Effect of different ash/organics and C/H/O ratios on characteristics and reaction mechanisms of sludge microwave pyrolysis to generate bio-fuels

Highlights

• High-ash sludge achieved a faster pyrolysis heating rate and higher temperature.

• High-organic sludge generated bio-gas with a higher yield and a higher LHV.

• Increases in H/C ratios enhanced the H₂ and aliphatic contents of bio-fuels.

• Decreases in O/C ratios resulted in higher CO and oxygenated compound contents.

• A mixture of high-ash and high-organic sludge can improve pyrolysis efficiency.

Abstract

To study the effects of different ash/organics and C/H/O ratios on bio-fuel characteristics and energy efficiency, four kinds of sludge with different properties were used for microwave pyrolysis (800 °C). Moreover, the microwave pyrolysis reaction mechanisms of different sludge were also explored. The results showed that high-ash sludge could accelerate the frequency of polar molecule rotation in the microwave field due to the presence of oxides with dielectric properties in ash, thereby achieving faster heating rates and higher temperatures. However, compared with high-organic sludge, high-ash sludge exhibited lower bio-gas yield and higher bio-char yield. As the H/C ratio increased from 0.127 to 0.148, the bio-gas yield increased from 15.41% to 40.01%, and the content of H₂ in bio-gas and aliphatics in bio-oil increased to 36.69 vol% and 26.54 wt%, respectively. When the O/C ratio was reduced to 1.31, the content of CO and oxygenated compound in bio-oil increased to 31.25 vol% and 40.04 wt%, which lowered the quality of the bio-oil. Those consequences also determined that a mixture of sludge with different ash/organic ratios could be pyrolyzed to obtain high-quality bio-fuels and high energy efficiency. Differences in C/H/O ratios in the mixed sludge greatly affected the microwave pyrolysis heating process, which affected the pyrolysis reactions and the quality of the bio-fuels. Therefore, this study provides a theoretical basis to elevate the quality of bio-fuels and reduce microwave pyrolysis costs.

Introduction

Compared with other treatment and disposal technologies of sludge, microwave pyrolysis technology has many advantages, including molecular-level uniform heating, short processing time and effective heat transfer, and a faster response (Mushtaq et al., 2014, Zaker et al., 2019). Thus, microwave pyrolysis technology is considered to be one of the most promising approaches for bio-fuels (e.g., bio-gas and bio-oil) recovery from sludge.

Previous studies have reported that experimental parameters, including microwave power (Song et al., 2017), temperature (Prathiba et al., 2018), and catalysts (Xie et al., 2019), can alter the distribution and component of products generated by sludge microwave pyrolysis. Particularly, Luo et al. (2020) have found that increasing microwave power and pyrolysis temperature enhance the generation of bio-gas and the reduction of bio-char. Bio-oil yields have been reported to first increase and subsequently decrease with increased microwave power and temperature (Mutsengerere et al., 2019). This is because high temperature promote the secondary pyrolysis of bio-oil and intermediate products to generate lighter oil and bio-gas (e.g., H₂ and CO) (Montingelli et al., 2016). Moreover, some studies (Ma et al., 2017a, Sun et al., 2018) have demonstrated that adding catalysts to the sludge microwave pyrolysis process can directionally regulate the reaction process, thereby selectively changing the distribution of products and effectively elevating the quality of bio-fuels. These changes attempt to enhance the yield and quality of bio-fuels via adjusting of experimental parameters, which ensures the stability of the chemical properties and composition of sludge.

However, some bottlenecks remain in the application of microwave pyrolysis technology to the industrialization of sludge pyrolysis. One of such key bottlenecks is the influence of the diversity of sludge sources and compositions on microwave pyrolysis, which restricts the industrialization of microwave pyrolysis technology. The properties and compositions of sludge, including the content of organics, moisture, and ash, vary greatly because of the differences in water sources (e.g., industrial wastewater, domestic wastewater), sewage treatment processes, and other factors (e.g., regions, seasons) (Gong et al., 2014). Accordingly, many studies have shown that differences in the concentration of organics, moisture, and certain metal oxides in sludge causes differences in the yield and quality of products (Fonts et al., 2009, Gong et al., 2014). Therefore, differences in sludge sources and composition will inevitably influence the characteristics of bio-fuels produced through sludge microwave pyrolysis, thereby affecting industrialization of this technology and sludge resource utilization efficiency.

