# Valorization of Insulation Cellulose Waste as Solid Biomass Fuel
## 1 Introduction  
Cellulose fiber (CF) is a popular material for the insulation of the external envelope of buildings. This is due to its favorable thermal and acoustic properties, low manufacturing cost and easy installation by blowing it in closed wall cavities or attics. CF is a natural, renewable and recyclable material with low embodied energy and carbon dioxide footprint. Moreover, it is a non-toxic resource and, thanks to its high hygrothermal conductivity, it can help control the building's indoor air humidity [1; 2; 3; 4; 5]. CF is usually manufactured from recycled newspaper which is calun in a fiberizer machine to separate the fibers. Because of the combustible and organic nature of this material, the addition of substances, usually boron salts known as borax, is required to increase its fire resistance and to avoid cellulose vermin and rotting. During manufacturing, once CF and borax are mixed, a separator removes the shorter fibers which are not adequate for applying as an insulator. As a result, a fine cellulose fiber powder (CFP) is generated, remaining as a useless waste because of its boron salts content. Based on actual circular economy thinking, CFP would not be considered a waste but a valuable resource to take advantage of, saving resources and avoiding its disposal in landfills. The manufacture of non-conventional fuel alternatives to fossil fuels is an application with sizeable economic and environmental benefits that would contribute to the vaporization of this waste [7,8]. Many published studies have demonstrated the ability of the manufacturing of non-conventional fuels for the valorization of different kinds of waste. Among these, different kinds of biomass stand out, like agricultural crop residues [9, 10, 11, 12], agroindustrial residues [13, 14, 15], forest biomass [16] and wood processing industry residues [17, 18]. Other wastes benefit from this advantageous method of valorization, such as sewage sludge [19, 20, 21] or waste paper [22]. CFP could become a green energy source based on its close composition to wastepaper. Table 1 shows the heating value ranges of different wastes used for the manufacturing of solid fuels, as well as the EN 14961-2 and EN ISO 17225-6 Standard heating requirements for woody and non-woody solid biofuels, respectively. Cellulose, the main constituent of CFP, has a lower heating value (LHV) of 14.570 MJ/kg, which suggests this waste's potential for valorization as a solid, non-woody biomass fuel for green energy production [26]. Before use, CFP requires a densification process because of its low density and powdery nature. Once densified, CFP could become a commodity able to be managed by the nowadays common solid biomass fuel handling, transportation, storage, feeding and combustion systems. Pelletization is the most usual biomass densification technology, Ref [27, 28, 29, 30]. During this process, solid particles, with or without binders, are bonded to each other by attracting forces, diffusion of molecules or ingredient crystallization. In this way, a hardened commodity with higher density, improved durability and improved combustion properties is produced [11, 19, 22, 23, 31, 32, 33, 34, 35, 36]. Although there are many works that demonstrate the suitability of different lignocellulosic materials for solid biomass fuels manufacturing [12, 37, 38, 39, 40, 41], there is a lack of knowledge about the potential of CFP with boron salts as an energy source. Therefore, the aim of the present study is to investigate the suitability of CFP for the manufacturing of a solid biomass fuel by means of the characterization of the key points of solid fuels: (1) physical properties and durability, (2) heating value and combustion characteristics and (3) ashes' production and potential emissions. The following parameters were analyzed: the ability of the CFP to be pelletized, the effect of binders on the pellet's physical characteristics and durability, the energy recovered from its combustion, its combustion properties and the ash production.