatile yield assumes that the volatile efficiency is a function of the temperature at which the fuel reacts. Volatility can be directly related to oxygen content or functional groups containing oxygen. Activation energies are highly important indicators of reactivity related to kinetic measurements. In general, low activation energies for volatility are associated with more reactive fuels. In this context, when volatile matter content is low, the reactivity is lower and burning of this fuel is more difficult.
Likewise, the activation energy required for char oxidation is another significant indicatior of the reactivity, where the low activation energy indicates that the fuel is more reactive.
Reactivity affects the values of many parameters relevant to the combustion of the solid fuels such as the ignition temperature of the fuel, heat release or burning time. For example; if the reactivity of the coal is higher, the heat release occurs faster.
Therefore, reactivity also affects many parameters such as temperature, pressure or boiler design parameters that affect the operation of the boiler and the heat transfer mechanism. For example, in pulverized coal burning systems, the bed height is larger than the grate coal combustion systems.
As scientific studies progress, a new approach to reactivity calculations is added every day. The calculations of this parameter, which must be taken into account in combustion systems, vary and they lead serious confusion in the determination of combustion conditions and in the proper selection of fuel.
For these reasons, the aim of this study is to minimize these disturbances and determine which of the various solid fuel types respond better to any distinct reactivity calculation method. Therefore, the experimental results of 9 different combustion studies which examined the combustion properties of different solid fuels were used. 
The values of  (dw/dt)%5, T%5, t%5, (dw/dt)w-ort, Tw-ort, tw-ort, (dw/dt)x-%5, Tx-%5, t x-%5,  TH-max, Hmax, T∆T- max, ∆Tmax, Ti, Tb, (dw/dt)max, Tmax, EA and other values required for Di, Db, Dw, S, R, Rm, Rc, X-Tb calculations were determined from TGA, DTA, DSC, DTG curves of the mentioned combustion experiments. Here, some values were compared as direct reactivity measurements, while some values were used in the reactivity formulations and graphs. In addition, reactivity and mass conversion graphs were prepared for each sample.
Firstly, the reactivity comparisons of each experimental set were carried out. According to the reactivities calculated with different approaches, the samples with the highest and lowest reactivity were determined from the samples with the highest and the lowest reactivity values. Then these values were compared with the other experimental sets and general conclusions were drawn.
In conclusion, results showed that one of the important factors determining the combustion reactivity of a fuel is the volatile matter content of the fuel. 
The thermal pretreatment temperature is a factor that directly affects the combustion reactivity of the chars. As a result of lignocellulosic biomass torrefication that is pre- treatment process where in the inert environment and the temperature below 300 ° C,  a significant increase in combustion reactivity has occurred. On the other hand, as a result of the carbonization process carried out at higher temperatures, a significant reduction in the combustion reactivity of the biochars was observed due to the significant losses in the volatile matter content of the biomass. It was determined that from the burning curves of biochars, (dw/dt)max values decreased and (Tmax) values increased slightly.
On the other hand, lignite coal has a high content of mineral matter. After carbonization it produced high combustion reactivity chars. This is due to the fact that the inorganic components forming the mineral substance become more concentrated in the chars and show a catalytic effect in the combustion medium. The sewage sludge chars obtained by the carbonization of wastes containing a very high percentage of inorganic material showed higher combustion reactivity than untreated sludge. 
Also, the reduction of the grain size of the solid fuels resulted in significant increases in all reactivity indices. 
When using a mixture of pure oxygen or oxygen-enriched air instead of dry air in the combustion environment, a significant increase in the combustion reactivity of solid fuels has been determined.
The reactivity parameter of (dw / dt)max is a more dominant parameter than parameters such as Ti, Tmax or Tb; also, reactivity indeces that are calculated with using (dw / dt)max parameter, this parameter was found to be highly determinant. In addition, it was determined that the Ti does not give reliable results every time.
However, it is difficult to see significant difference when comparing the burning properties of similar structured samples. In this situation; it was seen in the comparison of combustion reactivity of woody biomass such as ash tree, hybrid poplar and rhododendron. The interpretation of which biomass has a higher combustion reactivity may vary depending on the reactivity indicator taken into account. 
The most reactive sample in terms of Di, Db, Dw, S, R, and Rc indicators is found as hybrid poplar, while the Rm indicator indicates that the rhododendron more reactive. A similar situation was observed in the comparison of the combustion reactivities of 12 different lignite coals.
In the case of co-incineration of lignite and biomass or lignite and biochar, it was also determined that a number of unpredictable deviations occurred in the reactivity indicators. This can be explained by the fact that the components of the fuel mixture interact with each other to create synergistic or antagonistic effects.
This study show that, as an alternative to parameters such as combustion rate, temperature, conversion rate and duration which are read from combustion curves, quantitative values that are calculated based on various reactivity indices can be used as reliable approach to compare the combustion reactivities of very wide range of solid fuels.