A Waste-to-Energy Technical Approach: Syngas–Biodiesel Blend for Power Generation

Abstract

In this study, a technical analysis of synthesis gas (syngas) and biodiesel blend utilized in an internal combustion engine is presented. The experimental setup is composed of an engine workbench coupled with a downdraft gasifier which was fed with forest biomass and municipal solid waste at a blending ratio of 85:15, respectively. This research paper aims to contribute to the understanding of using fuel blends composed of synthesis gas and biodiesel, both obtained from residues produced in a municipality, since the waste-to-energy approach has been trending globally due to increasing waste generation allied with rising energy demand. The experiments’ controlling parameters regarding the engine are rotation and torque, exhaust gas temperature, and fuel consumption. The gasification parameters such as the oxidation and reduction temperatures, pressures at the filter, hood, and reactor, and the volume of tars and chars produced during the thermochemical process are also presented. Ultimate and proximate analyses of raw materials and fuels were performed, as well as the chromatography of produced syngas. The syngas produced from forest biomass and MSW co-gasification at a blending ratio in mass of 85:15 presented an LHV of around 6 MJ/m³ and 15% of H₂ in volume. From the experiment using syngas and biodiesel blend in the engine, it is concluded that the specific consumption at lower loads was reduced by 20% when compared to the consumption of the same engine operating with regular diesel. The development of co-gasification of forest and municipal waste may then be an interesting technology for electrical energy decentralized generation.

1. Introduction and Literature Review

A practical and laboratorial methodology was applied for integrating the waste-to-energy (WtE) approach developed in this paper, that is, the use of municipal solid waste (MSW) for power generation through the association of a gasification unit and an internal combustion engine (ICE). The co-gasification of forest biomass and common materials found in MSW such as plastic and cardboard, which were processed and transformed into pellets, thus becoming a refuse-derived fuel (RDF), was technically analyzed in terms of its feedstock characterization, operational parameters, and produced syngas. The engine operated with biodiesel produced from waste cooking oil (WCO) and was associated with the gasification unit.

This work is structured as follows: Section 1 briefly reviews the state-of-the-art from the related topics of this research, that is, waste collection and management status, syngas and biodiesel past utilizations and prospects for the future, use of biofuels in ICE, data of raw materials proximate and ultimate analysis, reference values for hydrogen percentage in volume of syngas from other materials, and, conclusively, the general advances in WtE in regions such as Europe and Latin America. Section 2 provides a methodology explanation about the gasification unit and the engine bench setup, and the calculation involved in the thermochemical process. Section 3 presents the results obtained from the practical experience aligned with theoretical calculations. Section 4 provides a discussion about some difficulties faced, perspectives and possible future studies, and the conclusion from the authors of this work.

Studies have been carried out regarding the use of biofuels in internal combustion engines since there is a common objective worldwide for reducing fossil-derived fuels and thus stimulating alternative solutions for lessening pollutant emissions. That is, a direct consequence of global warming resulting mainly from greenhouse gas emissions, such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and fluorinated gases (F-gases) [1,2]. Some authors have contributed to the technical and thermodynamic aspects of biofuels when they are used in unmodified engines [3]. Some of the main challenges found in recent studies are related to techno-economic aspects [4] (cost of fuel production, cost of electricity production, cost of investment and maintenance), energy efficiency and quality of fuels, environmental aspects (pollutant emissions, use of land, waste management) [5], and social aspects mainly those related to cities in which rapid development has been experienced [6,7].

A topic that is globally trending is waste-to-energy (WtE) as an alternative form of electricity supply. The WtE discussion is enriched by several studies that have proven that the proper management of residues may result in useful heat, power, or even as fuels that can be stored and thus commercialized in the energy market [8], which is also closely related to agriculture sector [9] and other industries. The production and use of residue-derived fuels (RDFs) have been a major discussion in the scientific community for representing one of the necessary steps when it comes to the waste valorization [10,11,12]. Among several sequential steps related to WtE, the collection of waste, and its management from disposal, is one of the most important and sometimes difficult mainly because it deals with public administration, community awareness, and governmental policies and investments mainly in fast-growing cities [13].

