Research articleAggravated fine particulate matter emissions from heating-upgraded biomass and biochar combustion: The effect of pretreatment temperature
Introduction
Utilization of more sustainable and environmentally-friendly sources of energy can help to abate the effect of greenhouse gas emissions. Biomass is an alternative source that is carbon-neutral with a potential to reduce CO2 and SOx/NOx emissions [1], [2]. As a big agricultural country, China has an abundant biomass energy resource, and its direct combustion is of great potential to reduce fossil fuel utilization and ensure energy-supply security. However, utilization of raw biomass for commercial power generation still faces several drawbacks to its implementation [3], [4]. These drawbacks include low bulk density, low energy density, high moisture content, high logistic cost and poor grindability [5], [6]. Therefore, for industrial use, many technologies have been proposed to upgrade biomass materials, overcome these drawbacks and improve the efficiency of biomass utilization. Heat pretreatments, including torrefaction and pyrolysis, are simple, promising and effective methods for biomass upgrading [7], [8].
Torrefaction is a mild thermolysis process at relative low temperatures of 200–300 °C, mainly to improve the thermo-chemical properties of biomass [9], [10]. In contrast, pyrolysis results in the deep decomposition of biomass materials at relatively higher temperatures, mainly converted into bio-fuels with high energy density, such as liquid (bio-oil), gas and solid sample (biochar) [11]. The solid products from both torrefaction and pyrolysis share significant improvement on fuel properties, with high energy density and good grindability, and are suitable for the utilization as alternative and supplementary fuels for power generation [12], [13].
Due to the high chlorine and alkali metal contents in biomass, direct combustion or co-firing with coal contributes significantly to PM1.0 (particulate matter with aerodynamic diameter < 1.0 μm) emission [14], [15], inducing ash deposition, fouling, and slagging [16], [17]. The heat pretreatment of biomass results in complex transformation to biochar and the release of a considerable amount of alkali and chlorine into the flue gas [18], [19]. However, the residual biochar pretreated at moderate temperatures still contains abundant species of alkali and chlorine, which may result in significant PM1.0 emission. Extensive studies on the characteristics of PM from raw biomass combustion [20], [21] or its co-firing with coal [22], [23] have been reported. Few studies were conducted on the PM emission from biochar combustion. Yani et al. [19] adopted a drop-tube furnace to investigate the behavior of PM10 emission from the combustion of biochar obtained through torrefaction at 220–280 °C. They observed bimodal mass-based particle size distribution (PSD) from biochar combustion, which is similar to that from raw biomass combustion. Nonetheless, when the biomass pretreatment temperature was further increased to 400–550 °C, a unimodal PSD was observed for the PM10 emission, and the PM1.0 (particulate matter with diameter < 1.0 μm) emission is negligible [24]. The biomass feedstocks used in these two works on biochar combustion contain low chlorine contents (< 0.2%). However, straw, which is the most part of biomass energy resource with a share of 72% in China [25], has a relatively high chlorine. The chlorine content in straw [26], [27], [28], [29] can be up to three times or higher than that in biomass used in previous studies [19], [24]. The chlorine species in biomass have been widely reported to be the main cause for the formation of fine particles during biomass combustion [14], [30]. These significant differences in the chlorine content of biomass may result in different characteristics of PM1.0 emission during biochar combustion. In addition, the effect of pretreatment temperature within a wide range on the PM emission from upgraded biomass and biochar combustion has not been fully explored. Therefore, a deep and systematic understanding on the fine particle emission from upgraded biomass and biochar combustion, is important for the high-efficient and environment-friendly utilization of biomass in China.
In this study, the behaviors of PM emission from the combustion of upgraded biomass and biochar prepared in a wide temperature range of 250–1000 °C, were carried out in a laboratory-scale entrained flow reactor at 1200 °C. The biomass pretreated below 300 °C is called “upgraded biomass”, while the biomass pretreated above 350 °C is called “biochar”. The fine particles were collected by a Dekali low pressure impactor (DLPI). Morphologies and chemical composition of the fine particles were analyzed by a scanning electron microscopy equipped with energy dispersive X-ray spectrometry (SEM-EDS). Based on the experimental results, the PM formation mechanism during upgraded biomass and biochar combustion is illustrated.
Section snippets
Fuel properties
The biomass used in the present study is the typical wheat straw from Baoji city of Shaanxi province. The proximate and ultimate analyses, and ash compositions are presented in Table 1, Table 2, respectively. As seen from Table 2, the chlorine and potassium contents are high, 6.15% and 27.68%, respectively. The straw samples were ground, sieved into diameter < 100 μm, and then put into sealing bag for use. The fuel samples were dried in an oven at 105 °C for 2 h before the preparation of upgraded
The yield of char and the evolution of K/Cl during straw pyrolysis
The yield of char as a function of temperature during the process of heat pretreatment is presented in Fig. 3(a). With the increasing in the pretreatment temperature from 250 °C to 350 °C, the char yield decreases from 55.5% at to 42.7%, and further decreases to 32% at 1000 °C. This trend of the temperature-dependent char yield is similar to previous research [7], while these yields are somewhat different. It might be due to the difference of fuel properties. Biomass mainly consists of cellulose,
Conclusions
- (1)
Pretreatment temperature significantly affects the yield and PSD of PM emissions, through changing the char yield and K/Cl contents in char. With increase in pretreatment temperature, the PM1.0 emission of upgraded biomass and biochar combustion first increases because of the enhancement of K/Cl in upgraded biomass and biochar, reaches the maximum at 500 °C, and then decreases. A linear relationship between the PM1.0 emission and Cl content in upgraded biomass and biochar was found.
- (2)
The
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 51676157, 5161101654 and 91544108), the National Key Research and Development Program of China (No. 2016YFC0801904), and the Fundamental Research Funds for the Central Universities.
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