## 2 Materials and Methods
### Sample Preparation and Operating Conditions  
Samples were prepared with Rotex industrial screener having 4.75 mm and 3.15 mm square hole sieves. Prepared and cleaned pellets retained by a 4.75 mm sieve of approximately 40 kg of sample size were poured directly into the feeding hopper. The pellets were then fed through the rotary valve to the pipeline and delivered to the receiving hopper by positive pressure lean phase pneumatic conveying method. The solid feed rate was altered with rotary valve speed, and the air mass flow rate was altered with the combinations of nozzles interfacing compressor for the supply of prime motive air. Three solid flow rates  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Load Cell** & **S-Type Tension/Compression** \\ \hline Rated load & 1500 kg \\ Output & 2 mV/V at rated load \(\pm\)0.1\% \\ Combined error & \(<\pm\)0.017\% of rated load \\ Output resistance & 350 \(\Omega\)\(\pm\) 3 \(\Omega\) \\ Resolution & 0.1 kg \\ \hline \hline \end{tabular}
\end{table}
Table 2: Specifications of load sensors.  
Figure 2: Full-scale industrial pneumatic conveying the test rig at The Wolfson Centre with a Line diagram showing bends in proximity at the end of the pipe with the full-scale test rig (pipe dimensions in the line diagram are not scaled).  
of approximately 0.2, 0.4, and 0.8 kg/s were tested under the effect of three air mass flow rates of 0.24, 0.27, and 0.3 kg/s, respectively. Two experiments were conducted for both pellets at each operating condition.  
The pellets were collected after experiments from the receiving hopper, and the entire batch was analyzed using the Rotex screener. The pellets retained by the 4.75 mm were considered clean pellets and sent back to the feeding hopper for a second consecutive run prior to the pellets being disposed of. The fines that passed through the 3.15 mm round hole sieve and the average particle size distribution after two consecutive runs were noted.  
Degradation of wood pellets is generally measured as Mechanical durability (DU) as per ISO 17831-1 at bench scale [32; 33]. It is the percentage weight of the pre-sieved pellet that survived or was retained by a 3.15 mm round hole after impact testing in a durability tester. Durability can be found by the equation given below:  
\[\text{DU}\;=\;\frac{M}{Mo}\;\times\;100(\%), \tag{1}\]  
where DU is durability, \(M\) is the mass of pellets retained by the 3.15 mm round hole sieve after impact, and \(Mo\) is the mass of pre-sieved clean pellets before testing.  
Mechanical durability depends upon various mechanical, physical, and chemical properties of pellets, such as surface cracks, moisture content, pellet length, raw material properties, pipeline features, particle velocity, impact load, and receiver type. There are two durability testers recognized by European Pellet Council as:  
* ISO standard Tumbling box durability tester
* Ligno durability Tester  
The durability of the pellets and the coffee beans in this study was tested by the ISO standard tumbling box tester, as shown in Table 1. In the test, 500 g of clean, prepared sample was tumbled for 10 min at a constant speed of 50 rpm. The retained mass above 3.15 mm was measured, and the durability was calculated as per Equation (1).