Determination of Time-to-ignition and Flaming Duration of Biomasses in a Mue Furnace

Measuring the energy characteristics of solid biofuels can help to determine the most suitable species for combustion. The objective of this study is to propose a new methodology for determining the ignition time and aming duration in lignocellulosic biomass. A mue furnace was used, instead of an epiradiador, to measure the variables. The optimal oven temperature was dened according to the average time-to-ignition of biomasses in the literature. Ten biomasses were analyzed to obtain their high heating value, volatile matter, xed carbon content, ash content, time-to-ignition, and aming duration. The results showed a high correlation between the biomass volatile content, time-to-ignition, and aming duration. In the literature, it is described that high levels of volatile materials accelerate the ignition of the material. Thus, the association between the volatile matter and the variables analyzed justies the use of the mue furnace methodology. Furthermore, biomasses with high levels of volatile matter have longer aming durations than other solid biofuels.


Introduction
Biomass can be used as a solid fuel. To assess the potential of solid biofuels, different energy variables can be measured through proximate analysis. This is a simple, low-cost, and widely used methodology [1,2] in which dry biofuel is divided into three content fractions: ash, volatile matter, and xed carbon [3][4][5][6].
The volatile matter of biomass consists of incinerable gases (hydrogen, light hydrocarbons, and tar) and non-incinerable gases (water and carbon dioxide) [7]. Incinerable gases increase reactivity and the rate of combustion, thus preventing high temperatures from being reached [8,9]. In non pyrolyzed biofuels, such as rewood and lignocellulosic waste, volatile materials can constitute 65-85% of the material.
The ammability of a solid fuel is one of the main variables in uencing the combustion process [10].
Flammability is related to the biofuel's ability to ignite, characteristics that maintain biofuel burning, speed at which biofuels burn, and amount of biofuel burned [11].
According to Kovalsyki et al. [12], parameters such as the ignition time (TTI) and aming duration (FD) provide the characteristics of the ammability of biofuels. The procedure to determine these variables requires an epiradiator, which radiates heat to the tested samples [13][14][15]. Mu e furnace equipment, commonly found in laboratories, also releases heat and can be used for the same procedure. Gill and Moore [16] tested a mu e furnace at 400°C to measure the TTI; however, the study was not published.
Thus, the objective of this study was to test a mu e furnace to validate a new methodology for determining the TTI and FD of lignocellulosic materials.

Collection of materials
The study was conducted in the Biomass and Bioenergy Laboratory at the Federal University of São Carlos, Sorocaba campus, at the geographical coordinates of 23° 34'52,72"S and 47º31'36,02" W.

Time-to-ignition and aming duration
The time-to-ignition (TTI) and aming duration (FD) were adapted from Silva et al. [10] and Kovalsyki et al. [12]. The equipment used was a mu e furnace (Bravac M2/3). Initially, a test was conducted to de ne the optimal mu e furnace temperature (600 °C, 700 °C, 750 °C, and 800 °C) using one type of biomass (T. grandis). The TTI (s) was determined at the time of the emergence of the ame. After the ame appeared, the crucible was positioned on the edge of the mu e furnace. The time between the appearance and the absence of the ame was called FD (s). Ten repetitions were performed for each sample.

Proximate analysis
Samples from each biomass were separated. One gram of each sample, with a particle size greater than an 18-mesh sieve, was placed in a calcined crucible with a lid. The sample was placed in a 900°C mu e furnace for 7 min [17]. After the material was cooled in a desiccator, it was possible to calculate the volatile matter according to below: VM Where mi is the initial dry mass, mf is the nal mass after 7 min, and VM is the volatile matter.
To calculate the ash content, one gram of each sample (granulometry greater than 18 mesh) was placed in a calcined crucible. The sample was placed in a mu e furnace. The heating (20°C.min-1) was from 27°C to 250°C, and the temperature then remained at 250°C for another hour. After the elapsed time, the mu e furnace temperature was raised to 550°C. Upon reaching this new temperature, the sample remained in the oven for another two hours. The test was considered complete when the sample reached a constant mass [18]. The ash content (A) was calculated as follows: Where mi is the initial dry mass, mf is the nal mass, and A is the ash content.
To calculate the xed carbon content (FC) was calculated as follows: FC Higher heating value The tests of higher heating value (HHV) were carried out with dry biomass and with a particle size greater than 18 mesh. The HHV of Biomass was estimated by the calorimetric pump (IKA C 200). The appliance uses 2 L of water per analysis. The water temperature was adjusted in a range from 18 to 23º C. The biomass was pressurized to 30 bar oxygen in a calorimeter vessel. These conditions attest to the requirements of the standard ASTM D5865 [2]. Table 1 presents the results of the TTIs at respective temperatures. The TTI of 39 s (750°C) for the biomass sample was the closest to the TTI values obtained by Silva et al. [10] and Kovalsyki et al. [12], which were 41 s on average. Therefore, in this study, a temperature of 750°C was established for testing. It was also the model that best presented the linear correlation coe cient ( Table 1, Fig. 1).

Results And Discussion
Protásio et al. [19], during a thermogravimetric analysis, found the rst peak of thermal degradation at a combustion temperature of 329°C for babassu bark, whereas for Pinus densi ora, the rst peak was found at 340°C. These values are close to the 750°C (337°C) model.
It is noteworthy that the spontaneous combustion of wood occurs at temperatures close to 280°C [20].
However, at the moment when biomass depolymerization occurs, exothermic reactions are still insu cient to observe the appearance of a ame. Flames appeared at temperatures above 337°C. Table 2 shows the results of burning and the higher heating value for different biomass species. C. nucifera presented the most optimal results for combustion, with a high TI (56 s) and short aming duration FD (41 s). According to Kovalsyki et al. [12], the higher the TTI and shorter the FD, the better is the fuel.
Ash values above 1% indicate the presence of impurities. Most of the material for the samples was collected from sawdust on a oor, where impurities may have been present in silica.
According to Rybak et al. [21], the TTI measures the interval from the moment the sample is introduced into a hot atmosphere until the appearance of a ame around the sample. This relationship was observed in the mu e furnace. Figure 3 shows the relationship between the content of volatile materials, TTI, and FD. Figure 3 (a) shows that the higher the volatile content, the smaller the TTI of the material, presenting a strong correlation between the variables (ρ = -0.93). This is consistent with the literature [7][8][9]. In addition, the higher volatile matter in biomass results in a higher reactivity and a lower ignition temperature for biomass compared to coal [22]. Rybak et al. [21] stated that the TTI varies according to the volatile matter. Figure 3 (b) shows that the lower the volatile content, the shorter the FD of the tested biomasses, showing a strong correlation (ρ = +0.92). In the tested samples, homogeneous ignition was observed; that is, when ignition occurs rst in the gas phase, the ame envelope around the sample prevents oxygen from reaching the sample surface [21,22].

Conclusion
Mu e furnace methodology is effective, relatively simple, inexpensive, and fast, allowing for the determination of the TTI and FD of different samples of biomass. The volatile material content of the samples determines the TTI and FD of the biomass.

Declarations
Figures Figure 1 Flowchart of the process to test the mu e furnace as a new methodology. Time-to-ignition (TTI); aming duration (FD); volatile matter (VM); xed carbon content (FC); ash content (A) Figure 2 Temperature versus time curves adjusted for different mu e furnace temperatures.

Figure 3
Volatile matter as a function of ignition time (a) and ame duration (b)