Combustion behavior of coal pellets blended with Miscanthus biochar

Highlights

• The Taguchi method was used to determine the optimal torrefaction parameters.

• Biomass cofiring can achieve GHG abatement and increase the share of renewable.

• The TGA analyses were used to examine the torrefaction and combustion feature.

• The ignition delay and burnout temperature of the blended fuels were investigated.

• The homo- and hetero-geneous reaction of blended fuels were discussed.

Abstract

To achieve reductions in CO₂ emissions, replacing fossil fuels with biomass in thermal power generation is becoming increasingly prevalent. In general, the fuel nature and combustion characteristics of biomass are distinct from those of fossil fuels. Biomass is typically subjected to torrefaction to improve its grindability, hydrophobicity, and heating value (HV). However, the pretreatment process is accompanied by fuel property alteration and an energy penalty. This is strongly associated with the operating envelope and combustion stability of biochar cofiring with coal. Therefore, in this study, the Taguchi method was used to calculate the optimal torrefaction parameters for maximum energy yield and HV. Thermogravimetric and fuel characteristic analyses were performed to examine the pyrolysis features and combustion behavior of the studied fuels. In addition, a blend of 50% Miscanthus biochar and 50% Australia coal was produced and pressed into pellets. The pellets were placed into a free-drop furnace to observe their combustion behavior. The results demonstrated that the ignition temperature and burnout temperature of the blended fuels could be effectively reduced, and that their fuel conversion rates and combustion characteristic index could be enhanced. The results can be applied to coal cofiring in large-scale boilers in the future.

Introduction

Climate change jeopardizes the survival of humans. However, total abdication of fossil fuel use to mitigate extreme increases in CO₂ emission is highly unlikely. The Paris Climate Change Conference in 2015 engendered unprecedented accomplishment, including changes in European government policies, the U.S.–China joint agreement, and support from business and technology communities [1]. Specifically, China agreed to limit its emissions by 2030 or earlier, if possible, and the United States pledged to reduce its emissions by 26%–28% below the 2005 levels by 2025. The European Union has already pledged to reduce greenhouse gas (GHG) emissions by 40% by 2030. Similarly, Taiwanese authorities ambitiously declared to reduce CO₂ emissions by 50% below the 2005 levels by 2050. However, Taiwan possesses nearly no energy resources, and the country relies on imports for nearly 98% of its energy requirements. The gross power generation reached 225,792 GWh in 2016, backed by considerable fossil fuel consumption to generate electricity [2]. The share of hydroelectric power in Taiwan is 1.5%. Thermal power accounts for a maximum share of 77.3% (174,533 GWh). The country’s energy consumption comprises 36.9% from coal, 4.4% from diesel fuel, and 36% from nature gas. Nuclear power accounts for 13.5%, whereas the share of renewable energy sources that include conventional hydropower, geothermal, solar, and wind power is 5.1%.

Taiwan authorities announced the new energy policy, which is aimed at phasing out nuclear energy by 2025. To meet energy policy commitments, Taiwanese authorities have an obligation to increase the contribution of renewable energy to 20% of electricity generation and meanwhile achieve the abatement of GHG emission. Biomass is considered an environmentally friendly fuel because of its advantage as a renewable and CO₂-neutral fuel [3]. The thermal utilization of biomass fuels can contribute to the reduction of CO₂ emissions because the same amount of CO₂ released through combustion is extracted from the air during the growth of biomass feedstock. In addition, the utilization of coal with biomass, especially for the partial substitution of fossil fuels during combustion or gasification-based processes, is an essential approach to reducing emissions and avoiding methane release from landfill biomass [[4], [5], [6]]. The GHG effect of CH₄ is 25 times more potent than that of CO₂ in terms of global warming impact [7]. Furthermore, cofiring of coal and biomass can reduce NO_x and SO₂ emissions primarily because of a reduction in the total amount of nitrogen and sulfur in the blended fuels [8].