Several discussions are being made about ways to stop global warming. This climatic urgency has been at the highlight of government proposals around the globe, being the United Nations (UN) the organization responsible for developing the 17 sustainable development goals (SDGs), which is an urgent call for action by all countries—developed and developing—in a global partnership [14]. Several studies correlate factors regarding MSW management such as gross domestic product (GDP), sustainability indicators, accounting behavior, and governmental strategies and policies. In a recent study, WtE technologies, such as gasification, incineration, and composting, among others, were strictly reviewed and analyzed in terms of technical and operational parameters, products obtained, and their advantages [8]. Figure 1 resumes some of the 17 SDGs that are related to the topic of WtE.

Regarding the collection of waste, several studies have shown the importance of logistic planning as well as effective selective collection [15,16]. Some materials found in municipal waste can either be recycled or sorted for energy recovery to be utilized in RDF or biofuels, as in the case of biodiesel produced from WCO [17]. Figure 2 shows a flowchart of waste management from collection to final energy use.

The material collected worldwide will vary according to several conditions such as income level, for instance, and policies for waste management. Countries that belong to the Organization for Economic Co-operation and Development (OECD) are responsible for around 35% of the world’s waste, and projections indicate the world’s waste production will reach 3.4 billion tons of waste by 2050 [8].

At a global level, the type of waste collected can vary according to each region. The composition of global waste is presented in

Table 1. Waste compositions sorted by world regions (adapted from [8]).

Material	East Asia and Pacific	Europe and Central Asia	Latin America and the Caribbean	Middle East and North Africa	North America	South Asia	Sub-Saharan Africa
Materials suitable for waste-to-energy
Food and green	53%	36%	52%	58%	28%	57%	43%
Paper and cardboard	15%	18.6%	13%	13%	28%	10%	10%
Plastic	12%	11.5%	12%	12%	12%	8%	8.6%
Rubber and leather	1%	1%	1%	2%	9%	2%	-
Wood	2%	1.6%	1%	1%	5.6%	1%	1%
Materials suitable for recycling/non-energetic recovery
Other	12%	21%	15%	8%	3.6%	15%	30%
Glass	2.6%	8%	4%	3%	4.5%	4%	3%
Metal	3%	3%	3%	3%	9.3%	3%	5%

.

An alternative way of waste treatment is the valorization of its energy content. Among several WtE technologies available, gasification technology is considered feasible for forest biomass energy recovery in higher-scale projects with low payback periods [19]. Meanwhile, to seek a positive net present value (NPV), the cost of biomass must be around 20 €/ton [20]. Gasification is an interesting process to transform waste into energy. Some materials such as cardboard, plastic, and rubber, which are common waste materials, can then be transformed into a gas which can be used as fuel in engines for transportation, for instance, or in rural areas where heavier machinery such as tractors, trucks, and others are fueled with diesel. The use of fuel blends between diesel and a biofuel such as syngas or biodiesel may be responsible for lowering fuel consumption and consequently reducing costs associated with power generation. That is, a decentralized power plant sized according to local energy needs and waste disposal could be an advantage, providing diversification of the energy matrix, promoting jobs, and, in an agricultural environment where heavy machinery is used, the gas produced could be used to reduce the direct consumption of diesel. Regarding biodiesel production, there are mainly three stages for its processing: preparation of raw materials, extraction of biodiesel oil (transesterification process), and refining of the produced biodiesel [21].

There are other technologies that we can apply in WtE, such as combustion of agriculture byproducts [22], heat recovery from wastewater from livestock [23], and biogas production from farms [24].

As can be observed from Figure 1, laboratory characterization is a necessary step for a successful WtE concept. The physical and chemical properties of several materials found in MSW, including WCO, can be analyzed through elemental analysis, Fourier transform infrared spectroscopy, and gas chromatography-mass spectrometry. Firstly, the proximate analysis provides moisture, volatile matter, fixed carbon, and ashes content on a wet basis, as shown in Figure 3, and the percentage of each content will determine the higher heating value (HHV) of each material or the quantity of tars and chars it will produce. For instance, materials with higher volatile matter content will produce more tars and chars than others, while materials with higher fixed carbon content will present higher HHV [18].

Alongside proximate analysis, there is the ultimate analysis of raw materials, a process used to obtain the percentage of carbon, hydrogen, nitrogen, sulfur, and oxygen in the solid raw material. Consequently, it is possible to obtain the HHV of each type of biomass or waste being analyzed. Figure 4 exposes the ultimate analysis of several feedstock materials in which a direct relation between the carbon content and the HHV is observed [18].

