Exergy Transfer Analysis of Biomass and Microwave Based on Experimental Heating Process

Abstract

Exergy transfer and microwave heating performances of wheat straw particles as affected by microwave power (250, 300, and 350 W), feeding load (10, 30, and 50 g), and particle size (0.058, 0.106, and 0.270 mm) were investigated and detailed in this study. The results show that when the microwave power increased from 250 to 350 W, the average heating rate increased in the range of 23.41–56.18 °C/min with the exergy transfer efficiency increased in the range of 1.10–1.89%. When the particle size increased from 0.058 to 0.270 mm, the average heating rate decreased in the range of 20.59–56.18 °C/min with the exergy transfer efficiency decreased in the range of 0.70–1.89%. When the feeding load increased from 10 to 50 g, the average heating rate increased first and then decreased in the range of 5.96–56.18 °C/min with the exergy transfer efficiency increased first and then decreased in the range of 0.07–1.89%. The highest exergy transfer efficiency was obtained at a microwave power of 300 W, feeding load of 30 g, and particle size of 0.058 mm.

1. Introduction

Biomass is considered a future renewable resource with a high potential for energy production due to the characteristics of net zero CO₂ release, environmental friendliness, biodiversity, and so on [1]. Biomass is an energy carrier with a large quantity [2], and it can help improve the ecological environment and maintain ecological balance [3].

Thermochemical conversion is currently the main route for converting biomass into heat, fuels, or chemicals [4]. In order to efficiently use biomass energy, researchers proposed many thermochemical technologies, such as (1) liquefaction to produce bio-oil [5]; (2) charring to produce biochar [6]; (3) combustion to release energy for electricity and heating; (4) pyrolysis to produce bio-oil, biochar, and gas [7,8]; (5) gasification to produce gas; and (6) baking to increase the calorific value and carbon content of biomass [9].

Pyrolysis is a thermochemical pathway for the conversion of biomass towards the production of charcoal, bio-oil, and syngas products [10,11,12]. Solid and gas products can be burned to provide energy for heat and power generation. For the liquid products, also known as bio-oil [13,14,15], its upgrading and utilization is a promising method for the production of clean fuels, valuable chemicals, and advanced materials [16,17]. Undoubtedly, a deep understanding of the biomass pyrolysis behaviors would significantly contribute to the control of the conversion process [18,19].

As a new pyrolysis technology, microwave pyrolysis has attracted wide attention [20] due to its characteristics of uniform heating, high heating rate, low energy consumption, and high energy efficiency [21]. As compared with the conventional (electrical) heating, microwave radiation will heat a substance uniformly instead of heating the outer surface first [22].

Wheat straw is known as one of the most available crop by-products used as a nutrient source for animal food and soil fertilizers [23]. Wheat straw is mainly composed of lignin (10–25%), cellulose (30–35%), and hemicellulose (15–25%) [24]. However, a considerable quantity of wheat straw is incinerated each year, releasing hazardous substances, such as carbon dioxide, polycyclic aromatic hydrocarbons, nitrogen oxides, and sulfur dioxide, to the atmosphere [25].

Up to now, many scholars are studying the microwave pyrolysis conditions and product characteristics of wheat straws [26]. Zhao et al. [27] studied the product distribution and generation mechanism of wheat straw based on microwave pyrolysis. Yemis et al. [28] studied the optimization of furfural and 5-hydroxymethylfurfural production from wheat straw by a microwave-assisted process. Many scholars have studied the microwave pyrolysis conditions and product characteristics of biomass [26]. As compared with the conventional energy analysis, the exergy analysis generally needs more for the related calculations. Therefore, the exergy transfer of the heating process of wheat straw particles during the microwave heating process was studied in this study. Basically, the heating process is the first step of all the thermochemical processes of biomass conversion, that is, drying, pyrolysis, gasification, and liquefaction; it is therefore of significant importance in the overall thermochemical process of biomass conversion. Therefore, the exergy transfer analysis of the initial heating process of biomass is significant. The objective of this study was to analyze the exergy transfer in the initial heating process of the microwave pyrolysis of wheat straw particles. The specific objectives were: (a) to investigate the effects of particle size, feeding load, and microwave power on the heating performances of wheat straw particles, and (b) to investigate the exergy transfer process of the heating process.

2. Materials and Methods

2.1. Material

The feedstock used in this study was wheat straw. Wheat straw was sourced from Lianyungang, China, and it was harvested in the summer of 2021. The wheat straw was ground into particles, and the sizes were 0.058, 0.106, and 0.270 mm. The wheat straw particles were stored in sealed containers at room temperature until they were used. The proximate analysis and ultimate analysis of wheat straw are shown in

Table 1. Proximate analysis and ultimate analysis of wheat straw.

