The production of quality fuels from the co-combustion of original, pyrolysed lignite and torrefied, pyrolysed, original biomass blends

Rapid increase in global greenhouse gas emissions (GHG) that is particularly produced by fossil fuel consumption has alarmed the world to focus on renewable energy sources and developing technologies that cause nearly zero emission. With 459 MtCO2e in 2013, Turkey is one of the top emitting countries in Europe due to the fact that the country's energy demand is increasing in parallel with population growth and industrialization. Turkey is almost lack of oil and large natural gas reserves and therefore depends on foreign energy sources. In order to decrease the share of imported resources in energy generation, strategies for securing energy supply and sustainable development were settled. Since the lignite coal is the major national fuel resource in Turkey, consumption of domestic coal was pointed out as the main goal by Turkish Energy Ministry and supported by government incentives. Co-combustion of coal and biomass renders possible utilization of Turkish lignites. Since Turkey is rich in young lignite deposits of which calorific value is quite low for an efficient and sustainable energy production, biomass addition to lignite has been marked as a reasonable solution to decrease foreign energy dependency of the country by increasing national primary energy production. Moreover, regarding to environmental concerns for increasing GHG emissions, co-combustion of biomass and lignite blends seems reasonable to prevent over-consumption of coal and diminish pollutant emissions thanks to the carbon neutral nature of biomass. Biomass addition to coal prior to burning has many justifiable reasons. Ash deposition and fouling on hot surfaces are commonly encountered problems when biomass is combusted alone and can be eliminated or decreased by co-firing of biomass/coal blends. Furthermore, emitted CO2 by burning of biomass is captured during photosynthesis that prevents greenhouse effect thanks to the fixation of atmospheric CO2. Many types of biomass species contain alkaline and alkaline earth elements in their composition that enable to capture released SO2 from combustion of coal, rich by sulfur content. On the other hand, undesirable effects of potassium that is one of the important element in most biomass materials, can be diminished as potassium reacts with sulfur coming from coal in the blend. Agricultural potential of Turkey that can be used as renewable energy source is substantial and has to be benefited due to its numerous advantages. According to the studies carried out by Forest and Water Affairs Ministry in 2002 to determine Turkey's biomass potential, Rhododendron was indicated as one of the suitable biomass species that can be used for energetic purposes. Hybrid poplar and ash-tree are also two of the most rapid growing woody biomass species along with their high economic value. Hence, these biomass materials were selected in this study to blend with Afsin-Elbistan (AE), Adıyaman-Gölbaşı (AG) and Çorum-Dodurga (CD) lignites which are used in many lignite-fired power stations in Turkey. Particularly, AE lignite is the most important domestic primary energy source in Turkey with a reserve of 3.4 billion metric tonnes. Since these lignites have low fixed carbon, low calorific value, high ash and sulfur contents, combustion of lignite leads to serious problems that are needed to be handled. Thus, blending of low rank coal and energetically important biomass materials for co-combustion processes enables to compensate undesirable characteristics of both biomass and lignites. In order to improve fuel characteristics of coal and biomass, thermochemical conversion methods such as pyrolysis, carbonization, gasification and liquefaction are applied. The aim of this study is to produce high quality fuel from co-combustion of the blends made of original, pyrolysed lignite samples and original, torrefied, pyrolysed biomass samples. Since understanding of the physical and chemical structure of lignite coal and biomass during thermal processes is essential for the design purposes and optimization of present conversion systems, burning test of the blends were performed using a thermogravimetric analyzer up to 900°C with a heating rate of 40°C/min under dry air atmosphere. Burning characteristics of the parent and blend fuels were determined from the TGA, DTG, DTA and DSC profiles obtained as an output of the thermal analyses experiments. Torrefaction of biomass samples at 200°C and pyrolysis of both biomass and lignite samples at 400°C and 750°C, respectively were carried out in the horizontal tube furnace under nitrogen atmosphere to prepare different types of blends that contain 10 wt% biomass and 90 wt% lignite. Proximate and ultimate analyses, calorific value measurements and SEM analyses were applied to parent fuels only excluding thermal analyses which was conducted for both parent and blend fuels in order to assess the co-combustion behavior of the blends and determine synergistic effects between biomass and lignites. Moreover, in order to obtain a deep information about the thermal decompositions and burning processes of the parent and blend fuels, kinetic study was carried out by applying the Borchardt & Daniels (B&D) kinetic analysis method. Kinetics parameters such as reaction order, heat of reaction, activation energy, and pre-exponential factor were calculated from the data obtained during a single linear heating rate scan and results were discussed.