Linyitlerin dolaşımlı akışkan yatakta hava, oksijence zengin hava ve oksiyanma ortamlarında yanma davranımlarının incelenmesi
Investigation of air, oxygen-enriched air and oxycombustion characteristics of lignites in circulating fluidized bed
Yazar: BERRİN ENGİN
Danışman: PROF. DR. HÜSNÜ ATAKÜL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Doktora, 2019
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Bu çalışmada linyit kömürlerinin dolaşımlı akışkan yatakta (DAY) hava ile, oksijence zenginleştirilmiş hava (OZH) ile ve oksiyanma (OY) koşullarında yanma davranışları incelenmiştir. Havayla yapılan deneylerde, hava fazlalık katsayısı (HFK) 1.1 -1.4 (0.02) aralığında seçilmiştir. Oksijence zengin havayla yapılan yanma deneylerinde havadaki oksijen oranı %21-28 arasında değişmiştir. Oksiyanma çalışmaları ise geri döndürülen baca gazları  (DBG)+ O2 karışımlarıyla gerçekleştirilmiştir. Çalışmada, Türkiye'nin değişik bölgelerinden toplanmış toplam 19 adet linyit kömürü kullanılmıştır. Bu kömürler L1-L19 olarak kodlanmıştır. Havayla yapılan yakma çalışmalarında bütün kömürler, oksijence zengin hava ile yakma ve oksiyanma çalışmalarında ise ikişer kömür kullanılmıştır.  Her üç yanma rejimi deneylerinde kullanılan kömürler Bursa-Orhaneli (L15) ve Manisa-Soma (L19) bölgelerinden alınan kömürlerdir. Yanma deneyleri 30 kW termal kapasitesindeki dolaşımlı bir akışkan yatak yakma sistemi kullanılarak gerçekleştirilmiştir. Sisteme bağlı bir ölçme ve kontrol ünitesi vasıtasıyla deneyler boyunca, sistem içerisinde değişik seviyelerdeki noktalarda sıcaklık ve basınç değişimleri ve hat üstü (on-line) çalışan iki gaz analiz sistemi kullanılarak baca gazı emisyonları tüm deneyler boyunca sürekli olarak ölçülmüş ve kaydedilmiştir. Hava ve yakıt besleme debileri yine bu sistem vasıtasıyla ayarlanmıştır.  Kullanılan kömürler Van Krevelen grafiğinde linyitleri tanımlayan H/C (0.08-0.12) ve O/C (0.20-0.35) sınırları içerisinde kalmakta, ancak oldukça geniş bir dağılım göstermektedir. 
Kömürlerin yanma süreçleri yatışkın koşullara ulaşıldıktan sonra, yanma stabilitelerini test etmek amacıyla bu koşullarda uzun süre (>8 saat) devam ettirilmiştir ve herhangi bir sorunla karşılaşılmamıştır. Havayla gerçekleştirilen yanma sürecinde en düşük sıcaklıklar dağıtıcı elek civarında, en yüksek sıcaklıklar ise kömür besleme noktasının üstündeki bölgede ölçülmüştür. Benzer bir durum oksijence zengin havayla ve oksiyanma koşullarında gerçekleşen yanma deneylerinde de geçerlidir. Hava fazlalık katsayısındaki değişim sıcaklık profilinde önemli bir değişiklik yaratmamıştır. Ancak sıcaklık profili yanma ortamı tarafından etkilenebilmektedir. Oksijence zengin havayla gerçekleştirilen yanma sürecinde yüksek oksijen oranlarında yanma yoğunluğunun, havayla yanmaya kıyasla,  daha alt bölgelere kaydığı görülmüştür. Sıcaklık profilinin üniformluk derecesini belirlemek için temel alınan ve yakıcıda ölçülen en yüksek ve en düşük sıcaklıklar arasındaki farkı gösteren TAY, havayla yanma sürecinde biri hariç diğer tüm kömürler için 30 - 80 oC gibi görece düşük olmuş ve dar bir aralıkta değişmiştir. Kömürlerin başlangıç deformasyon sıcaklıkları temel alındığında, güvenli yanma açısından en hassas ve en güvenli olan kömürler sırasıyla Kütahya -Ayvalı pano (L3) ve Çanakkale-Çan (L17) linyitleri olmuştur. 
