## 2 Materials and Methods  
In Greece, currently, there are two bins for the collection of municipal solid waste, namely the blue bin for the collection of recyclable waste (plastic, glass, paper, aluminum, or metal cars) and the green bin for the collection of the rest of the waste. The waste from the blue bin is separated in management plants. The plastic waste (PW) that was used in this study was the plastic fraction of the content of the blue bin. More precisely, plastic waste of Municipal Solid Waste (MSW) was collected from the integrated waste management plant in Kozani, Greece. These wastes contained typical plastic municipal waste such as water bottles, food packaging, packaging from cosmetic products, etc. The lignite (LIGA) sample was supplied by Agios Dimitrios Thermal Power Plant in Kozani, Greece. All samples were collected from the Western Macedonia area, located in Greece. All samples were air-dried for 2 weeks and grounded using the cutting mill SM100 Retsch. Then, the samples were dried in an oven at a temperature of 80 \({}^{\circ}\)C for 24 h to decrease the moisture content. Three blends with plastic wastes and lignite were prepared in different proportions: 30 wt.%, 50 wt.% and 70 wt.%. All samples/blends with lignite were analyzed with at least two replicates. The Gross Calorific Value (GCV) of the plastic waste sample (PW) and PW blends with lignite (LIGA) were measured using a Leco AC500 Isoperibol Calorimeter according to the ASTM D5865-13 standard [22]. The % difference of GCV of every sample/blend in comparison to the GCV of the LIGA sample was calculated by Equation (1). \[\Delta GCV_{sample}(\%)=\frac{GCV_{sample}-GCV_{LIGA}}{GCV_{LIGA}}\times 100 \tag{1}\]  
where:  
\(\Delta GCV_{sample}\): is the difference between the GCV of every sample/blend and the GCV of the lignite sample,  
\(GCV_{sample}\) : is the GCV of each sample/blend,  
\(GCV_{LIGA}\): is the GCV of the lignite sample. More details can be found presented in a previous study [23]. The theoretical GCV of blends was calculated (and compared with the experimental ones) as described in the previous study [24]. Briefly, the GCV of the blends were calculated from the following formula:  
\[GCV_{theor.}=W_{PW}\times GCV_{PW}+W_{LIGA}\times GCV_{LIGA} \tag{2}\]  
where:\(GCV_{PW}\): is the GCV of the PW sample. \(W_{PW}\): is the wt.% PW content of the blend. \(W_{LIGA}\): is the wt.% lignite content of the blend. Proximate analysis (determination of moisture, volatile, ash, and fixed carbon contents) was performed using a Leco TGA 701 analyzer according to the ASTM D7582 standard [25]. Thermogravimetric Analysis and Derivative Thermogravimetric Analysis (TG/DTG) were performed using a LECO TGA 701 analyzer as described previously [26]. Kinetic analysis (determination of the activation energy, \(E\), and pre-exponential factor, \(A\)) was performed based on the TG/DTG curves and data of the samples/blends as described in previous work [23]. Briefly, the Cumming approach, which is based on the Arrhenius equation, was used [27]:  
\[logK=logA-E/2.303\times R\times T \tag{3}\]  
\[K=-(dW/dt)/W \tag{4}\]  
where:  
\(K\): specific reaction rate. \(A\): pre-exponential factor. \(E\): Activation energy. \(T\): instantaneous absolute temperature. \(R\): universal gas constant. \(W\): mass of unburned combustible  
\(dW/dt\): instantaneous rate of mass loss. From the TGA data, the \(K\) can be found. The plot of \(logK\) versus \(1/T\) is linear. From the slope and intercept of this plot, the values of \(E\) and \(A\) can be found. The plot was made by using TGA data from stage III (see Figure S1 in Supplementary Material). Also, from the TGA data, the values of the comprehensive combustion index (CCI), the ignition index (\(D_{i}\)), and the burnout index (\(D_{b}\)) were calculated [28; 29]. The ignition index expresses the ignition behavior, e.g., how fast the fuel ignites, and the burnout index expresses the performance of the fuel with regard to incomplete char burnout [30].