Evaluation of the Effects of Using the Giant Miscanthus (Miscanthus × Giganteus) Biomass in Various Energy Conversion Processes

Abstract

The giant miscanthus (Miscanthus × giganteus) is one of the most essential energy plants. It also finds various alternative uses, including installing belts to prevent soil erosion. Biomass from such belts should be removed and rationally managed every year. The parameters of miscanthus biomass were investigated in terms of its suitability for combustion and anaerobic fermentation. Under the conditions of the experiment, miscanthus achieved a stable yield already in the second year of vegetation, mainly due to the high planting density. Energy parameters turned out to be typical for straw biomass (calorific value 18.06 MJ/kg). Relatively low ash melting temperatures (<1400 °C) and their chemical composition meant a high risk of contamination depositing on heating devices, which is often indicated as a shortcoming of biomass compared to hard coal. Miscanthus silage can be a valuable substrate for anaerobic digestion, but it requires a sufficiently early harvest, which affects the yield of biomass. The yield of energy in biomass obtained after drying plants was 163,623.6 MJ/ha. In contrast, the yield of energy from biomass collected in summer and processed into biomethane was much lower and amounted to 72,978.2 MJ/ha.

1. Introduction

Grasses from the Miscanthus family are important energy plants, mainly due to their high-yielding potential, durability, resistance to pathogens, and unfavorable environmental conditions [1]. The most outstanding prospects for disseminating energy crops are associated with the hybrid species of Miscanthus × giganteus Greef and Deuter, which was bred by crossing M. cacchariflorus and M. sinensis [2]. The plant’s advantages include a high-yield potential under various agroclimatic conditions [3], high heat of combustion [4], low ash content with a slight tendency to contaminate the surface of heating devices [5], spontaneous drying out after the end of vegetation [6], the possibility of harvesting with the use of typical agricultural machines, without the need to invest in specialized agrotechnical equipment [7].

The giant miscanthus has been indicated since the beginning of the 21st century as a species of great importance for biomass production for energy purposes [8,9,10]. Miscanthus biomass has been studied in terms of its use in various conversion processes into usable energy: combustion [11], pyrolysis [12,13,14], ethanol fermentation [15], and anaerobic digestion [16,17].

The biomass of grasses from the Miscanthus family, collected during the growing season, has different functional characteristics than those obtained after the above-ground parts have dried [6,18]. The differences include higher humidity, different yield structures [19], lower lignification expressed by the ratio/content of lignocellulosic fraction, and at the same time, higher digestibility [20]. Due to such proportions of components building biomass, it is much more helpful for biochemical conversion processes, especially methane fermentation [18,21,22,23]. It also proved rational to include miscanthus cultivation in the biogas plant operation system in terms of fertilization with digestate [24].

In addition to the energetic use of miscanthus, they also find a multidirectional alternative use. These grasses are planted in wetlands in wastewater treatment plants, where they collect pollutants and support the wastewater treatment process [25,26,27,28,29]. These plants purify sewage and remove pollutants (e.g., heavy metals) from the soil; hence, they are used in land reclamation and phytoremediation [30,31]. The ability of the bowl to collect pollutants from soil, water, and air cause these plants to fill protective strips around industrial plants and landfills [32]. The above-ground parts of the giant miscanthus are used to produce building materials [33,34]. Miscanthus straw is characterized by good absorbency, and therefore it is used as a litter for farm animals [35]. Dried giant miscanthus was tested for use as a feed additive for various animal species, both ruminants and monogastric animals [36]. Various species of the genus Miscanthus are characterized by ornamental value: they reach high height, have long fluffy inflorescences, some varieties have colorful leaves, and therefore, they are used as decorative plants in parks and gardens [10]. As can be seen from the above information, miscanthus is tested for suitability in various applications. Interactions of miscanthus with various elements of the environment were also assessed: soil [37], water deficiency caused by climate change [38], mycorrhizal fungi [39], and avifauna [40].