Havayla yanma sürecinde NOx ve SO2 gibi ana kirleticiler hava fazlalık katsayısı (HFK) tarafından etkilenmektedir. HFK artıkça NOx emisyonlarında bir artma eğilimi ortaya çıkmıştır. NOx emisyonu, genel olarak 100-400 mg/Nm3 aralığında değişmiştir. Kömürlerin %30'u için ise emisyonlar 200 mg/Nm3 altında kalmıştır. Kömürlerin yanma sürecinde SO2 emisyonları L7, L8 ve L19 haricindeki kömürler için 1100-20000 mg/Nm3 ölçülmüş olup geniş bir aralıkta değişmiştir. L7, L8 ve L19 linyitlerinin yanma sürecinde baca gazlarındaki SO2 miktarı ölçülmeyecek derecede düşük olmuştur. En yüksek SO2 emisyonları en yüksek yanabilir kükürt içeriğine sahip olan L17 (Syanar: %6.15) ve L18 (Syanar: %3.23) linyitlerinin yanması sırasında ölçülmüştür. Genel olarak SO2 emisyonu kömürlerin yanar kükürt içerikleriyle uyumlu olmuştur. Yani kükürt miktarı artıkça SO2 emisyonları artmıştır. Bu kirleticilerin birim enerji başına ölçülen miktarları, yanma rejimi tarafından da etkilenebilmektedir. Her üç yanma rejiminde kullanılan iki kömür, Bursa-Orhaneli (L15) ve Manisa-Soma (L19) için ölçülen NOx emisyonları (mg/MJ) hava ve oksijence zengin havayla yanma rejimlerinde göreceli olarak bir birbirine yakın olurken, oksiyanma koşullarında önemli ölçülerde düşmüştür. Bu düşme Orhaneli ve Soma linyitleri için, uygulanan yanma koşullarında,  sırasıyla %70-80 ve %50-60 aralığında değişmiştir.  Benzer bir durum SO2 emisyonları için de geçerlidir. Oksiyanma koşullarında, Orhaneli kömürünün yanması sırasında gözlenen SO2 emisyonları (mg/MJ), havayla yanmaya göre yaklaşık % 58, OZH ile yanmaya göre ise yaklaşık %60 daha düşük olmuştur. Soma linyiti için ise SO2 emisyonu hiçbir koşulda ölçülecek seviyelere çıkmamıştır. Kömürlerin yanma verimi ise %85-95 aralığında değişmiştir
Baca gazları geri döndürülerek kömürlerin oksiyanma süreci başarıyla gerçekleştirilmiştir. Bu süreçte baca gazlarındaki CO2 oranı O2/DBG karışımındaki oksijen oranı ile hızla yükselerek kuru temelde %96'ya kadar çıkarılabilmiştir.

Abstract:
Excessive dependency on imported fossil fuels is a serious threat to the energy security of Turkey. Turkey still imports up to 80% of her yearly energy needs which is a heavy burden on her national budget.  Lignite reserves are one of the major energy resources of the country. The currently determined reserves are about 15 billion tons and this figure is expected to increase as the new reserves are discovered in the recent explorations. However, the vast majority of these lignites, low calorific values, high ash, moisture and sulfur contents,  are classified as low quality coal according to the global criteria and can have significant environmental impacts when are used to generate energy. Therefore, the clean and efficient technologies are required for a cleaner energy production from these coals. Fluidized bed combustion technology (FBCT) is an advanced technology which has been widely studied and used in power generation based on fossil fuels. Research is still going on this technology, however, this technology is open to improvement for more efficient and cleaner burning of low grade and multiple fuels, including lignites and various biomasses. Although the studies on the fluidized bed combustion technology in Turkey date back to 1980s, they have mostly been confined to laboratory-scale and bubbling fluidized bed (BFB) combustion and remained limited.  Similarly, research on the combustion Turkish lignite in the circulating fluidized bed (CFB) also are very limited and did not produce suitable data that can be used for development of technology and design of the pilot- and industrial-scale combustor design. On the other hand, recently the focus of the research and development activities in power generation based on the coal has shifted to the oxygen-enriched combustion and the oxyfuel combustion aiming at combining power generation and tackling of CO2 emission problem. In Turkey, there have been few studies on oxygen-enriched and oxyfuel combustion of coal and biomass. Most of them, however, were conducted by using TGA method on microgram scales.  Therefore, there is need for research on the CFB combustion of Turkish lignites under air, oxygen-enrich air and oxyfuel combustion conditions in order to obtain data that can be used for design of these system and contribute to literature in the field.
