Electricity generation from pyrolysis gas produced in charcoal manufacture: technical and economic analysis

Abstract

Brazilian steel industry based on charcoal is unique in the world, making Brazil the largest world charcoal producer and the only one that produces green pig iron in large scale, a low-carbon and economic option to the traditional pig iron produced from coal coke. The renewable charcoal originates from the slow pyrolysis of eucalyptus wood produced in high productivity reforestation farms thought traditional batch reactors. Unfortunately, the state-of-the-art of charcoal making in Brazil do not comprise the pyrolysis gases utilization, wasting more than 30% of wood energy decreasing air quality and producing environmental and social impacts. Therefore, in this sense, the goal of this work is to evaluate the possibility to recovery this energy to produce electricity, and issues the topics related to the use of a very lean fuel, with flammability problems, which varies its composition over time, and the lack of water to be used as cooling mean for the thermodynamic cycles in the eucalyptus farms. The gases Lower Flammability Limit (LFL) was analyzed by the Calculated Adiabatic Flame Temperature (CAFT) method showing that in the first half of pyrolysis, the gases are generally under the LFL line, what could be solved by the gases pre-heating or mixing with richer fuel produced by kilns in advanced carbonization stage. Results showed an average gas Lower Heating Value (LHV) of 1323 kJ/kg. To overcome gas production instability a cluster system of 120 rectangular carbonization kilns working in synchrony was accessed through a computational tool, where it was found a minimum thermal power available of 57.5 MWth or 25.6% of initial wood energy content. Three energy conversion technologies were evaluated: the Steam Rankine Cycle (SRC), the Externally Fired Gas Turbine (EFGT) and the Organic Rankine Cycle (ORC). In this work, it was found that the efficiency of the SRC, EFGT and ORC conversion technologies are 24.0%, 19.3% and 23.7%, respectively. An economic analysis indicated the SRC as the most viable technology with a potential to generate 0.93 MWhel per ton of charcoal produced. Brazil has as estimated energy loss of 2.5 Mtoe (million tonne of oil equivalent) annually in the form of carbonization gases, which could be used to produce up to 5644 GWhel of electricity and reduce the Greenhouse Gases (GHG) emissions up to 15 Mton CO2–eq.