Selective Microwave Pretreatment of Biomass Mixtures for Sustainable Energy Production

Abstract

Methods for the improvement of regional lignocellulosic resources (wood and agriculture waste) were studied and analyzed using blends with optimized compositions and a selective pretreatment of the blends using microwaves to enhance their thermochemical conversion and energy production efficiency. A batch-size pilot device was used to provide the thermochemical conversion of biomass blends of different compositions, analyzing the synergy of the effects of thermal and chemical interaction between the components on the yield and thermochemical conversion of volatiles, responsible for producing heat energy at various stages of flame formation. To control the thermal decomposition of the biomass, improving the flame characteristics and the produced heat, a selective pretreatment of blends using microwaves (2.45 GHz) was achieved by varying the temperature of microwave pretreatment. Assessing correlations between changes in the main characteristics of pretreated blends (elemental composition and heating value) on the produced heat and composition of products suggests that selective MW pretreatment of biomass blends activates synergistic effects of thermal and chemical interaction, enhancing the yield and combustion of volatiles with a correlating increase in produced heat energy, thus promoting the wider use of renewable biomass resources for sustainable energy production by limiting the use of fossil fuels for heat-energy production and the formation of GHG emissions.

1. Introduction

In accordance with requirements of the European Climate Law, the EU economy is responsible for reducing its greenhouse gas (GHG) emissions by at least 55% by 2030, compared with 1990, promoting renewable energy production (solar, wind, water, bioenergy, etc.) and limiting the use of fossil fuels for energy production, aiming to become climate-neutral by 2050 [1]. The 2024 EU bioenergy sustainability report and directive [2] includes sustainability and savings criteria regarding greenhouse gas emissions for agricultural and forestry biomass, which are regarded as the main sources of renewable energy in the EU, accounting for about 59% of the renewable energy consumption in 2021. It is predicted that bioenergy production from biofuels will be responsible for the vast majority (95%) of renewable fuel growth to 2030 [3]. Among them, solid biomass accounts for 76.0% of the total biomass fuel consumption for heating in the EU [1]. Nowadays, the applicability of renewable biomass feedstocks (harvesting or agriculture residues) has great potential as an alternative to fossil fuels for green energy production, though the efficient use of them is limited due to the dissimilarity of their structure, elemental composition, and calorific value if compared with solid or gaseous fossil fuels. Differences in the structure of biomass feedstocks can be reduced by pelletizing or briquetting them and producing biofuels with a defined structure and composition for automated energy production. As the total amount of produced energy and its quality is also influenced by the differences in the elemental composition and calorific value of forestry and agricultural waste [4], research is being conducted to limit the impact of these differences on energy production. Since agricultural waste has a relatively low calorific value and high ash content compared to forestry waste, the use of agricultural waste for energy production reduces the total amount of energy produced and its quality compared to the amount and quality of energy produced by using wood biomass. Therefore, if biomass pellets or briquettes are produced from agricultural waste, their composition must be improved. Partial improvement of the main properties of biomass pellets and the energy produced by using agriculture waste can be achieved by mixing the pellets in various proportions with sawdust [5,6]. The preliminary research results allow us to conclude that a similar improvement and control of the produced energy can be achieved by mixing pellets produced from agriculture residues in different proportions with pellets produced from wood, as increasing the mass fraction of wood pellets in the mixture leads to an increase in the total amount of heat produced [7,8]. As a promising method for an additional improvement of biofuel characteristics to produce biofuels with well-defined compositions, increased energy density, reactivity, and reduced GHG emissions, the microwave (MW) pretreatment of biomass has been recognized and accepted [9,10,11]. In accordance with the deep research of the mechanisms responsible for the structural modification of biomass providing its irradiation with microwaves, it has been shown that MW pretreatment enhances the breakdown of lignocellulosic biomass components (hemicellulose, cellulose, and lignin) depending on their ability to absorb and convert the energy of high-frequency (f = 2.45 GHz) electromagnetic field oscillations into heat due to the dissipative effects (Q = ε′′E²) through the MW-induced molecular collisions caused by dielectric polarization [12,13,14]. MW pretreatment offers energy-efficient volumetric heating of lignocellulosic biomass and is regarded as a cost-effective process of biomass conversion, which depends on irradiation time, temperature, the type of biomass feedstock, and microwave power, offering an effective method for producing bioactive compounds and presenting a opportunities for industrial-scale applications [15,16,17,18]. The MW-induced activization of biomass thermal decomposition and thermochemical conversion is confirmed by research results of the effects of MW pretreatment on lignocellulosic wheat straw, peat, and wood pellets, which suggests that the breakdown of their components (hemicelluloses, cellulose, and lignin) results in increased porosity, reactivity, and an increased heating value of pellets with faster yield of combustible volatiles (CO, H₂, CH₄), as well as their ignition and combustion [19]. The microwave-induced activation of pretreated pellets suggests that pretreated pellets can be used to activate the thermal degradation of selectively activated mixtures of biomass pellets by activating the thermal and chemical interactions of the mixture components, which contributes to the thermochemical conversion of the mixtures, confirmed by research results [20,21], indicating the influence of synergetic effects on thermal decomposition and the thermochemical conversion of selectively activated mixtures. Synergy refers to the phenomenon when the interaction between different feedstocks during their thermochemical conversion produces a notable effect on the product yields that goes beyond the sum of their individual contributions with notable positive synergetic effects from volatile products for feedstocks of biomass with coal and polymers [22,23,24,25]. Synergistic effects have also been detected for mixtures of microwave-activated pellets of straw or wood with untreated peat pellets, providing a faster rise in temperature and heat power, realizing the combustion of volatiles and increasing the average value of produced heat. In addition, the synergistic effect significantly changes at various stages during the thermochemical conversion of selectively activated blends and by varying the mass fraction of pretreated pellets in the mixture compared to untreated pellets. Therefore, to provide efficient control, improvement, and stabilization of thermal decomposition, the combustion and heat-energy production processes of these selectively MW-activated mixtures are used as fuel, and research is carried out, which reveals the main factors promoting the formation of positive synergistic effects at different stages of the thermochemical conversion of these mixtures. The novelty of such research for selectively activated mixtures of straw and wood with an optimization of the microwave pretreatment regime and mixture composition will increase opportunities for the wider use of regional agriculture residues (straw) for renewable energy production, increasing its quality, which is the main goal with the practical application and economic feasibility of this research. Addressing the financial burden of MW pretreatment is one of the reasons for conducting this research, where we try to prove that it is not always necessary to carry out pretreatment for 100% of the pellets, and noticeable effects can be achieved by only pretreating some of them. In situations when solar electricity during the daytime incurs very low or negative costs, the MW treatment of biomass could be a good solution; we explore how to use it and store it in biomass.