In this study, the circulating fluidized bed combustion behavior of Turkish lignites in the air, the oxygen-enriched air and the oxyfuel combustion environments was investigated. Nineteen lignite samples taken from the major lignites reserves at different regions of Turkey were used.  All coals were supplied by Turkish Coal Enterprises (TKI).  They were designated as L1-L19. All of them were used in the air combustion and two of them were also used in the oxygen-enriched and oxyfuel combustion experiments. The combustion experiments were conducted in a circulating fluidized bed combustion system. It was consisted of a combustor, the fuel, sorbent and gas feeding units, the flue gas line, the ash removal, the gas recirculating, the gas analysis and the control units and had a 30 kW thermal capacity. Combustion/fluidization air was supplied by an air blower, oxygen was supplied from oxygen cylinders and flue gas was recycled by recirculation line and feed to the combustor through distributor plate. An induce fan attached to the flue gas line was used to provide a slightly negative pressure in the combustor. All air was fed to combustor as primary air without air staging. Temperature and pressure at different levels along the axis of the combustor and downcomer were measured. Similarly, along the courses of combustion experiments, the flue gas analyses were continuously done via two on-line gas analysis equipment. The flow rates of fuel, adsorbent, air, oxygen and carbon dioxide were measured and adjusted by using control system. All data obtained from measurements were recorded and saved in a computer attached to control unit. 
In the air-combustion experiments, equivalence ratio was chosen in the range of 1.1-1.4 (±0.02). Oxygen ratio in the air was changed in the range of 21-28% in the oxygen-enriched air combustion experiments. Oxyfuel combustion experiments were performed by mixtures of the recycled flue gases and oxygen. 
All coal samples were characterized by using the related appropriate standard analysis methods including proximate, ultimate, calorific and ash analyses, for their fuel and ash characteristics. Results showed that coal samples were of highly diverse nature. On the original basis, their approximate analyses yield:14-37% moisture, 20-40% volatile matter, 6-56% ash, 4-40% fixed carbon. Ultimate analyses: 20-70% carbon, 1.84-4.74% hydrogen, 0.69-2.44% nitrogen and 0.7-5.7% sulphur. The lower calorific values (LCV) varied in range of 5-22 MJ/kg and 6-27 MJ/kg on the original and the dry basis, respectively. 42% of the coal had LCV less than 15 MJ/kg on dry basis. 
The H/C and O/C ratios of coals varied in the range 0.042-0.083 and 0.08-0.38, respectively. Therefore, coal samples mostly were in O/C ratio range specified for lignites in the Van Krevelen graph, but highly scattered. Only two coals (L1 and L2) remained out of lignite O/C ratio range in the graph. Almost all coals, however were out of the H/C zone which was specified as lignite in the same graph.
