Combustion characteristics of torrefied biomass materials to generate power

Co-firing of biomass with coal for power generation is an effective way to lower the use of fossil fuels and to mitigate CO₂ emissions. However, high moisture content, high yields of volatiles, low density, and insufficient calorific value of biomass are the main challenges to burn them with coal in co-combustion systems. Also, co-firing systems usually suffer mainly from high volatile matter content of biomass that leads segregation of coal and biomass from each other during co-combustion. In this context, several woody biomass species such as olive milling residue (OMR), rhododendron (RH), and ash tree (AT) were subjected to torrefaction process at temperatures of 200, 250, and 300°C under nitrogen atmosphere to produce biochars that have lower content of volatiles and higher content of calorific value. The biochars were characterized by the proximate and ultimate analyses and the calorific value. It was found out that so significant improvements can be achieved in the calorific values that the biochars became almost comparable with sub-bituminous coals in terms of the calorific value. Besides, the burning reactivities of these biochars revealed that significant reductions took place in the burning reactivities through torrefaction. In addition, the burning characteristics of these biochars were investigated by thermal analysis methods.

I. Introduction

Biomass is the 4^th biggest primary energy source after oil, coal, and natural gas. However, the share of biomass in electricity generation is almost negligible and the most of the biomass energy is directed to either heat generation or biofuel production. On the other hand, power generation based on fossil fuels creates serious global problems such as the increasing greenhouse gas emissions and foreign dependency on supply of fossil fuels [1]. In contrast to fossil fuels, biomass energy sources can be easily found almost everywhere. Since biomass is regarded as an energy source which is renewable, sustainable, and CO₂-free, there have been miscellaneous efforts to incorporate biomass into existing power stations where coal is burned [2]. Co-firing systems where the blends of coal and biomass are burned have been of great importance to achieve this target. Co-firing of coal with biomass has been performed via three options such as direct co-firing, indirect co-firing, and parallel co-firing. Of which, the direct co-firing is by far the most common method that enables to use biomass up to 3% on energy basis [3]. Differences between the structures and the thermal reactivities of biomass and coal restrict to further increase the share of biomass at direct co-firing [4]. On the other hand, indirect co-firing covers gasification of biomass into a fuel gas to provide high degree of fuel flexibility, and the share of biomass could be enriched in this way up to 17% in Lahti plant in Finland. Besides, parallel co-firing systems rely on installation of a separate biomass boiler and utilization of the steam produced in the coal power plant system.