2. Materials and Methods

2.1. Materials

To conduct research and evaluate the effect of MW pretreatment on the thermochemical conversion of selectively activated mixtures (s* + w, s* + s, w* + s), commercial softwood (w) and wheat straw (s) pellets were used in this study, with average diameters of 6 mm and 8 mm, an average pellet length of 10–35 mm, and different chemical compositions and contents of volatile matter [19,26]. In the first phase of pretreatment, pellets in portions of 300 g were heated up to 350 K using convective heating in an inert atmosphere. In the second phase, unsteady MW pretreatment was performed in an argon atmosphere in a specially designed resonator to two different chosen temperatures (T = 473 K and T = 548 K) using a process of isothermal heating for 20 min [26]. The weighing of the raw and pretreated samples was performed to evaluate the changes in mass of the samples induced by MW heating and yields of solid and condensable fractions were controlled. MW pretreatment of pellets was used to produce MW-activated pellets (s*, w*) and selectively MW-activated mixtures by varying the mass fraction of pretreated pellets (s* or w*) in the mixture with raw pellets (s or w) in the range from 15% to 60%. Raw wheat straw and wood pellets have different moisture content levels, elemental compositions, and calorific values, which are significantly altered by the microwave pretreatment of these pellets [19]. In addition, wood and wheat straw pellets have different chemical compositions with different contents of hemicelluloses, cellulose, lignin, and volatile matter (

Table 1. Chemical composition and contents of volatile matter in raw wood and straw biomass [19,25,26].

Chemical Components	Wood	Wheat Straw
Hemicelluloses, %	22.78	28.1
Cellulose, %	43.2	35.4
Lignin, %	28.83	16.49
Volatile matter, %	77–78	67.8–74.8

) responsible for different yields of volatiles during their thermal decomposition and MW pretreatment.

Summarizing MW-induced changes in the main characteristics of pellets allows us to conclude that the MW pretreatment of wheat straw and wood pellets up to T_MW = 548 K causes the carbonization of pellets, increasing the carbon content in wheat straw pellets from 46.43% to 52.39% and the heating value (HHV) of pellets from 18.4 MJ/kg to 20.8 MJ/kg. For wood pellets, carbon content, respectively, increases from 50.59% to 56.18%, and HHV increases from 19.94 MJ/kg to 22.63 MJ/kg. In addition, increasing the pretreatment temperatures of wood and wheat straw pellets results in an enhanced yield of volatiles during their MW pretreatment and is followed by a decrease in the weight loss of pretreated pellets related to the yield of volatiles, while increasing weight loss related to char combustion. In addition, the MW-induced increase in the heating values of pretreated pellets [19] causes an increase in the amount of heat, which can be related to char combustion, while decreasing the amount of heat related to the combustion of volatiles [26]. To evaluate the effect of MW pretreatment on the elemental composition of biomass pellets, the content of C, H, N were measured according to the national standard (LVS EN15104:2011 [27]) using the elemental analyzer Vario MACRO. The HHV of pretreated samples was estimated using a regression equation and the data of elemental composition [26]. The MW-induced structural changes in pellets were estimated, providing measurements of the porous structure and specific surface area of pellets under different regimes of MW pretreatment. The porous structure of the pellets was estimated from N2 sorption/desorption isotherms determined using the sorptometer Quntachrome NOVA 4200e. Specific surface area was calculated with Quantachrome software based on Brunauer–Emmet–Teller (BET) theory. Degassing was conducted at room temperature for 120 h [26,28].

2.2. Batch-Size Pilot Device That Was Used for the Experimental Research of the Gasification/Combustion of Selectively Pretreated Mixtures of Biomass Pellets

The thermal decomposition and thermochemical conversion of microwave-activated (s*, w*) and raw (s, w) straw and wood pellet mixtures were studied using the previously developed original experimental device with average heat power 4–5 kW [19,20,21]. The device consists of a gasifier (1), which is charged with selectively activated mixtures and water-cooled sections of a combustor (4–6) (Figure 1).