A new concept of alternative use of miscanthus is planting it in anti-erosion belts to protect soils against water erosion and pollution [41]. Soil degradation phenomena caused by water erosion are multidirectional and complex, but they always lead to unfavorable changes in the physical and chemical properties of soils and thus, to lower yield potential [42,43,44,45,46,47]. Various methods are used to protect soils against water erosion, including installing field erosion belts planted with trees and shrubs. An alternative to anti-erosion belts made of trees and shrubs can be belts planted with miscanthus. After planting, the plants remain in the form of large clumps for several years only in the planted area. In addition, the above-ground parts of this grass dry up in late autumn, and leaves fall only in winter, which avoids crop contamination problems in crops adjacent to the belts. At the same time, the giant miscanthus, in the first year of vegetation, can produce a significant mass of underground organs: roots and short, vigorous rhizomes, which can quickly strengthen the top layer of soil, preventing erosion. Miscanthus restores the above-ground mass every year, and therefore it is advisable to collect the above-ground parts in early spring before starting the vegetation. Biomass can also be collected from plantations, including anti-erosion belts, provided that it is managed rationally during the growing season. Miscanthus is characterized by exceptionally high possibilities of regrowth after harvest [48].

The research aims to assess the suitability of giant miscanthus, obtained from plantings in anti-erosion belts, to be used for energy purposes in methane combustion and fermentation.

2. Materials and Methods

2.1. Characteristics of the Field Experiment

The study site is located in southeastern Poland in Czesławice (51°18’10” N, 22°15’09” E) in the Lublin Upland, in the central part of the mesoregion—Nałęczów Plateau. The experiment was established in an loess, eroded production field with a southeastern exposure, in the upper part of the slope with a slope of about 15%. In May of 2018, across the slope, a 30 m² (15 × 2 m) anti-erosion belt was separated, on which a perennial grass—Miscanthus × giganteus—was planted in a 25 cm × 25 cm grid. No mineral fertilizers and pesticides were used. More information about the location of the experimental object, environmental conditions and the method of setting up the anti-erosion belt planted with giant miscanthus are described in the article [41].

2.2. Study of Biometric Features and Yield

During each growing season, the height of the giant miscanthus was measured, and after the end of vegetation, the thickness of the shoots, the number of shoots per seedling, and the root system’s depth were determined. Biometric measurements were carried out on 20 randomly selected plants. The arithmetic mean, median, standard deviation, minimum and maximum value were calculated for the obtained results. The yield of dried-up aerial parts in working condition (fresh mass—FM) was measured annually in early spring (mid-March), sampling from 3 randomly selected plots of 1 m² each. In the tests, the content of dry matter (DM) was determined by the gravimetric method at 105 °C. The yield structure (share of shoots and leaves) in FM and DM was also determined. The paper presents the average values of three measurements.

Apart from the dried biomass yield, the miscanthus yield during the growing season was determined once in the third year of vegetation. For this purpose, in the third week of July, samples were taken from 3 plots, each of which had an area of 1 m². The biomass was weighed, and the dry matter content was determined.

2.3. Study of Energy Parameters

After the end of the third growing season and drying of the above-ground parts of giant miscanthus, samples were taken to determine the parameters necessary for thermochemical processes. The scope of the research included: humidity, ash content, volatile matter, higher and lower heating value, C, H, N, S, and Cl content (

Table 1. Methods of studying the energy parameters.

Determination	Symbol	Norm	Method
Total moisture	Mar	PN-EN ISO 18134-2:2017-03	gravimetric
Ash	Ad	PN-EN ISO 18122:2016-01	gravimetric
Volatile matter	Vd	PN-EN ISO 18123:2016-01	gravimetric
Higher heating value	HHV	PN-EN 14918:2010	calorimetric
Lower heating value	LHV	PN-EN 14918:2010	computational
Carbon	C	PN-EN ISO 16948:2015-07	measurement with an automatic IR analyzer
Hydrogen	H
Nitrogen	N		catharometric
Total sulfur	S	PN-EN ISO 16994:2016-10	measurement with an automatic IR analyzer
Chlorine	Cl		ion chromatography (IC)

).

The dry biomass was comminuted using a knife chopper into 10–30 mm particles, for which the bulk density was determined. Based on the yield and the calorific value, the energy yield contained in the biomass per area unit (MJ/ha) was calculated, and considering the bulk mass and the calorific value, the energy density of the raw material (MJ/m³) was calculated.

2.4. Study of the Impact on Heating Devices

To assess the impact of biomass combustion on heating devices, ash melting points were determined (according to CEN/TS 15370-1:2007, microscopic—photographic method), and the chemical composition of the ash was analyzed (

Table 2. Methods of ash composition testing.