All considered feedstock materials, which are RDF, forest biomass, plastic, wood, rubber, and seeds, among others, have been utilized in a fixed-bed downdraft gasifier. This type of gasification unit reaches high temperatures for the production of synthesis gas, or syngas, through the processes of pyrolysis, combustion and cracking, and reduction [18], which is filtered in a cyclone and gas filters before being injected into the ICE. Syngas composition may vary according to the material used as feedstock during the gasification process, or co-gasification when utilizing two feedstock materials [25,26]. A typical syngas composition should have around 10% in volume of hydrogen gas (H₂) [18], while presenting an HHV of around 5 MJ/m³ as given in

Table 2. Syngas compositions from different feedstock materials [18].

Gasification Feedstock (Blending Ratio)	H2 (%)	CO (%)	CO2 (%)	CH4 (%)	N2 (%)	LHV (MJ/m3)
Eucalyptus chips	12.9	20	10.7	1.6	55	5.1
EC + RDF (70:30)	12.4	17.8	10.8	3.1	56	5.4
Olive seeds + WP (90:10)	12.7	17.8	9.8	2.7	57	5.42
Miscanthus + WT (80:20)	6.0	10.6	9.7	3.8	55.7	3.64
Miscanthus + P&R (80:20)	5.9	8.3	11.1	3.7	52.5	3.09

.

The gasification process results in the production of chars and tars which are, respectively, the solid and liquid fractions of heavy hydrocarbons after incomplete combustion. Tars, defined as a mixture of organic compounds in the product stream that are condensable in the gasifier or in downstream processing steps [27], can severely damage engines causing blockage and consequently “gluing” the engine elements. However, the utilization of syngas as a complementary fuel in blends with diesel and biodiesel may lead to a reduction in particulate matter (PM) and NOx emissions in compression ignition engines while increasing brake thermal efficiency (BTE) [28]. Additionally, studies have shown that the main challenges of syngas utilization on dual-fuel engines show the possibility of achieving high diesel savings and efficiencies with low emissions [29]; furthermore, the use of syngas as a complementary fuel will increase the indicated power of the engine [30].

Hence, the alternative may cause an increase in efficiency while providing proper disposal of some waste material, not only the solid ones. Wastewater and organic waste have also been studied as major sources of biohydrogen, which can be achieved through other technologies, i.e., biological fermentation and microbial electrolysis cell [31]. The objective of this research paper is aimed at the use of a syngas–biodiesel fuel blend in an internal combustion engine for power generation. The syngas produced originated from the co-gasification of RDF produced from a mixture of MSW and forest biomass, and the biodiesel was produced throughout the transesterification process of waste cooking oil.

2. Methodology

Two experiments were performed separately:

• ICE utilizing different fuel blends between diesel and biodiesel.
• ICE operating with liquid–gas blend (biodiesel–syngas).

The engine bench test had fixed parameters such as the torque and the rotation. The equipment is composed of a 2 strokes diesel cycle engine with a power range from 2.5 to 7.5 kW, maximum torque of 12 N·m, and maximum speed of 6000 rev/min. The bench had a fixed volume of fuel for each test run with the utilization of a graduated volumetric pipette, which was fixed to 16 mL for all tests. For obtaining the fuel consumption of each fuel configuration, the time was controlled using a chronometer. The fuel configurations for each test are displayed in

Table 3. Fuel configurations in the engine bench test.

Fuel Configuration	Description
Diesel only	Market available fuel with 7% of biodiesel addition according to local legislation
Bio + diesel (50:50)	Fuel blend made of market diesel and laboratory-produced biodiesel
Biodiesel only	Laboratory-produced fuel from WCO through transesterification process
Biodiesel + syngas	Liquid–gas fuel blend through the admission of syngas in the air inlet of the ICE

.

