Oksijence zengin hava ve CO2 ortamında çeşitli yakıtların yanma özelliklerinin incelenmesi

TÜBİTAK MAG Proje01.11.2010Dünyadaki hızlı nüfus artışı ve ekonomik gelişmeler, enerji talebindeki artışı da tetiklemektedir. Kömür ispatlanmış rezervleri ve yıllık üretim miktarları açısından bakıldığında enerji üretiminde oldukça önemli bir paya sahiptir. Ancak dünyada artmakta olan enerji talebiyle, sera gazlarından olan CO2’nin emisyonunda da artış gözlenmiştir. Bu nedenle son yıllarda artan sera gazı salınımlarının azaltılması için temiz kömür teknolojileri geliştirilmekte ve alternatif teknolojiler üzerine çalışmalar yapılmaktadır. Bu teknolojilerden öne çıkan biri oxy-fuel teknolojisidir. Bu teknolojide yakıtların oksijence zengin hava veya karbondioksit ortamında yakılmasıyla baca gazındaki CO2 konsantrasyonu arttırılmakta ve bu sayede CO2 gazının baca gazından daha kolay bir şekilde ayrıştırılması sağlanmaktadır. Yakıtların oksijence zengin bir ortamda yakılmasıyla yanma sonrası açığa çıkan sera gazları azaltılmakta, yanma süresi kısaltılmakta ve yanma verimi arttırılmaktadır. 109M401 kodlu proje kapsamında, başlangıç aşamasında oxy-fuel teknolojisinin laboratuvar ölçekli bir sistemde denenmesi hedeflenmiştir. Buna göre, yapılan çalışmada endüstriyel uygulamalarda kullanılan ithal kömür, petrokok, yerli linyitler ve prina (zeytin artığı) gibi çeşitli yakıtların, azot ve karbondioksit ortamında piroliz; hava, oxy-fuel (%21 O2 - %79 CO2), oksijence zengin hava (%30 O2 - %70 N2) ve oksijence zengin oxy-fuel (%30 O2 - %79 CO2) ortamlarında, yanma davranışları TGA-FTIR birleşik sistemi kullanılarak incelenmiştir. Yapılan piroliz testlerinde tüm yakıtların her iki ortamda da 700o C’a kadar benzer kütle kayıp profilleri sergilediği, 700 o C’dan sonra ise kömür-CO2 gazlaşma reaksiyonundan dolayı profillerin farklılaştığı görülmüştür. Yakma testlerinde ise karbondioksitçe zengin oxy-fuel koşullarında, hava koşullarına oranla yanmanın geciktiği görülmüştür. Yanma atmosferinde oksijen konsantrasyonunun artmasıyla karakteristik sıcaklıkların düştüğü ve yanmanın daha kısa sürede tamamlandığı görülmüştür.Population growth and economical developments in the world triggers the increase in energy demand. Coal has very important share in energy production in terms of the amount of proven reserves and annual production. However, the growing world energy demand leads to higher emission of CO2which is one of greenhouse gases. For this reason, in recent years to reduce greenhouse gas emissions,clean coal technologies are being developed and studies have been continuously carried out. In recent years oxy-fuel combustion stands out as one of the prefered in carbon capture and storage (CCS) technologies. In this technology fuels are burned in oxygen enriched air or carbon dioxide atmosphereswhich lead to reduction in CO2 greenhouse gases with decrease in combustion time and to higher combustion efficiency. In 109M401-coded project, it is aimed to carry out laboratory scale tests on oxy-fuel combustion technology. According to the stated aim, pyrolysis tests were carried out under nitrogen and carbon dioxide environments while combustion characteristics were investigated under air, oxygen enriched air(30% O2-70% N2), oxy-fuel (21% O2-79% CO2)and oxygen enriched oxy-fuel (30% O2-70% CO2)conditions by using five different fuels that are widely used in the industrial applications such as imported coal, petroleum coke, indigenous lignites and olive residue in TGA-FTIR combined system. The pyrolysis testsreveal that all fuels display similar behaviour up to 700oC. However weight loss profiles differ after 700oC due to char-CO2 gasification reaction. Combustion tests reveal that oxy-fuel conditions lead slight delay in combustion, while oxygen enrichment shift combustion profiles to lower temperatures and leads to fall in characteristic temperatures

EVALUATION OF FLUX MODELS FOR RADIATIVE-TRANSFER IN RECTANGULAR FURNACES

Three flux-type models for three-dimensional radiative heat transfer were applied to the prediction of the radiative flux density and the source term of a box-shaped enclosure problem based on data reported previously on a large-scale experimental furnace with steep temperature gradients. The models, which are a six-term discrete ordinate model and two Schuster-Schwarzschild type six-flux models, were evaluated from the viewpoints of both predictive accuracy and computational economy by comparing their predictions with exact solutions produced previously. The comparison showed that the six-flux model based on angular subdivisions related to the enclosure geometry produces more accurate results and is computationally less expensive than the other two models

Dynamic simulation of fluidized bed combustors and its validation against measurements

