Pyrolysis behavior and economics analysis of the biomass pyrolytic polygeneration of forest farming waste

Highlights

• Biomass pyrolytic polygeneration can convert waste shell into high value products.

• Tri-state products varied with pyrolysis temperature.

• A lower operation temperature gave higher yield, but lower quality.

• The optimum pyrolysis temperature was 350–450 °C depending on the type of shell.

Abstract

The pyrolysis behavior of Chinese chestnut and Jatropha curcas shells (CNS and JCS, respectively) were investigated to determine the optimum operating temperature for biomass pyrolytic polygeneration systems. At low temperatures (250–450 °C), CO₂ was the main component of the pyrolytic gas, and high acidity oil was obtained. When the temperature increased to 550–650 °C, phenol-enriched oil and high LHV biochar (∼26 MJ/kg) were obtained; H₂ and CO yields increased. At high temperatures (750–950 °C), heavy-oil and high LHV pyrolytic gas (∼15 MJ/m³) were obtained. Meanwhile, the biochar showed a highly condensed aromatic ring system and low H/C (∼0.1) and O/C (∼0.05) ratios. CNS and JCS biochars showed different tendencies with regard to their structure evolution. An economic analysis was performed, which suggested that the optimum operating temperatures were 450 °C for CNS and 350 °C for JCS.

Introduction

Over the past few decades, the “Returning Farmland to Forest” program has played an important role in the increase in forest coverage in China, albeit at the expense of farmland. To improve the living standards of the consequently landless farmers, forest farming has been vigorously supported by the Chinese government (He and Sikor, 2015). Some of the crops widely grown in Chinese forest farming include the Chinese chestnut and Jatropha curcas, well known as a delicious nut and biodiesel precursor (Sun et al., 2017b), respectively. Generally, the seeds of these crops are harvested with their shells; once the product is separated, waste Chinese chestnut shell (CNS) and J. curcas shell (JCS) remain. Typical forest waste (branches, bark, sawdust, etc.) is regarded as an important biomass resource (Chen et al., 2018b). However, the utilization of this new forest farming waste has not been researched or regulated, leaving it to be directly burned, likely in the farmer’s residential area, contributing to air pollution. Hence, it will be beneficial to both farmers and the environment if these wastes were converted to bioenergy, biofuels, or biomaterials using modern biomass conversion technologies (Wang et al., 2017, Zhang et al., 2017).

The process of biomass conversion can be divided into biochemical and thermochemical methods (Zhang et al., 2018). The latter includes combustion, gasification, and pyrolysis. Biomass combustion for heat or electricity is a quick method, but further development of this method is restricted by downsides such as the production of polluting particulates, and the huge costs involved for feedstock collection and transportation (Wang et al., 2014). Gasification is another effective method of converting biomass waste to syngas (CH₄, H₂, CO, CO₂, etc.). However, the poor lower heating value (LHV; 3–5 MJ/m³) and high tar content of syngas from biomass gasification remain to be solved.

Pyrolysis can convert biomass into products of three states (tri-state) by heating in an inert atmosphere. These tri-state products, i.e., biogas, solid char, and liquid bio-oil, all have relatively high value or potential as intermediary products (Chen et al., 2012, Collard and Blin, 2014, Ji et al., 2017, Yang et al., 2016). Recently, biomass pyrolytic polygeneration of these products has been investigated as a promising conversion technology. Chen et al. studied the process of cotton stalk pyrolysis at laboratory scale, and found that 550–750 °C was a suitable operating temperature range (Chen et al., 2012). Yang et al. found that a commercial biomass pyrolytic polygeneration factory using retort reactors had high performance in converting straw to high-value products (Yang et al., 2016). Gao et al. investigated the effect of temperature during the pyrolysis of rapeseed stalk on the characteristics of the tri-state products, along with an economic analysis (Gao et al., 2017). They found that 350 and 650 °C were the optimum operating temperatures for char sold as solid fuel and activated char, respectively. However, these studies mainly focused on the pyrolysis of stalks; shells, as an abundant forest farming waste, were ignored. In addition, shells have high bulk density (∼0.42 g/cm³ for CNS and ∼0.49 g/cm³ for JCS) compared to stalks (∼0.11 g/cm³ for rapeseed stalk), which is good for transportation and economics Furthermore, CNS and JCS are easy to collect, as they are harvested with the seed product, while the collection of stalks faces problems due to their scattered distribution. Thus, the high bulk density and low collection cost of shell forest farming waste makes it suitable for biomass pyrolytic polygeneration.

Herein, two typical forest farming wastes were converted using a pyrolytic polygeneration system. The products were characterized by various analyses to speculate on potential applications. Furthermore, an economic analysis was performed based on the use of a moving bed. These data were used to elucidate the optimum processing condition for biomass pyrolytic polygeneration when using shells waste as feedstock. It is expected that commercial-scale biomass pyrolytic polygeneration can be utilized for the effective conversion of the abundant shell waste in China.