Article
Study of the kinetic behaviour of biomass and coal during oxyfuel co-combustion

https://doi.org/10.1016/j.cjche.2020.02.023Get rights and content

Abstract

In this study, the thermogravimetric analysis (TGA) method has been used to evaluate the kinetic behavior of biomass, coal and its blends during oxyfuel co-combustion. The thermogravimetric results have been evaluated by the Coats–Redfern method and validated by Criado's method. TG and DTG curves indicate that as the oxygen concentration increases the ignition and burn out temperatures approach a lower temperature region. The combustion characteristic index shows that biomass to coal blends of 28% and 40% respectively can achieve enhanced combustion up to 60% oxygen enrichment. In the devolatilization region, the activation energies for coal and blends reduce while in the char oxidation region, they increase with rise in oxygen concentration. Biomass, however, indicates slightly different combustion characteristic of being degraded in a single step and its activation energies increase with rise in oxygen concentration. It is demonstrated in this work that oxygen enrichment has more positive combustion effect on coal than biomass. At 20% oxygen enrichment, 28% and 40% blends indicate activation energy of 132.8 and 125.5 kJ·mol−1 respectively which are lower than coal at 148.1 kJ·mol−1 but higher than biomass at 81.5 kJ·mol−1 demonstrating synergistic effect of fuel blending. Also, at char combustion step, an increase in activation energy for 28% blend is found to be 0.36 kJ·mol−1 per rise in oxygen concentration which is higher than in 40% blend at 0.28 kJ·mol−1.

Introduction

Corn stover, an agricultural residue is a renewable and inexpensive resource widely available in China, and frequently used for the production of biochemical products [1]. It can also be combusted in boilers for power generation. On the other hand, coal is a nonrenewable fuel that finds application in power generation due to its plentiful reserves and being relatively inexpensive, contributing to 80% and 45% of the total electricity in China and the US respectively [2]. On the other hand, co-combustion of coal and biomass mitigates the environmental concerns of positive emission of gases such as CO2, NOx, SOx etc. caused by burning coal alone [3]. Moreover, the severe corrosion in the boiler heat exchanger caused by burning high alkali biomass can be avoided by co-combustion [4,5]. Other researchers have done studies with increasing biomass mix to coal. No challenges with regard to operations were found in the blend percentages of up to 15% blend on an energy input basis [3,6].

A further reduction in CO2 footprint can be achieved by oxyfuel combustion. Oxy-fuel combustion uses pure oxygen or the blend of pure oxygen and a recycled exhaust gas stream for oxidation instead of using air resulting into a high CO2 concentrated product gas that is suitable for CCS [7]. Galina et al. [8] reported that replacing N2 with CO2 improves oxygen combustion performance, reaction reactivity and kinetic parameters. The key issue in the oxy-fuel co-combustion is the behavior of coal and biomass blends under different oxygen enrichment levels. Cofiring under normal pressure and normal oxygen level has been studied extensively [8,9], but with high oxygen level, still need some research.

Thermogravimetric studies are used to understand the fuel and their blends kinetic behavior. Many studies are focused on co-combustion kinetics. The thermal effects, reactions and kinetics for biomass residues under nitrogen pyrolysis and oxidizing environments, utilizing a non-isothermal thermogravimetric (TGA) procedure have been studied. It was reported that the mass loss rates were significantly greater under oxygen enriched environments than pyrolysis conditions [10]. The thermal characteristics, kinetics and co-combustion behavior of coal, pine sawdust biomass and blended up to 80 wt% with biomasses have been evaluated with the help of a non-isothermal thermogravimetric method (TGA). Sawdust biomass was found to undergo a two-step combustion of devolatilization step (200–360 °C) and char combustion step (360–490 °C) [11]. The ignition temperature of fuel has been found to reduce with an increased level of oxygen [12]. Deng et al. [13] have reported a large distinction in activation energy between coal combustion conducted at different levels of oxygen concentration. On the other hand, the management of the solid municipal wastes (SMW) by co-combustion with coal have been proposed. Thermogravimetric analyses for the coals and their biomass blends were also conducted and kinetic factors were found to be decreasing with an increase in biomass to coal ratio [6].

So far, there is limited research found that addresses the effect of oxygen on the coal, biomass, their blends under oxyfuel co-combustion conditions. Moreover, there is a critical need to study fundamental mechanisms associated with oxy-fuel co-combustion of coal and biomass. Such information would provide theoretical guidance and technical support for scaling-up and deployment of the technology in the power industry. Therefore, this work performs thermogravimetric analysis on biomass to coal blends under oxy-fuel, oxygen enrichment combustion conditions.

Section snippets

Sample preparation

This study has used corn stover biomass and Zhundong lignite coal from Xinjiang province of China. The fuels were first dried and later ground to fine particles in a grinding mill. The final size of biomass was less than 250 μm and coal was 10% retained on 100 μm sieve (or 90% passing the sieve). This particle size is adopted to suit industrial PC boiler requirements [14,15]. The (ASTM-E0870-82R98E01) standard was used to perform the fuel's proximate and ultimate analyses and results are given

Theoretical Consideration

To fully understand the kinetic behavior of coal and biomass and their blends, the Arrhenius kinetic analysis approach is deployed. The general kinetic analyses for the thermal degradation of a solid fuel during combustion are given by Eqs. (1), (2), (3), (4), (5), (6). Thermal degradation of a solid fuel can be simplified by Eq. (1) [17] as:aA(s)kbB(s)+cC(g)where A represents the solid fuel, B represents all solid products and C represents all gaseous products emanating from A. The rate

Thermal degradation

Fig. 1, Fig. 2 show the TG and DTG curves for coal, biomass and its blends at 20%, 40%, 60% and 80% oxygen concentration for the constant heating rate of β = 25 °C⋅min−1 respectively. The combustion characteristic temperatures of fuels are summarized in Table 3. The first peak is defined by t01, tmax1 and tf1 corresponding to ignition temperature, maximum degradation temperature and final temperature after first peak. On the other hand, the second peak is defined by t02, tmax2 and tf2

Conclusions

The Coats and Redfern method has been successfully used to analyze the combustion kinetics of coal, biomass and its blends under oxyfuel combustion and whose model has been validated by the Criado's method. From the thermogravimetric analysis it can be concluded that: Combustion of fuels and their blends are achieved mainly by two major steps of devolatilization and char oxidation as seen from DTG curves. TG and DTG curves approach a lower temperature region with rise in Oxygen concentration.

Acknowledgements

Financed by the International Cooperation Foundation for China-USA (NSFC-NSF 51661125012).

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