A dynamic mathematical model of a continuous atmospheric bubbling fluidized bed combustor (ABFBC) has been developed on the basis of first principles and used to correlate data from a pilot-scale combustor, The model accounts for bed and freeboard hydrodynamics, volatiles release and combustion, char particles combustion and their size distribution, and heat transfer. The solution procedure of dynamic model employs method of lines (MOL) approach for the solution of the governing nonlinear partial differential equations. The initial conditions required for each sub-model was provided from the simultaneous solution of governing equations of dynamic model with all temporal derivatives set to zero. Predictive accuracy of both steady state and dynamic models was assessed by applying it to the prediction of the behavior of METU 0.3 MW ABFBC Test Rig, and comparing their predictions with measurements taken on the same rig at steady state and transient conditions. Transient data were obtained by imposing changes to coal and air flow rates and measuring corresponding changes in oxygen and carbon monoxide concentrations at the exit of freeboard and in temperatures along the combustor against time. Comparisons of measurements and predictions chow that the predictions of the model are physically correct and agree well with the measurements

A novel cfd code based on method of lines for reacting flows: Verification on methane/air diffusion flame

A novel parallel computational fluid dynamic (CFD) code based on method of lines (MOL) approach was developed for the numerical simulation of multi-component reacting flows using detailed transport and thermodynamic models. The code was applied to the prediction of a confined axi-symmetric laminar co-flowing methane-air diffusion flame for which experimental data were available in the literature. 1-, 5- and 10-step reduced finite-rate reaction mechanisms were employed for methane-air combustion sub-model. Steady-state velocity, temperature and species profiles obtained by all the mechanisms were validated against experimental data and were found to be in reasonably good agreement with measurements. The code was found to be a useful tool for the prediction and understanding of transient combustion systems

Performance of DOM and IDA with different angular discretization methods in 3-D absorbing-emitting-scattering media

Predicted accuracy and computationally efficiency of Discrete Ordinates Method (DOM) and Improved Differential Approximation (IDA) were evaluated by applying both methods to cubical enclosure problems containing purely isotropically/linearly anisotropically scattering/absorbing-emitting-isotropically scattering medium and comparing their predictions with benchmark solutions available in the literature. Performances of DOM and IDA with S-8, S-10 and T-4 quadratures were assessed by comparing their predictions against benchmark solutions. DOM with S-8 and IDA with T-4 were found to provide more accurate and efficient solutions compared to other discretization schemes. However, overall comparisons reveal that DOM S-8 produces higher accuracy and computational efficiency than IDA T-4 on all test problems under consideration

EXACT NUMERICAL-SOLUTIONS FOR RADIATIVE HEAT-TRANSFER IN CYLINDRICAL FURNACES

Exact expressions for the distributions of the components of radiative flux density and the radiative energy source term in terms of wall and medium temperature distributions have been formulated for an emitting absorbing medium of constant properties bounded by black walls of a cylindrical enclosure. The accuracy of numerical solutions has been tested on an idealized enclosure for which exact analytical solution of the expressions is possible and shown to have six‐figure accuracy. The exact expressions have then been solved numerically for an enclosure problem based on data reported previously on a large scale experimental furnace. The principal feature of the data is highly non‐uniform temperature distributions which are typical of the conditions encountered in industrial furnaces. These data have been chosen because of their practical importance and the non‐availability of exact solutions for such data. The resulting exact solutions have been tabulated and are intended to serve in the future as standards for testing the accuracy of the approximate predictions produced using various three‐dimensional flux models in cylindrical configurations

SENSITIVITY OF RADIATION MODELING TO PROPERTY ESTIMATION TECHNIQUES IN THE FREEBOARD OF LIGNITE- FIRED BUBBLING FLUIDIZED BED COMBUSTORS ( BFBCs)

Predictive accuracy and computationally efficiency of method of lines (MOL) solution of the discrete ordinate method (DOM) coupled with different radiative property estimation techniques (GG, SLW, SNBCK) are assessed by applying them to the prediction of incident radiative fluxes along the freeboard walls of a 0.3 MWt atmospheric bubbling fluidized bed combustor (ABFBC) and comparing their predictions with measurements generated previously from two runs one without and the other with recycle. Freeboard is treated as a three-dimensional rectangular enclosure containing a gray/non-gray, absorbing-emitting-isotropically scattering medium. Radiative properties of particles are evaluated by using geometric optics approximation. A comparative study is also provided between the source term distributions predicted by MOL solution of DOM with gray gas (GG), spectral line-based weighted sum of gray gases (SLW), and statistical narrow-band correlated-k (SNBCK) along the centerline of the freeboard for both runs. Comparisons reveal that the MOL solution of DOM with SLW model and geometric optics approximation provides accurate and computationally efficient solutions for wall fluxes and source term distributions in the freeboard of fluidized bed combustors containing particle laden combustion gases

Elektronik bilgi kaynakları için entellektüel mülkiyet ve dijital içerik hakları, düzenleme ve dijital ödeme sistemleri

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