Component	Symbol	Norm/Method
Silica	SiO2	IB_TL_21_07 on 28.05.2013 The analysis was performed with a plasma spectrometer Thermo iCAP 6500 Duo ICP
Iron	Fe2O3
Aluminum	Al2O3
Manganese	Mn3O4
Titanium	TiO2
Calcium	CaO
Magnesium	MgO
Sulfur	SO3
Phosphorus	P2O5
Sodium	Na2O
Potassium	K2O
Barium	BaO
Strontium	SrO
Chlorides	Cl	PN-EN 196-2:2013-11 titration method
Carbonates	CO2	PN-EN 15936:2013-02

).

The analyses were carried out in 3 replications (expanded uncertainty u for the coefficient k = 2 and the confidence level of 95%). The ash research results were used to evaluate the influence of miscanthus biomass combustion on heating devices. The following indicators were used [49,50,51,52]:

• Indicator c_m:

$${\mathrm{c}}_{\mathrm{m}}=\frac{{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{MgO}+{\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}+{\mathrm{P}}_2{\mathrm{O}}_5}{{\mathrm{SiO}}_2+{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{Ti}}_2\mathrm{O}}$$

(1)

c_m—the ratio of basic to acidic oxides [50].

2.

Base-acid ratio B/A:

$$\mathrm{B}/\mathrm{A}=\frac{{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{MgO}+{\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}}{{\mathrm{SiO}}_2+{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{Ti}}_2\mathrm{O}}$$

(2)

It is the ratio of base to acidic oxides. If B/A < 0.4 or B/A > 0.7, the tendency to fouling is low, if 0.7 > B/A > 0.4—the tendency to fouling is high [49].

3.

Slagging factor R_S:

$${\mathrm{R}}_{\mathrm{S}}=\left(\mathrm{B}/\mathrm{A}\right)\cdot {\mathrm{S}}^{\mathrm{d}}$$

(3)

where: B/A—base-acid ratio, S^d—sulfur content in the fuel (% DM).

If R_S < 0.6—tendency to impurity formation is low, if 0.6 < R_S < 2.0—medium, if 2.0 < R_S < 2.6—high, if R_S > 2.6—very high [49,52].

4.

Simplified indicator R_(b/a):

$${\mathrm{R}}_{\left(\mathrm{b}/\mathrm{a}\right)}=\frac{{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{MgO}}{{\mathrm{SiO}}_2}$$

(4)

The lower the R_(b/a) index value, the higher the melting and flow point of the ash, and thus the lower the risk of slagging. At R_(b/a) < 0.15 these temperatures exceed 1600 °C [50].

5.

Silica percentage S_R:

$${\mathrm{S}}_{\mathrm{R}}=\frac{{\mathrm{SiO}}_2· 100}{{\mathrm{SiO}}_2+{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{MgO}}$$

(5)

A high value of the S_R index means a high slag viscosity and thus a low slag tendency. If S_R > 72—the slag shows a low slag tendency, if 72 ≥ S_R > 65—medium, if S_R ≤ 65—low [50]. If the S_R is 72–80, the propensity to pollute is low, if the S_R is 65–73—medium, and if it is 50–65—very high [49].

6.

Indicator F_u:

$${\mathrm{F}}_{\mathrm{u}}=\left({\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}\right)\cdot {\mathrm{c}}_{\mathrm{m}}$$

(6)

The F_u index determines the fuel’s propensity to the formation and subsequent sintering of contaminants on the heated surfaces and the tendency to form slag initiating deposits. If F_u ≤ 0.6, the fuel does not tend to form contamination, if 0.6 < F_u ≤ 40—has a high tendency, and if F_u > 40—a very high tendency to form and sinter contamination [50].

7.

Iron calcium ratio:

$$\mathrm{IC}=\frac{{\mathrm{Fe}}_2{\mathrm{O}}_3}{\mathrm{CaO}}$$

(7)

If IC < 0.3 or IC > 3.0, the propensity to pollutant formation is low, if 0.3 < IC < 3.0—high [49].

8.

Iron plus calcium:

I + C = Fe₂O₃ + CaO

(8)

If the sum of Fe₂O₃ + CaO oxides does not exceed 10%, the fuel has a low propensity to pollute [49].

9.