The thermal decomposition of selectively activated mixtures and the formation of an axial flow of volatile components (CO, H₂, CH₄) are initiated in the gasifier using a propane flame flow (2), which provides a heat supply into the upper part of the biomass layer and is switched off after the ignition of the pellets. The axial flow of volatiles is supplied into a water-cooled combustor, downstream of which the combustion of volatile components is completed. To initiate and support the gasification process, a primary air supply at the base of the gasifier is used with an average flow rate of 40 L/min. To provide complete combustion of volatiles, secondary swirled airflow is used with an average flow rate of 60 L/min (3). The gasifier is charged with a selectively activated mixture, the total mass of which in the gasifier reaches 350–500 g. The concentration of MW-activated pellets in the mixture is changed within the range from 15% to 60%. The thermal decomposition of mixtures was initiated with the propane flame when they were charged into the gasifier.

2.3. Methods of Measurements

The thermal decomposition of selectively activated mixtures yielding volatiles resulted in the weight loss of pellets, charged in the gasifier. The weight loss of pellets is estimated by providing continuous measurements of changes in the levels of biomass layer height in the gasifier with an accuracy of 2%, using an originally developed device (Figure 1) and methodology, described in [19,20,21]. To evaluate the composition of volatiles produced during the thermal decomposition of selectively activated mixtures of gas, a sampling probe was used, which was inserted into the axial flow of volatiles through the orifice at the outlet of the gasifier, providing a continuous analysis of gas flow composition using the gas analyzer Testo 350 [19,20,21].

To estimate the effect of MW pretreatment of wood* and straw* additives in mixtures on combustion characteristics, local measurements of the flame temperature, produced heat, and emission compositions at different stages of volatiles’ thermochemical conversion were carried out. The measurements of the temperature were performed online, with 1 s intervals. A Pt/Pt/Rh thermocouple was inserted in the reaction zone of the flame, and the data were recorded with Pico TC-08 logger with accuracy ±5%. The placement of the thermocouple was through the orifice intended for diagnostic tools in the distance L = 280 mm downstream from the secondary air-supply nozzles.

The calorimetric measurements were provided using the Data Translation DT9805 data acquisition module and processed with the Quick DAQ program Version 2014, recording the inlet and outlet temperatures of the cooling water. The temperature of the continuous cooling water flow was measured with the thermo sensors AD 590, which have accuracy of ±1% using 1 s intervals. The heat output transferred from the device was calculated from the data of differences in the cooling water temperature at the inlet and outlet of each of 3 sections. The cooling water flow was maintained at a fixed value for the experiment and the accuracy of the mass flow was estimated at ±2.5%.

The average duration of the thermochemical conversion of selectively activated mixtures with an average mass load of 350–500 g depends on mixture composition and varied between 3000 s and 3500 s. The average values of processes developing at different stages of thermochemical conversion—during the primary precombustion stage and the self-sustaining stage—were estimated from the kinetics of heat power, indicating that the primary precombustion stage lasts between 400 s and 1000 s, when heat power produced in the device gradually increases and approaches its maximum value (min–max), which then starts the self-sustaining combustion of the selectively activated mixture and lasts between approximately 1000 and 2000–2500 s (max end).

Following on from the results of previous research, the MW pretreatment of biomass pellets (wood, wheat straw), which are produced from biomass waste (agriculture and forestry residues), changes their structure, porosity, reactivity, and elemental and chemical composition, as well their calorific value [19,20,21], with a direct influence on the kinetics of thermal decomposition of mixtures, the yield of volatiles, and heat production, promoting changes in the average values of the weight loss rate (dm/dt, g/s), the yield of volatile compounds (CO, CO₂), and produced heat.

A kinetic study of the thermal decomposition and devolatilization behavior of pretreated and raw biomass pellet mixtures with different elemental and chemical compositions of the mixture components is important to assess the contribution of each component in the development process and the weight loss rate of the mixture. At this moment, the understanding of the kinetics of such kinds of biomass mixtures is far from clear, and the average values of the weight loss rates for different mass fractions of pretreated pellets in the mixture can be estimated using a simple additive approach:

dm/dt = C* (dm/dt)* + C₀ (dm/dt)₀

(1)

where dm/dt, g/s—weight loss rate of the mixture, (dm/dt)*, g/s—weight loss rate of MW pretreated pellets, (dm/dt)₀, g/s—weight loss rate of raw pellets, C*—mass fraction of pretreated pellets, and C₀—mass fraction of raw pellets in the mixture.

Using the single first-order reaction model represents a starting point in process modeling [29], when dm_i/dt for each component can be expressed by accounting for its rate constant, depending on the activation energy E_a and temperature T of its thermal decomposition:

dm/dt = k(1 − m)

(2)

m = (m₀ − m_t)/(m₀ − m_f)

(3)

k = A∙exp (−E_a/RT)

(4)

where m is the converted mass, k is the reaction-rate constant, m₀ is the original mass of the test sample, m_t is the mass of the test sample at time t, m_f is the final mass at the end of thermal decomposition, E_a is the activation energy of a thermal decomposition component, T is the temperature, A is the pre-exponential factor, and R is the universal gas constant.

