## 1 Introduction  
Biomass is the unique carbon-based renewable energy source and a potential supplement to fossil fuels in the worldwide energy system [1, 2]. Biomass, composed of cellulose, hemicellulose, and lignin, can be converted into useful fuel gases or chemicals via gasification [3, 4]. Pyrolysis is the first step during biomass gasification, which includes a set of complex reactions such as primary pyrolysis and secondary reactions of char forming, depolymerization, and fragmentation [5, 6]. Tar is one of the primary products, taking part in the subsequent coding reaction and gasification reactions. The cooling reaction forms tiny carbon particles, corroding the equipment and clogging the pipes during industrial applications [7, 8]. Tar formation also decreases the conversion rate of biomass to fuel gas, impacting negatively the energy efficiency of a gasification system [9]. Tar is formed from free radical reactions during pyrolysis, which is influenced by the operating conditions and biomass compositions [10, 11]. Researchers have studied the free radical reaction during biomass pyrolysis using model compounds to explain the formation mechanisms of tar, gas, and coke. [PERSON] [12] studied the initial reaction mechanisms of cellulose pyrolysis and revealed the detailed free radical reactions with the aid of ReaxFF molecular dynamics, which agreed well with the cellulose pyrolysis-GC-MS data. [PERSON] et al. [13] also investigated the reaction pathways of hemicellulose models by the density functional theory. With respect to free radical analysis during biomass pyrolysis, lignin or lignin model compounds are usually used to predict the characteristics of free radicals. [PERSON] and co-workers [14; 15; 16] used \(\beta\)-O-4 dimeric phenolic compounds to propose a complex reaction pathway dominated by free radical reactions. [PERSON] et al. [17] reported the pyrolysis of methoxy substituted \(\alpha\)-O-4 dimeric phenolic compounds to investigate the effects of methoxy groups on lignin pyrolysis; free radicals were detected by electron paramagnetic resonance (EPR). [PERSON] et al. [18] used in-situ EPR to detect free radicals to resolve the difficulty of directly observing reaction intermediates from lignin pyrolysis. However, biomass pyrolysis behavior is more complex than the model compounds or lignin due to the interaction among cellulose, hemicellulose, and lignin. [PERSON] and [PERSON] [19; 20] investigated the interaction of biomass components and its influence on pyrolysis. They found that pyrolysis characteristics of biomass cannot be predicted through summation of the individual components. In spite of a number of studies of the pyrolysis mechanism, tar formation dominated by free radical reactions is still not clearly understood. Pyrolysis on a catalyst may have an influence on free radical variation converting tar to fuel gas. Active alumina is widely used as a catalytic support [21; 22; 23; 24] for tar removal or a heat carrier in a fluidized catalytic cracking reactor [25], which embodies high efficiency for heat transfer and accelerates the water to gas reaction. [PERSON] [26] showed the catalytic effect of composite alumina on the pyrolysis of sewage sludge in a fluidized bed reactor. The composition in aluminum drops (AD) is similar, containing alkali metal, Mg, and Fe, which may be used as catalyst for C-C and C-O bond activation [27; 28; 29] during biomass pyrolysis. However, the mechanism of the AD catalytic effect on biomass pyrolysis is rarely reported. In order to expatiate the formation and removal of tar and the catalytic effect of AD, AD catalyzed biomass pyrolysis is conducted and the free radical reactions are scrutinized. To the best of our knowledge, this specific research area has received little attention and few works are published. This study compares the results of thermogravimetric analysis (TG) and \({}^{13}\)C-NMR to elaborate the molecular structural evolution of an AD catalyzed biomass. Tar composition is analyzed by GC-MS.