Proximate Analysis (wt.%)				Ultimate Analysis (wt.%)
M	V	A	FC a	C	H	N	S	O a
11.00	56.87	17.14	14.99	36.47	5.93	1.39	0.18	56.03

^a Calculated by difference.

.

2.2. Experimental Setup

Figure 1 shows the schematic diagram and photo of the microwave heating experimental setup. The experimental setup is composed of four main parts: (a) a microwave heating system, (b) a cooling system, (c) a temperature measuring system, and (d) an auxiliary system. The microwave heating system is mainly composed of a microwave oven to supply energy and a quartz flask to load feedstocks. The microwave oven (0–1000 W, 2450 MHz) was manufactured by the Shanghai Longyu Microwave Equipment Company (Shanghai, China), and the furnace cavity was 300 mm in length, 300 mm in width, and 350 mm in height. The quartz flask was manufactured by the Donghua Quartz Products Company (Lianyungang, China), and the chamber volume was 150 mL. The cooling system mainly consisted of three glass tubes and a cooling tank filled with water for cooling, and the cooling system was used to cool the vapor produced during the heating process. The temperature measuring system was mainly composed of a K-type thermocouple and a digital thermometer, and the temperature measuring system could be used to measure the temperature of feedstocks. The auxiliary system mainly consisted of a vacuum pump to vacuum the whole system. The furnace cavity was filled with insulation cotton, which wrapped the quartz flask. Due to the poor microwave adsorption ability of the insulation cotton, the microwave or heat absorbed by insulation cotton could be ignored.

2.3. Experimental Procedures

To investigate the heating performances of wheat straw in the microwave chamber, some experimental procedures were followed: (1) The wheat straw particles were weighed and then put into the quartz flask. (2) The experimental system was connected, as shown in Figure 1; then the vacuum pump was turned on to vacuum the whole experimental system for 10 m. (3) The insulation cotton was filled in the microwave chamber, and the initial temperature was measured. (4) During the experiment, the vacuum pump was kept in working mode, and the temperatures were recorded every 30 s. (5) When the experiment was finished and the quartz flasks and quartz tubes were heated in the muffle furnace at 800 °C for 2 h to clean up the adhering wheat straw particles, all the glass tubes were cleaned for the next experiment.

Table 2. Experimental design.

Group	Microwave Power (W)	Particle Size (mm)	Feeding Load (g)
1	250	0.058	30
2	300	0.058	30
3	350	0.058	30
4	350	0.106	30
5	350	0.270	30
6	350	0.058	10
7	350	0.058	50

shows the experimental design, and the experiments were repeated three times.

2.4. Exergy Analysis

The specific heat capacity of wheat straw was calculated by using the equation proposed by Guo et al. [29],

$$C=a_1{e}^{-(\frac{T-b_1}{c_1})}+a_2{e}^{-(\frac{T-b_2}{c_2})}$$

(1)

where

C is the specific heat capacity of biomass, kJ/(kg·°C);

T is the temperature of biomass, °C;

a₁, a₂, b₁, b₂, c₁, and c₂ are the fitting parameters.

During the microwave heating process, the input electrical energy was converted into microwave energy first, and then it was converted into thermal energy of the feedstock to make the temperature of wheat straw particles increase. This formula was used to calculate the heat absorption of wheat straw during the microwave heating process:

$$Q={\displaystyle {\int }_{T_0}^T{c}_p}m\mathrm{d}T$$

(2)

where

Q is the heat absorption of biomass, kJ;

c_p is the specific heat capacity of biomass, kJ/(kg·°C);

m is the mass of biomass, kg;

T is the temperature of biomass, °C;

T₀ is the initial temperature of biomass, °C;

The biomass microwave heating process was studied in this study, and there was no chemical reaction. The chemical exergy of biomass did not change before and after the heating process. In order to simplify the calculation process, it was assumed that the reaction system was performed in a vacuum after evacuation.

The exergy input during the process includes: (a) microwave exergy, (b) biomass chemical exergy, (c) biomass thermal exergy, and (d) air exergy.

The exergy output during the process includes: (a) biomass chemical exergy, (b) biomass thermal exergy, and (c) air exergy. The biomass chemical exergy can be estimated using the equation summarized by Zhang et al. [30].

An exergy transfer model is simplified for the microwave heating process, and it is shown in Figure 2. The input microwave exergy is calculated through:

$$E{x}_{\mathrm{m}}=P\cdot t$$

(3)

where

Ex_m is microwave exergy, kJ;

P is microwave power, kW;

t is heating time, s.

Figure 2. Exergy transfer analysis of biomass and microwave.

Figure 2. Exergy transfer analysis of biomass and microwave.