More accurate and specific models are required using the distributed activation energy model [30,31] to describe the thermochemical conversion of mixture components, when the process development involves many independent, parallel rate processes assuming them as irreversible first-order reactions in relation to the amount of unreacted material. By analogy, the rate at which the volatiles are produced by a particular reaction then can be defined according to the mass balance on the reactant species:

dVi/dt = k_i (V_i^t − V_i)

(5)

V_i^t − V_i = V_i^t ∙ [exp(A_i ∙ exp (−E_i/RT))]_i

(6)

where V_i^t is the final quantity of volatile matter for the generic species, i, and k_i is the rate constant of the reaction expressed according to the Arrhenius law.

In accordance with continuity equation, the weight loss of lignocellulosic biomass during its thermal degradation follows the formation of the axial flow of volatile components at the outlet of the gasifier, which are responsible for the formation of the reaction zone downstream of the combustor and the heat-energy production. As follows from (1)–(4) and the differential thermal gravimetry data [28], the thermal decomposition and weight loss rate of biomass mixtures is highly influenced by the activation energy and heating values of the components, promoting the development of oxidative volatilization and char-conversion processes (Figure 2a–c). For low-temperature MW pretreatment conditions (T < 473 K), the MW pretreatment of biomass pellets enhances the endothermic processes of biomass drying, with enhanced yields of chemically bonded water and organic emissions, a breakdown of the polymerlike structure of the main biomass components (hemicelluloses, cellulose, and lignin) [32,33], and yields of condensable and non-condensable products with a relatively slight increase in their activation energy and calorific value (Figure 2a–c) [34]. For wood and wheat straw, the yield of condensable and non-condensable products, which starts at T = 473 K, consisted of about 80% physically bonded water and significantly increased with increasing pretreatment temperature. The estimation of the changes in the content of physically bonded water in off-gases during high-temperature MW pretreatment (T = 573 K) revealed that the wheat straw is the most thermolabile biomass for which, after MW pretreatment, only 42.1% of the initial biomass remained as a solid fraction [34]. Increasing the MW pretreatment temperature with a more intensive decomposition of biomass pellets results in a pronounced increase in the activation energy and calorific value of selectively activated biomass components (Figure 2b,c) by limiting oxidative volatilization while enhancing the char conversion of pretreated pellets (Figure 2a) [28].

3. Results and Discussion

3.1. The Effect of Additives of MW-Pretreated Straw (s*) to Raw Wood (w) Pellets on the Processes of Thermal Decomposition and Thermochemical Conversion in the Selectively Activated Mixture s* + w

To achieve the efficient thermal decomposition and thermochemical conversion of selectively activated mixtures using the MW pretreatment of mixture components, it is necessary to consider its influence on the kinetics of the mass loss of the mixtures, the formation of volatile compounds, and the process of heat-energy production, accounting for MW-induced changes in the activation energy of oxidative volatilization, char conversion, and the heating values of pretreated pellets. Therefore, studies have been conducted on mixtures of different compositions, activating straw or wood pellets in MW pretreatment and producing selectively activated mixtures s* + w, s* + s, w* + s with an evaluation of the effect of activated pellets on the thermal decomposition of the mixture and heat-energy production.

To assess the effect of MW pretreatment of wheat straw pellets on the thermochemical conversion of the selectively activated mixture s* + w, studies were conducted to evaluate the influence of wheat straw pretreatment on changes in the weight loss of the mixture, the yields of volatile matter, and heat-energy production by changing the MW pretreatment temperature of wheat straw and the mass fraction of activated wheat straw pellets in the mixture. The effect of these changes on the formation of the primary precombustion stage and self-sustaining combustion of the selectively activated mixture were evaluated using two different temperatures of wheat straw pretreatment (T_MW = 473 K and T_MW = 548 K).

Considering main factors that influence the development of thermal decomposition and the thermochemical conversion of mixture s* + w, it should be noticed that for low-temperature MW pretreatment conditions of wheat straw pellets (T_MW = 473 K), the activation energies for pretreated straw oxidative volatilization (E_a = 120.9 kJ/mol) and char conversion (E_a = 318.6 kJ/mol) are higher than that for raw non-pretreated wood pellets (E_a = 108.7 kJ/mol and 135.2 kJ/mol, respectively) {27]. The higher activation energy of pretreated straw for char conversion suggests that additives of pretreated straw to raw wood can predominately influence the development of mixture volatilization, the intensity of which depends on chemical composition of mixture components. Comparing the chemical composition of wheat straw and wood biomass (Table 1) shows us that wheat straw pellets have a higher content of hemicellulose than raw wood, with a lower content of cellulose and lignin. Therefore, additives of pretreated straw pellets to raw wood result in an increase in the content of hemicellulose in mixture, decreasing contents of cellulose and lignin. A higher content of hemicellulose in the mixture with a preliminary breakdown of its polymeric structure during the low-temperature MW pretreatment of wheat straw [13,14] suggests that mixture volatilization is influenced by the enhanced exothermal decomposition of hemicellulose which occurs at temperatures between 470 K and 580 K with a peak value of the mass loss rate and yields of volatile components (CO₂, CO, H₂) at T = 540 K [34]. However, from the data of the TG analysis, it follows that the initial temperature of the active decomposition of pretreated pellets shifts towards a higher temperature in comparison with that for raw wood, and can slightly delay the thermal decomposition of pretreated pellets [28].

Increasing the pretreatment temperature of wheat straw to 548 K activates the yield of volatile components during the MW pretreatment of wheat straw, increasing the activation energy of pretreated pellets (Figure 2b). This suggests that MW pretreatment can result in a decrease in the weight loss (Figure 2a) and yield of volatile components during the thermal decomposition of a selectively activated mixture, while increasing the weight loss during the char conversion of pretreated pellets, which for the higher pretreatment temperature is comparable with that for oxidative volatilization (Figure 3a).