Alkali index:

$$\mathrm{AI}=\frac{\left({\mathrm{K}}_2\mathrm{O}+{\mathrm{Na}}_2\mathrm{O}\right)\cdot \mathrm{A}}{{\mathrm{LHV}}_{\mathrm{d}}}$$

(9)

where: A—ash content determined at 550 °C (%), LHV_d—lower heating value of dry mass (MJ/kg).

If the AI > 0.17, the formation of pollutants is likely; if the AI > 0.34, then pollutants are formed [52].

10.

Sintering index:

$$\mathrm{SI}=\frac{\mathrm{CaO}+\mathrm{MgO}}{{\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}}$$

(10)

If SI > 2, the slagging risk is low; if SI < 2—high [52].

11.

Bed agglomeration index:

$$\mathrm{BAI}=\frac{{\mathrm{Fe}}_2{\mathrm{O}}_3}{{\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}}$$

(11)

It concerns the possibility of sediment formation on fluidized beds. If BAI < 0.15, agglomerates are formed [52].

12.

Slagging index:

$$\mathrm{T}=\frac{\mathrm{HT}+4\cdot \mathrm{DT}}{5}$$

(12)

where: HT—hemisphere temperature (°C), DT—deformation temperature (°C).

The slagging risk is low if T > 1340, medium when T is 1230–1340, high when T is 1150–1230, and T < 1150 is a substantial slag risk [49].

2.5. Biogas Efficiency Study

During the third year of giant miscanthus vegetation (in the third week of July), samples of the above-ground parts in the vegetative stage of development (before the formation of inflorescences) were taken and crushed to the form of 10–30 mm long chaff with a chopper. In a 3-L glass jar, the chaff was kneaded to remove the air. Then, a tightly closed jar was placed in a dark, cool room, where the biomass was ensiled. After six weeks, tests on the suitability of silage for use in a biogas plant at the Department of Biosystems Engineering at the Poznań University of Life Sciences were carried out. The tests were performed in 3 repetitions according to DIN 38414. The tests included: dry matter and organic dry matter content in silage, pH, conductivity, biogas and biomethane efficiency as a function of time, and evaluation of biomethane content in biogas.

3. Results

3.1. Biometric Parameters of a Giant Miscanthus

Miscanthus seedlings, planted at a distance of 25 × 25 cm, developed adequately. When establishing the anti-erosion belt, most seedlings produced single shoots, 30% produced two each, and 10%—3 shoots each. Plants produced only short stalks, and the average height measured from the base of the stem to the end of the longest leaf, assessed after the end of the first growing season (in October), was 98.4 cm (

Table 3. Biometric parameters of a giant miscanthus.

Specification	Tested Parameter
	Height [cm]			Number of Shoots [pcs.]			Shoot Thickness [mm]			Root Length [cm]
	Year			Year			Year			Year
	I	II	III	I	II	III	I	II	III	I	II	III
Mean	98.4	202.7	264.2	1.5	7.3	7.8	4.3	5.1	8.6	16.9	20.1	30.2
Minimum	75	184	187	1	1	3	2	3	6.5	12	13	15.0
Maximum	128	226	312	3	12	12	6.5	6.5	10.7	23	27	39.0
Standard deviation	17.0	11.6	48.5	0.7	2.8	2.5	1.2	0.9	1.2	3.4	4.1	6.9
Median	100.0	201.0	291.0	1.0	7.5	7.5	4.4	5.3	8.8	16.5	19.5	29.0
Variation coefficient	0.17	0.06	0.18	0.46	0.39	0.33	0.28	0.18	0.15	0.20	0.20	0.23

). The average thickness of the shoots was 4.3 mm. Plants developed root systems to a maximum depth of 23 cm, and their average length was 16.5 cm.

In the second year after planting, the plants sprouted up to 12 shoots per plant (Table 3), while in the third year, the maximum value did not change, but no single-shoot plants were found. In the second year of vegetation, the plants developed tall (202.7 cm) and thick (5.1 mm) stems, which are a characteristic feature of this species. The root systems also penetrated the soil deeper, reaching 20.1 cm. In the third year of vegetation, the plants reached the biometric parameters typical for this species: the height of some shoots exceeded 3 m, their average thickness was 8.6 mm, and the roots reached an average of 30.2 cm deep into the soil.