The output biomass thermal exergy and the lost exergy can be calculated through:

$$E{x}_{\mathrm{bio}}={\displaystyle {\int }_1^2(1-\frac{T_0}{T})\delta Q}$$

(4)

$$E{x}_{\mathrm{lost}}=E{x}_{\mathrm{in}}-E{x}_{\mathrm{bio}}$$

(5)

where

Ex_bio is biomass thermal exergy, kJ;

Ex_lost is lost exergy, kJ.

Exergy analysis mainly focuses on the ratio of biomass exergy and microwave exergy during the microwave heating process, and the ratio is defined as the exergy transfer efficiency:

$${\eta }_{\mathrm{xt}}=\frac{E{x}_{\mathrm{bio}}}{E{x}_{\mathrm{m}}}\times 100\%$$

(6)

where

η_xt is the exergy transfer efficiency, %.

The weight loss of wheat straw particles is slight at temperatures below 250 °C [31]; therefore, when the wheat straw particles were heated to around 250 °C, the microwave oven was turned off and the experiment was stopped.

3. Results and Discussion

3.1. Effect of Microwave Power

3.1.1. Heating Performance

Figure 3 shows the heating performances of wheat straw at different microwave powers (250, 300, and 350 W), while the chamber volume (150 mL), feeding load (30 g), and particle size (0.058 mm) were fixed. The results show that with the increase in heating time for different particles, the temperature of wheat straw particles increased gradually, and this was because the wheat straw particles absorbed more microwave energy when there was a longer heating time [32].

When the microwave powers were 250, 300, and 350 W, the times required to increase the temperature to 252.2, 258.2, and 254.7 °C were 9.5, 7.5, and 4 min, respectively, and the average heating rates were 23.41, 30.55, and 56.18 °C/min, respectively.

With the increase in microwave power, the average heating rate of wheat straw particles gradually increased. This was because with the increase in microwave power, the energy density in the microwave field increased, and the wheat straw particles absorbed more energy [33].

3.1.2. Exergy Transfer

Figure 4 shows the exergy transfer flowcharts at different microwave powers. The chamber volume (150 mL), feeding load (30 g), and particle size (0.058 mm) were fixed. In Figure 4a, when the microwave power was 250 W, the biomass exergy was 1.57 kJ, accounting for 1.1% of the microwave exergy (142.5 kJ). In Figure 4b, when the microwave power was 300 W, the biomass exergy was 1.63 kJ, accounting for 1.21% of the microwave exergy (135 kJ). In Figure 4c, when the microwave power was 350 W, the biomass exergy was 1.59 kJ, accounting for 1.89% of the microwave exergy (84 kJ).

Figure 5 shows the percentages of biomass exergy and lost exergy at different microwave powers. When the microwave power increased from 250 to 350 W, the percentage of biomass exergy increased from 1.10% to 1.89%, whereas the percentage of lost exergy decreased from 98.90% to 98.11%. These were due to the fact that with the increase in microwave power, the time required to increase the temperature of wheat straw to around 250 °C gradually decreased, and the consumption of microwave energy gradually decreased.

3.2. Effect of Particle Size

3.2.1. Heating Performance

Figure 6 shows the heating performances of wheat straw at different particle sizes (0.058, 0.106, and 0.270 mm), while the chamber volume (150 mL), feeding load (30 g), and microwave power (350 W) were fixed. The results show that with the increase in heating time for different particles, the temperature of wheat straw particles increased gradually, and this was because the wheat straw particles absorbed more microwave energy when there was a longer heating time [32,33].

When the particle sizes were 0.058, 0.106, and 0.270 mm, the times required to increase the temperature to 254.7, 249.6, and 255.4 °C were 4, 8.5, and 11 min, respectively, and the average heating rates were 56.18, 25.87, and 20.59 °C/min, respectively.

The wheat straw particles with a particle size of 0.058 mm had the fastest heating rate, and the wheat straw particles with a particle size of 0.270 mm had the lowest heating rate. This was because with the increase in particle size, the stack density decreased [32,33]. The wheat straw particles with a particle size of 0.270 mm had the lowest stacking density, and the volume of the same mass and the largest heat dissipation area were the highest, and the heat lost was also the highest [32].

3.2.2. Exergy Transfer

Figure 7 shows the exergy transfer flowcharts at different particle sizes. The chamber volume (150 mL), feeding load (30 g), and microwave power (350 W) were fixed.

In Figure 7a, when the particle size was 0.058 mm, the biomass exergy was 1.59 kJ, accounting for 1.89% of the microwave exergy (84 kJ). In Figure 7b, when the particle size was 0.106 mm, the biomass exergy was 1.55 kJ, accounting for 0.87% of the microwave exergy (178.5 kJ). In Figure 7c, when the particle size was 0.270 mm, the biomass exergy was 1.62 kJ, accounting for 0.70% of the microwave exergy (231 kJ).