In addition to MW-induced changes in the activation energy of pretreated pellets, which influence the development of the processes of volatilization and char conversion in mixture, the additives of pretreated wheat straw pellets to raw wood cause changes in the elemental composition and calorific values (HHV and LHV) of the mixture. For low-temperature (T_MW = 473 K) wheat straw MW pretreatment conditions, the calorific value of straw pellets increases from 18.4 MJ/kg for raw pellets to 18.69 MJ/kg for pretreated wheat straw and the HHV of pretreated straw pellets is still lower than the calorific value of raw wood pellets (19.9 MJ/kg) (Figure 2c). This suggests that for the low-temperature pretreatment conditions of wheat straw, increasing the mass fraction of pretreated wheat straw additives to raw wood can result in a decrease in the calorific value of the mixture. Increasing the MW pretreatment temperature of wheat straw to T_MW = 548 K, when the calorific value of straw pellets increases to 20.76 MJ/kg and exceeds the calorific value of raw wood pellets (Figure 2c), the influence of pretreated wheat straw additives on the calorific value of the mixture is nonidentical. Hence, for high-temperature MW straw pretreatment, the calorific value of the mixture tends to increase when increasing the mass fraction of pretreated pellets in the mixture.

A set of MW-induced changes in wheat straw activation energy for oxidative volatilization and the heating value influence the kinetics of the thermal decomposition of the selectively activated mixture (s* + w) during the primary precombustion stage of the mixture, and during its self-sustaining thermochemical conversion. From the results of the kinetic study, it follows that increasing the mass fraction of pretreated additives in the mixture and the pretreatment temperature of wheat straw causes a faster increase in the weight loss of the mixture and produced heat, reaching their maximum values, with a faster transition from the primary precombustion stage to the self-sustaining combustion of the selectively activated mixture (Figure 3a–d) resulting in changes in the average values of the weight loss rates, yield of volatiles, and produced heat at different stages of the thermal decomposition and thermochemical conversion of the mixture (s* + w) (Figure 4a–d).

Considering the effect of MW pretreatment of wheat straw on the average values of the weight loss of the mixture, yields of volatiles (CO₂, CO), and the amount of heat produced allows us to conclude that the additives of pretreated wheat straw to raw wood significantly influence the development of the primary stage of the mixture thermal decomposition (min–max). A preliminary analysis of the main factors that influence the development of the primary stage of the mixture’s thermal decomposition suggests that the thermal decomposition of the mixture (s* + w) starts with the fast exothermic decomposition of hemicellulose in raw wood pellets, which have lower activation energy and higher calorific value than pretreated wheat straw pellets (Figure 2b,c) and can be initiated at lower temperature than pretreated straw pellets [28]. The higher activation energy of wheat straw pellets over raw wood promotes a decrease in the weight loss rate of the mixture to a minimum value when the mass fraction of pretreated pellets in the mixture approaches 15–30% (Figure 4a). The exothermic decomposition of hemicellulose in raw wood pellets, with an increase in heat output, activates the thermal interaction between raw wood and pretreated wheat straw pellets by increasing the temperature of pellets and enhancing the exothermic decomposition of hemicellulose in pretreated straw pellets. The higher content of hemicellulose in wheat straw and the increasing temperature of pellets results in increasing yields of volatiles (CO₂, CO) and the amount of produced heat up to the peak values, occurring when the mass fraction of pretreated straw in the mixture approaches 30%. It should be noted that the development of the exothermal decomposition of hemicellulose is followed by an increase in the temperature of the mixture components. The yield of volatiles tends to decrease to a minimum value with a correlating decrease in calorific value, while the weight loss of the mixture increases up to the peak value, when the mass fraction of pretreated pellets approaches 45–60%. This suggests that the enhanced exothermic decomposition of hemicellulose by increasing additives of pretreated wheat straw in the mixture promotes an increase in the mixture temperature to above 540 K, which corresponds to the peak value of the yield of volatiles. Increasing the temperature of the mixture above 540 K limits the volatilization of hemicellulose by enhancing the char conversion of solid residues [34] (Figure 4a–d). For such conditions, the development of thermal decomposition at a higher mass fraction of pretreated wheat straw in the mixture promotes the development of the surface reactions (7)–(9) responsible for an increase in the yields of CO and CO₂ when the mass fraction of pretreated straw in the mixture increases above 45%.

C + 0.5O₂ → CO   ∆H = −123.1 [KJ/mol]

(7)

C + H₂O ↔ CO + H₂   ∆H = +118 [kJ/mol]

(8)

C + O₂ ↔ CO₂  ∆H = −394 [kJ/mol]

(9)

The development of processes during the primary stage of thermal decomposition influences their development during the next stage of thermal decomposition, which refers to the self-sustaining burnout of volatiles. As can be seen from Figure 4a–d, the enhanced volatilization of the mixture during the primary precombustion stage with a correlating increase in the heat output limits the yield of volatiles and heat output during the self-sustaining burnout of volatiles and the total amount of heat produced during the thermochemical conversion of the mixture.