On average, the combustion of coals lasted more than 8 hours after steady-state was attained. All coals maintained a stable and steady combustion and no problem (such as sintering and agglomeration) was observed under the operating conditions applied. In the combustion process with air, the lowest temperatures were measured in the vicinity of the distributor plate and the highest temperatures were measured in the region above the coal feed point. In a typical profile, the temperature increased with height from the distributor plate, reached a maximum level in the region at about 100-250 cm from the distributor plate and then gradually and slightly decreased toward to the exit of the combustor. Generally, a similar trend in temperature profiles was observed in the oxygen-enriched air and oxyfuel combustion tests. Some changes seen in the profiles of these two combustion processes, however, indicated that the temperature profile can be affected by the combustion atmosphere. In oxygen enriched combustion at high oxygen concentrations the combustion density has shifted to the lower region of the combustor, compared to the air burning. Temperature profiles of the oxyfuel combustion resembled to that of the air combustion but the temperatures in the combustor were higher in comparison to both the air and oxygen-enrich combustion. The change in equivalence ratio, on the other hand, did not significantly affect the temperature profile. The temperature difference (TAY) between the regions where the highest and the lowest temperatures were measured was used to quantify the uniformity degree of the temperature profile in the combustor. In the air combustion TAY varied in the range of 30-80°C for 18 out of 19 lignites. TAY was slightly higher in the oxygen-enriched and oxyfuel combustion of two coals which burned in tree environments. The operating safety of the combustor burning lignites was evaluated based on difference between the initial deformation temperature and the maximum temperature coal particles attained  (safety margin) during combustion. In the air combustion, the operating safety margins of coals varied in a wide range of 88-562 °C.  Kütahya-Ayvalı pano  (L3) and Çanakkale-Çan (L17) had the lowest and the highest safety margins, respectively.  For the two coals, Orhaneli (L15) ve Soma (L19), test in three combustion environments, the safety margin varied in descending order as:  air combustion > oxygen enriched combustion > oxyfuel combustion.                                     The main emissions such as NOx and SO2 are affected by equivalence ratio (ER) in the air combustion. As ER increased, NOx emissions also tended to increase. Generally, the NOx emission varied in the range of 100-400 mg/Nm3. For 30% of the coals, emissions were below 200 mg/Nm3. In the combustion process of coals, SO2 emissions mostly were as high as 1,100-20,000 mg/ Nm3 and changed over a wide range except three coals. SO2 emissions in the flue gases were too low to be measured during the combustion of three lignites, namely L7, L8 and L19. The highest SO2 emissions in flue gases were measured, as may be expected, for the coals with the highest contents of combustible sulfur, such as L17 (Scomb: 6.15%) and L18 (Scomb: 3.23%).  In general, the SO2 emissions were o consistent with the combustible sulfur content of the coals. In other words, SO2 emissions increased as the amount of combustible sulfur in the coals increased or vice versa. The emissions of pollutants per unit energy generated also affected by combustion regimes. The NOx emissions (mg/MJ) measured from combustion of two coals, Bursa-Orhaneli (L15) and Manisa-Soma (L19) which were tested in three combustion regimes, were similar in the air and the oxygen-enriched air combustion regimes, but significantly decreased under the oxyfuel combustion conditions. Decrease in NOx emissions were about 70-80% and 50-60%  in the oxyfuel combustion of Bursa-Orhaneli (L15) and Manisa-Soma (L19), respectively, in comparison to that of the air combustion under the conditions applied. A similar trend is valid also for SO2 emissions. In oxyfuel combustion atmosphere, SO2 emissions per unit energy (mg/MJ) from Orhaneli lignite (L15)  were about 58% lower than of the air combustion and about 60% lower than that of the oxygen-enriched combustion. For Soma lignite, the SO2 emissions have never been high enough to be measured under conditions applied. 
In the air-combustion experiments, different limestone samples were used as adsorbents in order to retain the sulfur oxides in-situ (in bed) formed during combustion of lignites in the circulating fluidized bed. Different Ca/S ratios were determined and SO2 retaining efficiencies were calculated.  
Combustion efficiencies calculated varied in the range of 85-95%. 
The oxyfuel combustion processes of coals have been successfully carried out by recirculation of flue gases (RFG). During the course of oxyfuel combustion of coals, the CO2 concentration in the flue gases rapidly increased with increase in oxygen concentration in the O2/RFG  mixture and reached to 96% on dry basis.