Currently, attention is shifting to biofuel options for energy generation because they do not require high-quality farmland and thus minimize the risk of deforestation or competition with food crops. Such options include biofuels produced from food or farm waste [9], thus representing second-generation biofuels produced from nonfood biomass or recovered organic materials such as wood, Miscanthus grass, switchgrass, crop residuals, forestry waste, and perennial grasses [10]. Growing trees and perennial grass on degraded land can facilitate the reduction of soil erosion, restoration of soil fertility, and prevention of the invasiveness of nonindigenous species. Accordingly, Miscanthus, considered a perennial energy crop, is widely available and easy to grow in Taiwan. Miscanthus does not require irrigation water and is not susceptible to pests or diseases. During photosynthesis, Miscanthus engages the C4 pathway, enabling efficient carbon fixation, thus rendering it a carbon-rich energy crop [11]. In particular, raw Miscanthus has considerable energy density that is slightly lower than woody biomass, and extensive drying or pyrolysis process can evidently improve its combustibility [12]. Compared with other energy crops, Miscanthus is advantageous for obtaining high productivity and carbohydrate containment [13].

In decades, co-firing of coal with biomass for electricity generation has received increasing attention because its implementation can accommodate varying amounts of available biomass [[14], [15], [16]]. In addition, such cofiring does not require large investments in retrofitting an existing coal-fired power plant [[16], [17], [18]]. Lau et al. [19]pointed out that the optimum torrefaction temperature is determined to be at 250 °C, giving the torrefied oil palm frond a high heating value of 26.62 MJ/kg, while maintain an energy yield of 92.7%. Hu et al. [20] conducted a case study at Taipower, a major electricity supplier in the Taiwanese energy market. It concluded that Miscanthus is more economical than switchgrass in terms of the production cost and the land required to generate biopower for the same levels of biomass co-firing. The Drax power station in England is regarded as the cleanest and most efficient coal–biomass cofiring power generation system in the world; it can produce 4000 MW_e to meet 7% of the UK electricity demand [21]. In the Netherlands, the Amer power station operates a waste-wood gasifier connected to a 600-MW_e coal-fired power station with 42% net electric efficiency [22]. Although a biomass cofiring rate of more than 20% in coal-fired furnaces is currently feasible and technically achievable, the typical biomass share currently is below 5% and rarely surpasses 10% on a continual basis. A total 10% of biomass co-combustion could achieve CO₂ abatement from 45 million to 450 million ton per year by 2035 [23]. Low percentages of biomass can be co-fired easily [24]. Thus, it is applicable to a limited range of biomass types and to very low biomass to coal cofiring ratios that is typically less than 5% by mass [25]. However, when the percentage of biomass is increased, limitations are observed at distinct locations in the involved equipment and processes. The fundamental physical and chemical differences between biomass and coal represent limitations or require adjustments. For example, differences in the flow characteristics of biomass and coal necessitate distinct types of logistic installations. Milling characteristics are highly dissimilar because nearly all biomass types have a fiber structure and are extremely difficult to grind [26]. Moreover, combustion behavior is considerably disparate, not only because of differences in chemical composition but also because of differences in particle size. Air–fuel ratios must be adjusted for biomass.

The main difference between biomass and coal is in terms of their fuel properties and particle sizes, which can influence ignition and cofiring characteristics, rendering the task of designing burners and controlling the biomass co-combustion process difficult. Alternatively, high biomass shares in biomass co-combustion furnaces engender several technical challenges including sustained availability of biomass, low grindability of biomass, slagging, fouling, and corrosion [27]. Several technical constraints associated with biomass co-combustion application are predictable. For example, biomass particles are larger than coal particles, and the fibrous structure of biomass feedstock results in relatively high energy consumption during grinding. In addition, lignocellulosic biomass has high moisture content in raw material due to hydroxyl group that form hydrogen bonds to retain additional water [28], which ultimately affects the overall process efficiency. All such constraints originate from the inherent properties of biomass materials, which result in low thermal efficiency and high GHG emissions. Accordingly, upgrading the raw biomass feedstock to meet the requirements of current coal-fired furnaces prior to cofiring is necessary. Various thermochemical conversion technologies such as thermal pyrolysis or gasification may be used to convert raw biomass into biofuel [29], bio-oil [30], or syngas and improve combustion properties [31]. Biochar is the main product of torrefaction, a mild pyrolysis process performed in the temperature range of 200–300 °C under oxygen-free conditions [32]. Torrefaction facilitates the storage of solid fuel and increases its heating value (HV) [33].