Increasing the pretreatment temperature of wheat straw MW to T_MW = 548 K causes an enhanced volatilization of hemicellulose in wheat straw pellets with an additional loss of volatile components during the MW pretreatment of wheat straw. As a result, the activation energy for the oxidative volatilization of pretreated wheat straw increases (Figure 2b), decreasing the weight loss rate of the mixture and yield of CO₂ to the minimum value during the primary stage of mixture thermal decomposition, when the mass fraction of wheat straw additives in the mixture approaches 15–30% (Figure 4b,c). A more pronounced decrease in the yield of CO₂ by increasing the temperature of MW pretreatment of wheat straw, with a less noticeable decrease in the yield of CO, suggests that the enhanced carbonization of pellets could cause notable variations in the yield of CO and CO₂, because of the process of the exothermic partial oxidation of carbonized pellets (10), (11) and the simultaneous development of Boudouard’s reaction, which is endothermic (12), increasing the yield of CO, while slightly decreasing the yield of CO₂ [9]:

CO + 0.5O₂ → CO₂   ∆H = −283 [KJ/mol]

(10)

C + 0.5 O₂ → CO   ∆H = −123.1 [KJ/mol]

(11)

C + CO₂ → 2CO  ∆H = +159.9 [KJ/mol]

(12)

It should be noticed that in contrast to low-temperature wheat straw pretreatment for conditions of high-temperature pretreatment, the calorific value of pretreated pellets increases and exceeds the HHV of raw wood (Figure 2c). For such conditions, when increasing the mass fraction of wheat straw additives in a mixture above 45%, the yield of combustible volatile CO at the outlet of the gasifier and the total amount of heat produced during the primary precombustion stage of the mixture’s thermal decomposition tends to increase, with a correlating increase in the heat produced for the self-sustaining combustion of volatiles, which proceeds at nearly stoichiometric combustion conditions in reaction zone.

As can be seen from Figure 4a–c, the development of the processes which occur during the primary precombustion stage significantly influence the development of the mixture’s thermal decomposition during the self-sustaining burnout of volatiles (min-end). For low-temperature wheat straw pretreatment conditions, increasing the total amount of produced heat and yield of volatiles up to peak values during the primary precombustion stage promotes a slight decrease in the produced heat and yield of volatiles, reaching the minimum values during self-sustaining combustion conditions (min–max) with slight changes in the air excess ratio in a range between α = 0.97 and α = 1.09 and the temperature in reaction zone between T = 1100 K and T = 1150 K. In contrast, high-temperature wheat straw pretreatment (T = 548 K) conditions, increasing the mass fraction of wheat straw additives in the mixture above 45% promotes a correlating increase in the yield of volatile components and heat production during the self-sustaining burnout of volatiles (max-end), increasing the total value of produced heat (min-end). However, the enhanced volatilization of mixture achieved by increasing the mass fraction of pretreated straw promotes a slight decrease in the air excess ratio in reaction zone α = 0.95–0.96, indicating that an enhanced yield of CO can promote a slight increase in the content of unburned volatiles in the products.

3.2. The Effect of Additives of MW-Pretreated Straw (s*) to Raw Straw (s) Pellets on the Processes of Thermal Decomposition and Thermochemical Conversion in the Selectively Activated Mixture s* + s

Analogous to the main factors that influence the thermal decomposition of mixture s* + w, MW-induced changes in activation energy, heating value, volatilization, and the char conversion of pretreated wheat straw influence the kinetics of the thermal decomposition and thermochemical conversion of the mixture s* + s, which depend on the mass fraction of pretreated pellets in the mixture and the temperature of MW pretreatment (Figure 5a–d).

A more detailed analysis of the influence of MW pretreatment of wheat straw pellets (s*) on process development during the thermal decomposition and thermochemical conversion of the selectively activated mixture (s* + s) is provided, with an estimation of the average values of weight loss rate, heat output, and the yield of volatile components for different stages of process development using two temperatures for the MW pretreatment of wheat straw pellets (T_MW = 473 K and T_MW = 548 K) and by varying the mass fraction of pretreated wheat straw pellets in the mixture from 15% to 60% (Figure 6a–e).

