## 1 Introduction  
The total amount of biomass used is now the fourth largest after coal, oil and natural gas. Biomass has the natural advantage of being a carbon reduction alternative as it is a renewable resource. To lessen CO\({}_{2}\) and NO\({}_{x}\) emissions, coal has been co-fired with biomass in coal-fired boilers [1]. Xinjiang Province has huge biomass reserves, due to its developed cultivation and high vegetation cover. Common biomass in Xinjiang includes cotton stalks, wood and corn stalks. At the same time, the large amount of oil sludge from oil fields is in urgent need of treatment in Xinjiang. Oil sludge is a hazardous waste from petrochemical processes including oil production and refining [2, 3]. Globally, oil sludge reserves reached 300 million tones by the end of 2021 [4]. Disposing of oil sludge is difficult due to its complex composition and difficulty in decomposition. Many methods have been proposed for treating oil sludge, such as incineration (direct combustion), extraction, biological treatment and solidification. Direct combustion is an effective method for treating oil sludge, but it produces large quantities of toxic gases and NO\({}_{x}\)[5, 6]. The involvement of coal blending with biomass or oil sludge for carbon reduction can address the above issues [7, 8, 9]. Blended combustion provides a good way to utilize biomass and oil sludge extensively. Blending can also reduce pollutant emissions (SO\({}_{x}\), NO\({}_{x}\) and soot) [10,11]. In addition, pollutants from oil sludge could be effectively treated using the mature flue gas treatment equipment of boilers [12]. Therefore, blending combustion of coal with biomass or oil sludge has received increasing attention [13,14]. Because of the shortcomings of seasonality and poor dispersion, current biomass utilization in China is still dominated by small-scale direct-fired units with a relatively low efficiency. Compared to coal, biomass and oil sludge have different compositions and structural characteristics. Before tri-fuel is applied, a precise understanding of the combustion characteristics of coal, biomass and/or oil sludge blending is particularly significant [15]. The blending combustion process of coal with different biomass fuels has been investigated by many researchers [16, 17, 18]. [PERSON] et al. [18] and [PERSON] et al. [17] performed thermogravimetric and kinetic analyses on blended fuel with different mixing ratios. The results indicated that when the mixing ratio was 70% coal:30% corn straw, the highest exergy was obtained. And different degrees of synergistic effects occurred in the blended fuels. [PERSON] et al. [16] investigated the effect of mixing ratios of bituminous coal and rice husk in a circulating fluidized bed on the concentration distributions of the gas product along the vertical upward direction of the furnace chamber. [PERSON] et al. [19] analyzed the migration pattern of S during biomass blending with coal in a circulating fluidized bed. The results showed that blending biomass was helpful in dislodging SO\({}_{2}\) from the flue gas. When the mixing ratio of biomass increased, the S content in the ash and slag residue increased and the SO\({}_{2}\) emission decreased. [PERSON] et al. [20] evaluated the combustion process of blended fuels with polyethylene at different mixing ratios. The kinetics of the blended fuel were analyzed using the Coats-Redfern method. The combustion mechanism of the blended fuel was a diffusion reaction. And blending biomass was beneficial to improving the combustion reactivity and reducing the activation energy. In earlier studies [21, 22, 23], the combustion characteristics of fuel mixed with sludge were studied. Oil-bearing sludge (oil sludge), as a sludge with a special composition and structure, has not received much attention [24]. [PERSON] et al. [25] explored the combustion characteristics of oil sludge, ginkgo seeds and a blended fuel composed of them via thermogravimetric analysis and kinetic analysis.