3.2. Biomass Yield

In the first year, the average yield of miscanthus biomass, determined as received, did not exceed 400 g/m², which per 1 ha gave less than 4 Mg FM. In the second and third years of growth, the harvest was similar and amounted to 11.62 and 12.01 Mg FM/ha, respectively. Miscanthus loses a significant part of water during the winter and naturally achieves a high dry matter content, which in the first, second, and third years was 69.38%, 79.30%, and 75.50%, respectively. Based on the FM yield and its moisture, the DM yield in the following years was calculated at 2.35, 9.39, and 9.06 Mg DM/ha, respectively.

The evaluation of the yield components made it possible to establish that the leaves constituted the dominant part of miscanthus in the first year of vegetation, the percentage of which in FM and DM amounted to 64.45 and 72.41%, respectively. In the second and third years, these proportions changed as the plants developed tall, thick shoots that dominated the yield structure of both FM and DM (Figure 1).

Miscanthus biomass yield determined during the third year of vegetation (July) was 3470 g FM/m², with a moisture content of 80.4%. Calculated per field unit, it yielded 34.70 Mg FM/ha and 6.80 Mg DM/ha.

The DM yield obtained in summer was almost 25% lower than that collected after the plants had dried because in the summer, the shoots were further elongated, and new leaves were formed. However, to obtain a substrate suitable for anaerobic fermentation, it is not advisable to delay harvesting, as plants become woody as they grow, and their biomass becomes less digestible for fermentation bacteria.

3.3. Energy Parameters

Elemental analysis of miscanthus biomass showed that the share of carbon was 48.96% DM, and hydrogen 5.98% DM (

Table 4. Giant miscanthus biomass parameters important from the energetic point of view.

Parameter	Status *	Unit	Value	Extended Uncertainty u
Mar	r	%	8.8	0.2
Ad	d	%	2.76	0.10
Vd	d	%	82.0	3.8
HHV	d	J/g	19,290	700
	r	J/g	17,590	830
LHV	d	J/g	18,060	700
	r	J/g	16,260	860
C	d	%	48.96	1.40
H	d	%	5.98	0.27
N	d	%	0.20	0.01
S	d	%	0.03	0.00
Cl	d	%	0.045	0.008

*—r—as received; d—dry.

). These elements determine the heating value of the fuel, which was LHV 18.06 MJ/kg DM. Low carbonization of the examined biomass resulted in a high content of volatile matter (82.0% DM), which should be considered when composing mixtures for co-combustion. The content of elements unfavorable from the point of view of environmental pollution (nitrogen and sulfur) was not high and amounted to N—0.20% DM and S—0.03% DM. Miscanthus also contained a small amount of chlorine (0.045% DM), which often causes chloride corrosion in boilers used to burn biomass.

A parameter indirectly related to the energy use of biomass is its bulk mass (volumetric density), as it determines the size of rooms or tanks for storage and means of transport. The density of chaff from giant miscanthus in the working condition was 58.3 kg/m³. Considering the bulk mass and calorific value, it was calculated that the energy density of miscanthus biomass was 947.96 MJ/m³.

3.4. Ash Properties

The ash content in the giant miscanthus biomass was 2.76% of dry mass. Acid oxides predominated in the ash composition: silica (SiO₂) and aluminum oxide (Al₂O₃), while the basic oxides of calcium (CaO), potassium (K₂O), magnesium (MgO), and iron (Fe₂O₃) were in the minority (

Table 5. Composition of ash from the combustion of a giant miscanthus biomass.

Component	Content [% DM]	Extended Uncertainty u
SiO2	61.3	3.3
Fe2O3	0.48	0.04
Al2O3	1.25	0.11
Mn3O4	0.39	0.04
TiO2	0.33	0.04
CaO	8.27	0.71
MgO	2.49	0.26
SO3	2.02	0.26
P2O5	8.16	0.53
Na2O	0.44	0.08
K2O	12.50	1.36
BaO	0.04	0.01
SrO	0.04	0.01
Cl	1.22	0.28
CO2	0.13	0.02
Sum	99.06	3.73

). The results of the ash composition analysis were used to assess the impact of the biomass burned on the condition of heating devices [18].

The melting temperatures of miscanthus biomass ash < 1500 °C (

Table 6. Ash melting points (reducing/oxidizing atmosphere).

Parameter	Unit	Value
Shrinkage temperature, SST	°C	750
Deformation temperature, DT	°C	900
Hemisphere temperature, HT	°C	1300
Flow temperature, FT	°C	1370

) indicate that the ash tends to settle on the heating surfaces of the boilers, and as a result, the heat exchange may be disturbed. Also the subsequent temperatures ranges of state ash change (shrinkage, deformation, hemisphere, and flow) were relatively low.