The co-gasification experiments were performed in a fixed-bed gasification unit which was fed with a mixture of forest biomass pellets and material found in MSW, namely, cardboard and plastic from packaging and bags. The proportion used for the co-gasification of the pellet–MSW blend was 85:15, that is, 85% by mass of the mixture consisted of forest biomass pellets while 15% of it was shredded plastic and cardboard from MSW. The reason for choosing this pellet–MSW mass ratio is based on the experience gained from previous gasification tests. It was observed that a higher level of uniformity in the geometric characteristics of the solid feedstock material significantly improves the efficiency of the gasification process, particularly during the step when the mixture is introduced into the reactor through a screw feeder. Utilizing industrialized pellets as a solid material mixture consistently yields higher efficiency compared to using ground plastic and paper, which can result in irregular and varied sizes of material pieces.

A small size shredder machine was used for the pre-treatment of the waste material, with the purpose of reducing it to small pieces. The shredded material was then mixed with the forest biomass pellets before being added to the gasification unit hopper as shown in Figure 5.

A picture of the gasification unit is shown in Figure 6.

The scheme illustrated in Figure 7 represents the proposed waste-to-energy generation system, which is composed of two main equipment: a gasification unit and an internal combustion engine (ICE). The fixed-bed downdraft gasifier is fed with the residues blending (85% biomass + 15% MSW) in the hopper (1), which are dried in a heat exchanger (2). In the Imbert-type reactor, the thermochemical process of gasification happens due to the pyrolysis (3), oxidation (4), and reduction (5) zones resulting in the production of the synthesis gas (syngas). Ash and char (6) are solid byproducts formed during gasification, which were collected during the experiment and quantified thereafter. For security and monitoring, there is a flare (7) for burning the syngas at the beginning of the process and in case of an engine stop. Particle separation is performed in a cyclone filter (8) and the produced syngas recirculates inside the heat exchanger (2) before being filtered (9) and injected into the ICE (10).

Simultaneously, the ICE is fueled with biodiesel resulting in a liquid–gas fuel mixture with the syngas. The engine test bench provided the following parameters: biodiesel consumption (mL), rotations per minute (RPM), exhaust gases temperature (°C), and torque (N·m). The gasifier unit stores all data regarding temperatures and pressures of equipment, and to obtain the syngas flow, an anemometer was used, which measured the flow speed (m/s) and its temperature (°C).

Proximate and ultimate analyses were performed at the laboratory for the characterization of the forest biomass (pellets) and the MSW material. A PerkinElmer STA 6000 thermogravimetric analyzer (PerkinElmer Inc., Shelton, CT, USA) was used, using a heating rate of 20 °C/min in an oxidative atmosphere (for proximate analysis), and ultimate analysis was determined using a Thermo Fisher Scientific Flash 2000 CHNS-O analyzer (Thermo Fisher Scientific, Waltham, MA, USA).

The elementary analysis allows us to know the elementary composition of the raw material. Elements of interest include nitrogen (N), carbon (C), hydrogen (H), sulfur (S), and oxygen (O). Quantities of C, H, N, S, and O were determined using a Thermo Fisher Scientific Flash 2000 CHNS-O analyzer.

Thermogravimetric analyses were used to determine the concentration of moisture, volatile matter, and fixed carbon and ash in the raw material samples. The equipment used for the analysis of the mentioned parameters was a thermogravimetric analyzer PerkinElmer, STA 6000, using a heating rate of 20 °C/min. The content of each type of matter was determined from the thermogravimetric profile (variation of sample mass versus temperature), considering the inflection points of the derivative as a function of time.

The high calorific value (HHV) of the fuel was calculated using the IKA C 2000 calorimetry equipment (IKA-Werke GmbH & Co. KG, Staufen, Germany), through the complete combustion of the samples in an adiabatic environment.

The syngas samples collected during the experiments were analyzed by gas chromatography. The gases were analyzed in a Varian 450-GC Gas Chromatograph (Agilent Technologies, Palo Alto, CA, USA) with a thermal conductivity detector (TCD) (used for identification and quantification of the gaseous constituents CO, CO₂, H₂, CH₄, and light hydrocarbons present).