Sludge organic elements mainly contain carbon (C), hydrogen (H), and oxygen (O). The differences in sludge properties reflect differences in concentration of C, H, and O, which directly affects bio-fuel distribution of pyrolysis products and components of bio-fuels (Gong et al., 2016). Ma et al. (2018) have found that the content of organics in high-ash sludge was 38.12% and the contents of C, H, and O were 14.29 wt%, 3.14 wt%, and 16.94 wt%, respectively. The yields of bio-gas, bio-oil and bio-char, under the condition of 1000 W, were 16.93%, 2.98%, and 78.27%, respectively. However, Zuo et al. (2011) reported that the yields of bio-gas, bio-oil, and bio-char were 63.20%, 7.30%, and 29.50% under the same pyrolysis condition. These differences in bio-fuel yields likely occurred because the sludge used by Zuo et al. (2011) possessed a higher organic content (75.50%) and the C, H and O contents (39.40 wt%, 5.70 wt% and 24.40 wt%, respectively) were higher than those of high-ash sludge. Thus, different organic contents in sludge have a crucial influence on the distributions of sludge-derived bio-fuels. However, there is a lack of systematic studies on the effects of the organic content of sludge on the bio-fuels produced by microwave pyrolysis. Moreover, the contents of C, H and O in sludge also cause differences in the composition of bio-gas and bio-oil. Similarly, few reports have investigated in this aspect.

Microwave heating occurs through the polarization and rearrangement of molecules under high-frequency electric fields, which leads to internal collision and friction between polar molecules, thus generating heat (Motasemi and Afzal, 2013). Materials with different molecular polarity have different microwave reception capacities, and different heating rates in microwave pyrolysis. In turn, this changes the reaction pathway and ultimately affects the products. Studies (Mesroghli et al., 2015, Mushtaq et al., 2016) have shown that the oxides of Si, Fe, Al and other elements in sludge ash possess dielectric properties, which can act as microwave absorbents and help increase heating rates and pyrolysis temperatures. However, the low ash content in sludge results in poor dielectric properties, impeding to reach high temperatures when pyrolyzed sludge alone. Therefore, it is necessary to add a microwave absorbent to enhance the microwave absorption capacity of sludge. When an absorbent is added, the absorbent particles are selectively heated and their heat is transferred to neighboring particles, resulting in bulk heating of the sludge (Ellison et al., 2020). Once this process begins, the sludge particles partially transform into bio-char, becoming good microwave absorbents themselves. Menéndez et al. (Menéndez et al., 2002) reported that bio-char can act as an in-situ microwave absorbent, significantly increasing the dielectric properties of sludge. However, differences in organic contents and C, H, and, O in sludge affect the yield and composition of bio-char, which also affects the conversion of microwave energy to heat energy and ultimately the energy efficiency. Most studies have focused on the bio-char and energy consumption of a single sludge source under different pyrolysis conditions, while there are few studies on the effect of sludge with different properties on bio-char and energy consumption under constant pyrolysis conditions. If a correlation between different sludge properties and the yield, quality of bio-fuels, and energy efficiency of microwave pyrolysis can be elaborated, the effect of sludge properties on the energy efficiency will be explored from an energy perspective. This provides a theoretical basis to optimize the efficiencies of microwave pyrolysis of different sludge sources.

For studying the effects of diverse ash/organics and C/H/O ratios on the properties of bio-fuels produced by sludge microwave pyrolysis and energy utilization, four kinds of sludge with different properties were examined. First, the characteristics of temperature-rising curves and product distributions of sludge microwave pyrolysis were studied. Afterward, the components of bio-fuels were analyzed from the perspective of the ratios of organic elements. Then, the chemical energy of bio-fuels was calculated and the chemical energy efficiencies of sludge microwave pyrolysis were compared. Finally, reaction mechanisms were discussed based on the microwave pyrolysis characteristics of sludge with various properties. Therefore, this study provides a theoretical basis for the implementation of high-efficiency sludge microwave pyrolysis to reduce its cost while generating high-quality bio-fuels.