The values of indicators confirm it for assessing ash’s tendency to contaminate the surfaces of heating devices (

Table 7. Indicators of the assessment of the risk of contamination of heating surfaces of boilers.

Indicator	Pattern No.	Value	Deposition Hazard
cm	1	0.51	—
B/A	2	0.38	low
RS	3	12.82	severe
R(b/a)	4	0.18	severe
SR	5	84.51	low
Fu	6	6.66	high
IC	7	0.06	low
I + C	8	8.75	low
AI	9	1.98	severe
SI	10	0.83	high
BAI	11	0.04	severe
T	12	980	severe

). The high probability of sediment formation on the fluidized beds was also determined. Deposits and impurities on the heat exchange devices hinder this exchange, which reduces the efficiency of the installation. They can also cause corrosion and erosion phenomena in heating boilers.

3.5. Biogas Efficiency

Miscanthus biomass, collected in the vegetative phase, hardened correctly and observed no mold or abnormal changes. The silage reaction was 5.12, its conductivity was 1.54, the dry matter content was 18.3%, and the dry organic matter content (ODM) was 91.5% DM.

Biogas yield from giant miscanthus silage was 547.0 m³/Mg ODM (

Table 8. Biogas yield from giant miscanthus silage.

No.	Fresh Mass—Efficiency		Dry Matter—Efficiency		Organic Dry Matter—Efficiency		Methane Content [%]
	Accumulated Methane [m3/Mg FM]	Accumulated Biogas [m3/Mg FM]	Accumulated Methane [m3/Mg DM]	Accumulated Biogas [m3/Mg DM]	Accumulated Methane [m3/Mg ODM]	Accumulated Biogas [m3/Mg ODM]
1.	51.75	90.51	282.86	494.72	309.15	540.70	57.18
2.	51.54	92.61	281.71	506.30	307.89	553.23	55.65
3.	52.95	91.58	289.44	500.54	316.34	547.06	57.82
Mean	52.08	91.56	284.67	500.52	311.13	547.00	56.88

). The biomethane content in biogas was high (56.88%), which, combined with the high yield of biogas, resulted in a significant yield of biomethane per mass unit. Converted to a field area unit, with the yield of the fresh mass of 34.7 Mg/ha, the yield of biogas and biomethane amounted to 3177.13 and 1807.18 m³/ha, respectively.

Figure 2 and Figure 3 show the course of the fermentation process and the time when the process was most effective: HRT (Hydraulic Retention Time) was 26 days from the start of methane fermentation. Although the substrate was kept in the reactor up to 59 days from the initiation of the process, the increase in the amount of biogas from the 27th day was negligible.

3.6. Energy Yield

The conversion of the chemical energy contained in the giant miscanthus biomass into usable energy can be carried out thermochemically (incineration) or biochemically (anaerobic fermentation). These processes require different handling of biomass during the growing season: to obtain a substrate for biogas production, plants should be harvested in the vegetative phase, and the best combustion parameters have dried biomass harvested even a few months after the end of vegetation (in Poland, about 3–4 months after the plant development processes have ceased).

Considering the DM yield per 9.06 Mg/ha and its LHV of 18.06 MJ/kg (Table 4), it was calculated that by harvesting the biomass after drying of the plants, 163,623.6 MJ could be obtained from 1 ha of the field. On the other hand, the harvest of green biomass in the amount of 6.80 Mg DM/ha allows for the production of 1935.76 m³ of biomethane (Table 8). Assuming the calorific value of methane at 37.7 MJ/m³, it was calculated that the energy yield, in this case, was 72,978.2 MJ/ha. The yield of energy obtained by fermentation of miscanthus biomass turned out to be 55.4% lower than the energy contained in the dry biomass burned. It should be considered that the above calculations do not consider the efficiency of power devices, which may affect the results obtained in natural conditions.