In general, the pulverized coal/biomass combustion can be modeled as four-step process: drying, devolatilization, volatile combustion and char burning [34,35]. The coal/biomass particle undergoes the drying process. With an increase in the temperature, the inception of devolatilization process starts. During the devolatilization, obvious mass loss occurs due to the release of volatile matter. The quantity and composition of the volatiles depend on the coal and biomass ingredients and the particle size and temperature. The volatiles are burned out in the gas phase. After the devolatilization, only char and ash remain in the solid particle. Eventually, char oxides to carbon monoxide or carbon dioxide depending in the particle size and temperature, and ends up with ash [36]. For practical application, there are several studies that have discussed the direct co-combustion of alternative fuels in small-scale furnaces. Bhuiyan and Naser [37] numerically examine the co-firing of biomass with coal in an oxyfuel condition in a small-scale furnace, and optimize the biomass share and recycled ratio to achieve stable radiative, convective heat transfer and burnout performance. Gubba et al. [38] investigated the co-firing of straw and coal in a 300 MW_e pulverized fuel fired boiler. The prediction of the temperature profile, NO_x formation, and char burnout was in good agreement with the reported measurement. Dong et al. [39] discussed the co-firing of coal and product gas from biomass gasification in a 600 MW_e tangential pulverized coal fired boiler. The result demonstrated a decrease in NO_x emission of approximately 50–70%. Karampinis et al. [40] discovered that a decrease in NOx emission of up to 10% can be achieved when cardoon is co-combusted with coal. Agraniotis et al. [41] evaluated various coal and solid recovered fuel (SRF) co-combustion modes in a 600 MW_e with regard to the evaluation of different co-combustion scenarios. Mikulčić et al. [42] studied different biomass co-combustion shares in a cement calciner, and numerically investigate the thermos-chemical reaction occurring inside the calculated calciners and to make improvements. In addition, Hu et al. [43] pointed out that bio-char pellets had reduced ignition temperature, wider temperature range, and higher oxidation activity compared with the raw bio-char. The releasing and combustion of volatiles from the added organic binders led to a small peak before char combustion on the TG curves of the organic pellets.

According to the preceding descriptions, most studies have focused on assessing the technical feasibility of biomass cofiring; however, few studies have examined the effect of biochar cofiring on flame characteristics. Therefore, the current study focused on the fuel properties of pulverized coal and Miscanthus floridulus biochar mixtures and their combustion phenomenon. Thermogravimetric analysis (TGA) was performed to observe the thermal behavior of the fuels. Furthermore, the blended fuels were pelletized in a cylindrical die under controlled conditions, and the corresponding combustion process and flue-gas emission were examined using a laboratory-scale free-drop furnace for single pellet combustion. The dominance of homogeneous and heterogeneous reactions could be further explained by matching the mass-loss rate and flue-gas emission of single pellet combustion. The purpose of this study is not to propose a new and innovative measurement approach, but to delineate the interrelation between biochar cofiring feature and torrefaction condition as well as optimize the appropriate torrefaction condition for Miscanthus as exemplified. Meanwhile, seeking a balance between the performance loss of biomass co-combustion and the energy loss of biomass pretreatment is the basic task to maximize the advantages of biomass cofiring.