Statistical evaluation of the carbonization results of hazelnut shell

Abstract

Carbonization experiments has been conducted on the samples of hazelnut shell (Corylus Avellana) to determine the effect of carbonization variables such as temperature, heating rate, and particle size on the biochar yield. A statistical design technique was applied by use of a two-level factorial design matrix to interpret experimental results. Carbonization experiments were carried out at a static gaseous atmosphere in a Jenkner type fixed bed retort system which is a cylindrical stainless steel fixed bed reactor with a length of 270 mm and an inner diameter of 130 mm. The reactor was heated externally by an isolated electrical furnace where temperature was measured by a Ni-Cr-Ni thermocouple inside the bed. During the carbonization the inner and outer temperatures of the reactor were controlled continuously with a temperature control instrument. Before heating, the system was flushed with dry nitrogen for 30 minutes to remove all traces of oxygen. After carbonization at final temperature which lasted 30 minutes, the final weight of samples was determined to calculate the biochar yield. It was found that the biochar yields of samples were changed depending on the carbonization conditions. Amprical relations between the biochar yield and the carbonization conditions were developed. Biochar yield of hazelnut shell was decreased with the increasing temperature and heating rate and increased with the increasing particle size. The regression equation clearly showed that since the coefficient of temperature is the highest among all the variables, the effect of this variable on the char yield is the strongest. The decrease in the char yield at the higher temperature is connected with secondary decomposition of the char and this situation is consistent with the increase in the volatile matter. The heating rate affects the amount and rate of volatile matter evolution from the sample during carbonization. The increase in the rate of volatile matter evolution also prevents the formation of further reactions which increase the char yield. An increase in heating rate decreases the restricted effects of mass transfer and increases the decomposition of the biochar into liquid and gas product. Contrary to the effect of the temperature and heating rate, particle size variable has a positive influence on the the biochar yield values which means that biochar yield values are increased with increasing particle size and decreased with decreasing particle size. A possible reason for the observed decrease in biochar yield with decreasing particle size could be that smaller particles are heated more uniformly and thus heat and mass transfer are facilitated. This results in a higher mass loss and reduction in biochar yield. Larger particles may cause higher heat transfer resistance that results in lower interparticle temperatures and lower volatile evoluation. The higher the volatile residence time in the larger particles can result the greater possibility of secondary reactions. By taking into account the above results, the optimum carbonization conditions for maximum biochar yield were determined as T=773 K, $R_t$=5 K /min and $D_p$=1-1.4 mm. The raw material and biochar product obtained at maximum yield condition were characterized and compared according to their fuel properties. For this purpose gross calorific value and proximate analysis of samples were performed and compared. The raw material has very high volatile matter and very low ash value. Biochar obtained in this study is a carbon-rich fuel and contains 6.58% ash, 30.26% volatile matter, and 63.16% fixed carbon. In comparison with raw material, biochar sample can be characterized as carbon rich solid fuel with high content of fixed carbon and low volatile matter content. Gross calorific values of raw material and biochar samples were found as 18.33 MJ $kg^{-1}$ and 29.08 MJ $kg^{-1}$, respectively. Biochar has a higher calorific value than the apricot stone due to its low moisture and high fixed carbon content. This study proved that experimental design methodology could efficiently be applied for evaluation of analytical parameters affecting biochar yield and that it is an economical way of obtaining the maximum amount of information in a short period of time and with the few number of experiments. Moreover, this study shows that the hazelnut shell can be a resource for biochar production. After further investigations, biochar can be used for various industrial processes. High amount of hazelnut shell can be obtained in Turkey. Not only hazelnut shell but also other shells can be used as a resource of carbonization for production of alternative fuels and activate carbon. It can be concluded from the overall results of this study that carbonization creates a different application area for hazelnut shell.

Özet

Çalışmada, farklı karbonizasyon koşullarının katı ürün verimi üzerine olan etkisini belirlemek amacıyla, fındık kabuğunun (Corylus Avellana) karbonizasyonu gerçekleştirilmiş ve sıcaklık, ısıtma hızı ve tanecik boyutunun karbonizasyon katı ürün verimi üzerine olan etkileri araştırılmıştır. Karbonizasyon deney sonuçları, iki seviyeli faktoriyel tasarım matrisine göre, istatistiksel olarak değerlendirilmiştir. Karbonizasyon deneyleri, sabit yataklı Jenkner tipi retort kullanılarak, statik gaz atmosferinde gerçekleştirilmiştir. Karbonizasyon sonucu elde edilen katı ürünün verimini en fazla etkileyen parametrenin, karbonizasyon sıcaklığı olduğu belirlenmiştir. Karbonizasyon sıcaklığından sonra sırasıyla, tanecik boyutu ve ısıtma hızı da katı ürün verimini etkilemektedir. Katı ürün verimi, artan sıcaklık ve ısıtma hızıyla azalırken, artan tanecik boyutuyla birlikte artış göstermiştir. Statik gaz atmosferinde yürütülen karbonizasyon deneyleri sonucunda, en yüksek katı ürün verimine sıcaklığın 723 K, tanecik boyutunun 1-1.4 mm ve ısıtma hızının 5 K/dak olduğu koşullarda ulaşılmıştır. Çalışmada, faktoriyel tasarım tekniğiyle, karbonizasyon sonucu oluşan katı ürünün verimini etkileyen parametrelerin istatistiksel olarak değerlendirilmesi yanında, ayrıca, fındık kabuğu ve karbonizasyon sonucu en yüksek verimde elde edilen katı ürünün, kısa analiz ve ısıl değer sonuçları karşılaştırılmış ve katı ürünün ısıl değerinin ve sabit karbon içeriğinin fındık kabuğuna kıyasla arttığı, uçucu madde içeriğinin ise azaldığı gözlenmiştir. Sonuç olarak, karbonizasyon sonucu elde edilen katı ürünün, yüksek enerji içeriğine sahip, çevreye dost, katı bir yakıt olarak değerlendirilebileceği belirlenmiştir.