4. Discussion

The vegetation of giant miscanthus plants was normal in the conditions of Central and Eastern Europe. The five-fold increase in the number of shoots produced by a single plant, noted between the first and second years of vegetation, in the third year was only 6.8%. The number of shoots produced by miscanthus can amount to several dozen heads/plant, and this value depends mainly on their density [53]. The planting density (25 × 25 cm) used in the present study was not typical for miscanthus plantings for energy purposes, whereas in Poland, the density of 1–8 plants/m² is used [48]. However, the experiment aimed to protect the soil from erosion and create a compact, turfed strip. The results of our own research confirms the ability of this species to self-regulate the number of shoots, as written by other authors [48,54,55]. Other biometric parameters: shoot height and thickness as well as root length in the third year of vegetation reached parameters similar to those reported in the literature [2,9].

Grasses from the Miscanthus family are perennial plants that can be used for several years [56]. In the first year after planting, the seedlings are adopted, and the yield obtained is negligible, as found in the authors’ research. Yu et al. [6] also found a low yield of various Miscanthus genotypes in the first year of cultivation. The optimal yield is most often obtained in the third year of plantation and ranges from a dozen to over 20 Mg/ha [48,57,58]. In our research, the yield of this order (11.62 Mg/ha FM) was obtained already in the second year, which could have resulted from the plant density used. In the third year, the yield did not increase significantly.

Like that of other species intended for combustion, the high proportion of dry matter in the grass biomass is a favorable feature [1,6,11]. The plant can naturally get rid of a significant amount of water thanks to drying tissues, draining water, and transferring nutrients to the underground parts. Dry biomass does not require significant expenses for drying, which reduces the costs of its production [10,48]. Humidity of giant miscanthus biomass, collected in early spring, below 25%, allows it to be compacted or stored without additional drying. Due to the low humidity, the calorific value of the biomass in the working condition was relatively high (16.26 MJ/kg), and in the dry condition it was 18.06 MJ/kg. These values are lower compared to wood biomass, and comparable with other perennial species, such as Virginia mallow or Jerusalem artichoke [8,10,52,59].

The results of our own research confirmed the risk of contamination of heating devices during biomass combustion. The giant miscanthus ash melting points were comparable with other types of biomass [51,52,60] and lower than that of hard coal. Although the ash content in the biomass was low (2.76% DM), its composition may cause deposition of pollutants, slagging and sintering in boilers, which limits heat exchange and reduces the efficiency of power equipment. This is confirmed by the results of calculations of the pollution risk indicators for the flat boilers [5,52,61]. However, ashes from miscanthus combustion can be used as biochar to improve soil properties [62].

The yield of biogas from miscanthus silage was a half lower than that obtained from the most commonly used substrate used in agricultural biogas plants—maize silage. As can be seen from various sources [63,64], the methane yield of maize ranges from 105 to 125 m³/Mg FM, depending on the variety, harvest phase, silage quality, etc. [65,66,67]. An argument favoring miscanthus compared to maize is its durability: as a perennial culture, it does not require annual plantings, and after reaching the third year of vegetation, it does not require care. Literature data on miscanthus fertilization indicate that it can be cultivated with a limited supply of mineral fertilizers [24,68,69].

5. Conclusions

The giant miscanthus biomass, which is created as an added value in protecting soils against erosion, can be used for energy purposes in thermochemical and biochemical conversion processes. The specificity of the anti-erosion belt prompted the use of high plant density (16 seedlings/m²), which contributed to the self-regulation of the number of shoots already in the third year of vegetation.

Due to the natural ability of grasses to dry out and lose moisture, a typical miscanthus harvest time in Poland is the end of winter and the beginning of spring. The evaluation of the energy parameters of the Miscanthus giganteus biomass confirmed the validity of such a procedure, as the plants were characterized by low humidity, maintaining a high calorific value of 18.06 MJ/Mg DM. Like other types of biomass, miscanthus tends to pollute heating devices, which was confirmed by calculating most pollution risk indicators.

An alternative way to convert the energy contained in miscanthus biomass is anaerobic digestion. It requires harvesting during the vegetation period when the plants have not reached total yield, negatively affecting biomass yield. Although the miscanthus chaff had cured correctly, it obtained a much lower biogas yield per mass unit. Comparing the energy yield obtained in thermochemical and biochemical processes speaks in favor of combustion.

In this experiment, miscanthus biomass was sourced from a belt whose primary function was to prevent erosion. Therefore, it should be considered whether the harvest of miscanthus in the vegetative phase intended for biogas production will not limit its anti-erosion function. It requires research to assess the effects of summer harvesting on plant development, durability, yield, and soil protective effect.