Evaluating the main factors that can influence the thermal decomposition of a mixture (s* + s) with a nearly equal chemical composition of mixture components indicates that the thermal decomposition of the mixture is predominately influenced by the MW-induced activation of hemicellulose with higher yields of CO₂ [34], occurring when the calorific value (HHV*) and activation energy (E*_a) of pretreated wheat straw pellets exceeds the values for raw straw (Figure 2b,c). In addition, the initial temperature of the active decomposition of pretreated pellets shifts towards a higher temperature in comparison with that for non-treated wheat straw, and slightly delays the beginning of thermal decomposition for pretreated pellets [28]. This suggests that the thermal decomposition of mixture s* + s starts with an exothermal decomposition of hemicellulose in non-pretreated, raw wheat straw pellets, promoting the yield of CO₂. The thermal interaction between raw (s) and MW-pretreated wheat straw pellets (s*), which have higher calorific value, activates the thermal decomposition of the mixture, increasing the weight loss of the mixture to a maximum value, when the mass fraction of pretreated pellets in the mixture approaches 15–35% and causes a simultaneous increase in CO₂ emissions to their maximum value, while decreasing the yields of CO and H₂ emissions (Figure 6a–d). This correlation suggests that an increase in the yield of CO₂ is also slightly influenced by the partial thermochemical conversion of combustible volatiles during the development of mixture gasification. The increase in the weight loss of the mixture and yield of CO₂ to the maximum value is followed by a gradual decrease when increasing the mass fraction of pretreated wheat straw pellets in the mixture, which is observed under low- and high-temperature wheat straw pretreatment conditions (Figure 6a). A simultaneous decrease in the weight loss of the mixture and the yield of CO₂ testifies that a higher energy input in gasification zone is needed to support the volatilization of pretreated pellets with a higher activation energy than raw pellets, promoting a decrease in heat output when the mass fraction of pretreated pellets exceeds 45% for low-temperature conditions and 15% for high-temperature pretreatment conditions. As can be seen from Figure 6a, the dependence of the weight loss rate of a mixture on the mass fractions of pretreated pellets cannot be estimated with linear approximation, and indicates the deviation of dm/dt = f(straw*) from linearity. Such a deviation confirms the formation of a positive synergistic effect related to the thermal and chemical interaction between the mixture components, which is caused by the increase in the reactivity, calorific value, and activation energy of the MW-pretreated pellets, contributing to changes in the weight loss rate of the mixture with the deviation of dm/dt from linearity. As can be seen from Figure 6a–d, for low-temperature wheat straw pretreatment, similar changes in the average values of the weight loss rate and the yields of volatiles by increasing the mass fraction of pretreated pellets in a mixture to 30–45% are observed not only for the primary stage of a mixture’s (min–max) thermal decomposition, but also for the self-sustaining burnout of a mixture (min-end) and for the total process of a mixture’s thermochemical conversion. This allows us to conclude that additives of pretreated straw enhance the thermal decomposition of a mixture, which predominately refers to the development of char conversion of solid residues, the formation of which is observed during the thermal decomposition of hemicellulose and lignin [28,34] by increasing the output of produced heat. As can be seen from the results of the kinetic study (Figure 5b), an evident increase in heat output is observed at the end stage of a mixture’s self-sustaining burnout.

A more pronounced influence of MW pretreatment on the yields of CO₂, CO, and H₂ is detected under high-temperature wheat straw pretreatment conditions. MW pretreatment enhances the carbonization of pellets and the loss of volatile components during MW pretreatment, resulting in enhanced char conversion, while the limited oxidative volatilization of pretreated pellets increases the yields of combustible volatiles to their maximum values, while decreasing the yield of CO₂ to minimum values during the development of the primary stage of a mixture’s thermal decomposition (Figure 6b–d). The higher activation energy of pretreated pellets causes a decrease in the weight loss rate and heat output during the primary stage of thermal decomposition by increasing the mass fraction of pretreated pellets in the mixture, while enhancing heat output during the self-sustaining burnout of the mixture. The increased heating value of the pretreated pellets enhances the exothermal process of char conversion with an increased yield of combustible volatiles during mixture gasification, completing their burnout in the reaction zone for nearly stoichiometric combustion conditions (α = 1.05–1.15), increasing the total amount of produced heat energy per mass of pretreated pellets and the average temperature in reaction zone from T = 1050 K to T = 1150 K.

3.3. The Effect of Additives of MW-Pretreated Wood (w*) to Raw Straw (s) Pellets on the Processes of Thermal Decomposition and Thermochemical Conversion in the Selectively Activated Mixture w* + s

The results of the kinetic study of the weight loss rates under conditions of the non-isothermal heating of mixture w* + s have shown that, similar to the development of the process for the mixtures s* + w and s* + s, additives of pretreated pellets (wood*) to raw pellets (straw) result in the faster thermal decomposition of a mixture, with a faster rise in the weight loss rates and the heat produced up to their peak values, determining a faster transition to the isothermal regime of thermal decomposition and the self-sustaining burnout of a mixture with increased weight loss and produced heat, which depends on mass fraction of pretreated pellets (w*) in the mixture and temperature of MW pretreatment (Figure 7a–d).