Figure 8 shows the percentages of biomass exergy and lost exergy at different particle sizes. When the particle size increased from 0.058 to 0.270 mm, the percentage of biomass exergy decreased from 1.89% to 0.70%, whereas the percentage of lost exergy increased from 98.11% to 99.30%. These were due to the fact that with the increase in particle size, the average heating rate decreased, and the time required to increase the temperature of wheat straw to around 250 °C gradually increased, and the consumption of microwave energy gradually increased.

3.3. Effect of Feeding Load

3.3.1. Heating Performance

Figure 9 shows the heating performances of wheat straw at different feeding loads (10, 30, and 50 g), while the particle size (0.058 mm), chamber volume (150 mL), and microwave power (350 W) were fixed. The results show that with the increase in heating time for different particles, the temperature of wheat straw particles gradually increased, and this was because the wheat straw particles absorbed more microwave energy when there was a longer heating time [32,33].

When the feeding loads were 10, 30, and 50 g, the times required to increase the temperature to 220.1, 254.7, and 251.8 °C were 32, 4, and 15.5 min, respectively, and the average heating rates were 5.96, 56.18, and 14.17 °C/min, respectively.

With the increase in feeding load, the average heating rate of wheat straw increased first and then decreased. This was because when the feeding load was less than or equal to 30 g, microwave energy could not be fully absorbed by the wheat straw particles, and with the increase in feeding load, the heat dissipation area of per unit mass decreased and the average heating rate gradually increased [32]. When the feeding load was greater than 30 g, the wheat straw particles fully absorbed the microwave energy, but the microwave energy was absorbed by per unit mass of wheat straw particles, which gradually decreased, and the average heating rate gradually decreased [33].

3.3.2. Exergy Transfer

Figure 10 shows the exergy transfer flowcharts at different feeding loads. The chamber volume (150 mL), particle size (0.058 mm), and microwave power (350 W) were fixed. In Figure 10a, when the feeding load was 10 g, the biomass exergy was 0.46 kJ, accounting for 0.07% of the microwave exergy (672 kJ). In Figure 10b, when the feeding load was 30 g, the biomass exergy was 1.59 kJ, accounting for 1.89% of the microwave exergy (84 kJ). In Figure 10c, when the feeding load was 50 g, the biomass exergy was 2.51 kJ, accounting for 0.77% of the microwave exergy (325.5 kJ).

Figure 11 shows the percentages of biomass exergy and lost exergy at different feeding loads. When the feeding load increased from 10 to 50 g, the percentage of biomass exergy increased from 0.07% to 1.89%, and then decreased to 0.77%, whereas the percentage of lost exergy first decreased from 99.93% to 98.11% and then increased to 99.23%. These were due to the fact that when the feeding load was less than or equal to 30 g, microwave energy could not be fully absorbed by the wheat straw particles, and with the increase in feeding load, the heat dissipation area of per unit mass decreased, and the average heating rate gradually increased [32]. When the feeding load was higher than 30 g, the wheat straw particles absorbed more of the microwave energy, but the microwave energy was absorbed by per unit mass of wheat straw particles, which gradually decreased, and the average heating rate gradually decreased [33].

4. Conclusions

The exergy transfer analysis of biomass and microwave during the experimental heating process was detailed. Some conclusions were obtained.

When the microwave powers were 250, 300, and 350 W, the times required to increase the temperatures of wheat straw to 252.2, 258.2, and 254.7 °C were 9.5, 7.5, and 4 min with average heating rates of 23.41, 30.55, and 56.18 °C/min and exergy transfer efficiencies of 1.10%, 1.21%, and 1.89%, respectively.

When the particle sizes were 0.058, 0.106, and 0.270 mm, the times required to increase the temperatures to 254.7, 249.6, and 255.4 °C were 4, 8.5, and 11 min with average heating rates of 56.18, 25.87, and 20.59 °C/min and exergy transfer efficiencies of 1.89%, 0.87%, and 0.70%, respectively.

When the feeding loads were 10, 30, and 50 g, the times required to increase the temperatures to 220.1, 254.7, and 251.8 °C were 32, 4, and 15.5 min with average heating rates of 5.96, 56.18, and 14.17 °C/min and exergy transfer efficiencies of 0.07%, 1.89%, and 0.77%, respectively.

The highest exergy transfer efficiency was obtained at a microwave power of 300 W, feeding load of 30 g, and particle size of 0.058 mm.

The heating processes and efficiencies can be adjusted by controlling the microwave powers, particle sizes, and feeding loads used. Additionally, biomass properties (i.e., biomass type, moisture content, compositions, etc.) may affect the heating processes and efficiencies. Detailed exergy analyses are also required.