To consider the effect of MW pretreatment of wood pellets on the thermal decomposition of a selectively activated mixture (w* + s), average values of mass loss, yield of volatiles (CO, CO₂) and heat production at different stages of process development were evaluated by varying the MW pretreatment temperature and the mass fraction of pretreated wood pellets in the mixture. The thermal decomposition of a mixture is influenced by differences in the chemical composition of mixture components (Table 1), and additives of pretreated wood cause a decrease in the content of hemicellulose and an increase in the content of cellulose in the mixture. As a result, increasing the mass fractions of pretreated wood pellets can promote changes in the yields of CO₂ and CO, decreasing the yield of CO₂ and increasing the yield of CO during the thermal decomposition of a mixture. Taking into account that the pretreatment of wood pellets results in an increase in the calorific value and activation energy of pellets (Figure 2b,c), the thermal decomposition of a mixture occurs when the calorific value and activation energy of the oxidative degradation of pretreated wood pellets is higher than that for raw straw, but lower for the char-conversion stage [28]. In addition, the pretreatment of wood pellets slightly delays the thermal decomposition of pellets, and the thermal decomposition process of a mixture starts with the fast decomposition of hemicellulose in raw wheat straw and is followed by the thermal decomposition of pretreated wood pellets. The thermal decomposition of a mixture is influenced by the thermal and chemical interaction between the pellets. Therefore, additives of pretreated wood with higher calorific value enhance the exothermal processes of a mixture’s thermal decomposition both for the low-temperature and high-temperature MW pretreatment conditions of wood pellets, promoting the chemical conversion of volatile components and solid residues. Enhancing the thermal decomposition of a mixture by increasing the mass fraction of pretreated wood pellets in the mixture causes an increase in the weight loss of the mixture, and the maximum value of the weight loss rate is achieved for 60% of pretreated wood pellets in a mixture (Figure 8a). A similar increase in the weight loss rate by increasing the mass fraction of pretreated wood pellets in a mixture is observed during the primary precombustion stage, as well as for the conditions of the self-sustaining combustion of volatile components and during the entire process of thermal decomposition (Figure 8a). The deviation of dm/dt from linearity when part of the mass fraction of pretreated pellets in the mixture increases confirms the influence of a thermal and chemical interaction between pretreated and raw pellets. An increase in the activation energy of wood pellets during their MW pretreatment (Figure 2b) and changes in the chemical composition of a mixture by increasing the mass fraction of pretreated wood pellets in a mixture slows down the oxidative volatilization of the mixture, decreasing the yields of CO₂ and CO and heat output to minimum values, when the mass fraction of pretreated wood in the mixture approaches 15–30% (Figure 8b–d). A further increase in the mass fraction of pretreated wood in mixtures is followed by changes in the yields of volatile components and heat output, which can be related to the thermal decomposition of cellulose by enhancing the yield of CO and the development of competitive reactions (10)–(12) of char conversion. A more pronounced increase in heat output for the primary stage of a mixture’s thermal decomposition and the self-sustaining burnout of volatile components is observed by increasing the temperature of wood pretreatment, wherein MW pretreatment activates the development of char conversion, decreasing the yield of CO₂ to minimum values during the primary stage of a mixture’s thermal decomposition while increasing the yield of CO. Exothermal and endothermic reactions of char conversion (10)–(12) also occur, with interrelated changes in the yields of volatiles’ components and heat output (Figure 8b–d) with nearly stoichiometric conditions in the reaction zone α = 0.9–1.13 and for temperature T = 1030–1130 K.

4. Conclusions

From the results of the experimental study of the effects of the selective MW pretreatment of wood or straw pellets on the thermal decomposition of a mixture of raw and pretreated pellets by varying pretreatment conditions and the mass fraction of pretreated pellets in mixtures (s* + w, s* + s, w* + s), the following conclusions can be drawn:

First, the MW pretreatment of pellets causes changes in their heating values and the energy that is required to activate the thermal decomposition of the main components of lignocellulosic pellets (hemicellulose, cellulose, and lignin) (Figure 2). This occurs by varying the process of oxidative volatilization, the char conversion of pretreated pellets, and the yield of volatile components at different stages of the thermal decomposition of mixtures, which depends on the temperature of the MW pretreatment and the mass fraction of pretreated additives in the mixture.

For all mixtures, increasing the additives of pretreated pellets in the mixture from 15% to 60% results in faster thermal decomposition with the faster weight loss of mixtures and the faster thermochemical conversion of selectively activated mixtures. The most pronounced decrease in the duration of thermochemical conversion (by 360 s) was detected for the mixture s* + w using pretreatment temperature 473 K, while a less effective decrease (by 75 s) was observed for the mixture w* + s under T = 473 K.

Next, the additives of pretreated pellets in the mixture cause an increase in the heat produced, with the most pronounced increase observed for the mixtures w* + s (by 28% for T_mw = 548 K) and s* + s (by 24% for T_mw = 473 K), while less noticeable changes in the heat produced were observed for the s* + w mixtures when, under a pretreatment temperature of T = 548 K, additives of pretreated straw slightly reduced the produced heat (by −1.5%).

The thermal decomposition of selectively activated mixtures starts with changes in the weight loss rate of mixtures and the deviation of its dependence on the mass fraction of MW-pretreated pellets from linearity, confirming the influence of the thermal interaction between pretreated and raw pellets on the thermal decomposition of mixtures with positive synergistic effects on weight loss rate, which can be related to MW-induced changes in the elemental composition of pretreated pellets increasing the carbon content in pretreated pellets and their calorific value responsible for an increase in heat output and thermal interaction between the pellets.

The MW-induced changes in the weight loss rate of mixtures and the yield of volatile components (CO₂, CO) are influenced by the difference in chemical composition of the mixture’s components responsible for the difference in the contents of hemicellulose, cellulose, and lignin in pretreated and raw pellets, as well as the yield of volatile components during the MW-induced thermal decomposition of mixtures. An increase in the mass fractions of pretreated straw pellets in the mixture causes an increase in the content of hemicellulose by enhancing the yields of CO₂, while increasing the additives of pretreated wood promotes an increase in the content of cellulose with a higher yield of CO.

MW-induced changes in the yield of CO₂ and CO suggest the influence of chemical interaction between the development of endothermic and exothermic surface reactions on components during the thermochemical conversion of a mixture and heat-energy production, which depend on mixture composition, the content of pretreated pellets in the mixture, and the temperature of MW pretreatment, promoting changes in weight loss rate of the mixture and the composition of produced gas.

Analyzing the effects of the MW pretreatment of biomass pellets on the thermal decomposition of mixtures and the thermochemical conversion of selectively activated mixtures, it can be inferred that additives of MW pretreated pellets to raw pellets noticeably improve the thermal decomposition of the mixture, the yield of volatile components, and their thermochemical conversion, thus facilitating the control of heat-energy production with a wider and sustainable use of renewable energy resources—particularly agriculture and wood residues.