A flowmeter was used to estimate the volumetric air mass entering the reactor, which was located at the equipment’s air intake. Because the equipment did not have a flowmeter to measure the synthesis gas produced, an equation was used, in which the amount of volumetric air entering the reactor was combined with the percentage of nitrogen present in the synthesis gas, as exposed in Equation (1).

$$V_{syngas}=V_{air} · \left(\frac{0.781}{N_2}\right)$$

(1)

where V_syngas is the volumetric flow of syngas in Nm³/h, V_air is the volumetric flow of air in Nm³/h, 0.781 is the nitrogen percentage in the air, and N₂ is the nitrogen percentage in the syngas. The equivalence ratio is defined as the actual air/fuel ratio (used in the gasification) to the stoichiometric air/fuel ratio for combustion, as it is expressed in Equation (2).

$$ER=\frac{(A/F)}{{(A/F)}_{stoic}}$$

(2)

where ER is the equivalence ratio, (A/F) is the same mass ratio but under the experimental conditions that were adopted, and (A/F)_stoic is the mass ratio of air/fuel at stoichiometric conditions. The cold gas efficiency is calculated as follows in Equation (3) [32].

$$Cold gas efficiency=\frac{{LHV}_{syngas} \times gas production rate }{{LHV}_{biomass}}\times 100\%$$

(3)

The gasifier parameters that were analyzed during the experiments and were displayed in the equipment are the oxidation temperature (T_rst), the reduction temperature (T_red), and the pressure at filter (P_filt) and in the reactor (P_react).

At the end of the experiment, the produced chars (V_chars) and tars (V_tars) were weighted, and their production rate is given related to the volume of syngas produced.

Finally, considering the ICE control volume from Figure 7, it is possible to apply the first law of thermodynamics or the law of conservation of energy. Equation (4) is formulated to express this balance of energy.

$$E_{Fuel}+{\dot{m}}_{air} · {cp}_{air}· T_{air}=\dot{W}+{\dot{m}}_{eg}· {cp}_{eg}· T_{eg}+E_{jw}+L$$

(4)

where $E_{Fuel}$ is energy available in the fuel mixture (kW), ${\dot{m}}_{air}$ is the mass flow (kg/s), ${cp}_{air}$ is the calorific value of the air (kJ/kg·K) at the temperature $T_{air}$ (K). $\dot{W}$ is the power delivered by the engine (kW), ${\dot{m}}_{eg}$ is the exhaust gases flow (kg/s), ${cp}_{eg}$ is the calorific value of the exhaust gases (kJ/kg·K), $T_{eg}$ is the temperature of exhaust gases (K), $E_{jw}$ is the energy from the engine jacket water (kW), and L refers to loses due to friction and radiation (kW). It is possible to calculate the energy available in the fuel mixture $E_{Fuel}$ (kW) considering Equation (5).

$$E_{Fuel} = {\dot{m}}_{syngas} · {LHV}_{syngas}+{\dot{m}}_{biodiesel} · {LHV}_{biodiesel}$$

(5)

where ${\dot{m}}_{syngas}$ and ${\dot{m}}_{biodiesel}$ are the mass flow rates (kg/s) and ${LHV}_{syngas}$ and ${LHV}_{biodiesel}$ are the lower heating values (kJ/kg) of syngas and biodiesel, respectively. Considering the conservation of mass, it is possible to assert the following equality from Equation (6).

$${\dot{m}}_{syngas}+{\dot{m}}_{biodiesel}+{\dot{m}}_{air}={\dot{m}}_{eg}$$

(6)

where the sum of mass flows in the inlet of the ICE, i.e., ${\dot{m}}_{syngas}$ (kg/s) and ${\dot{m}}_{biodiesel}$ (kg/s) (fuel) added to ${\dot{m}}_{air}$ (kg/s) (air) result in the mass flow of exhaust gases ${\dot{m}}_{eg}$ (kg/s). The power delivered by the ICE, i.e., mechanical energy from the axis $\dot{W}$ (kW), is determined by Equation (7).

$$\dot{W}=\frac{T · N · \frac{2\pi }{60}}{1000}$$

(7)

where T is the torque (N·m) and N is the engine’s rotation in RPM (rotations per minute), which were controlled parameters available in the engine bench test. The energy produced $E_P$ (kW) can then be estimated using Equation (8), considering the association of the ICE with a generator.

$$E_P=\dot{W} · \eta$$

(8)

where t (h) is the period of operation and $\eta$ the generator efficiency, considered of 95%.

Finally, the electric efficiency of the ICE can be calculated according to Equation (9).

$${\eta }_{ele}=\frac{E_p}{E_{fuel}}$$

(9)

During the testing process, samples of syngas were collected in Tedlar bags. These collected samples were subsequently subjected to analysis utilizing a Varian 450GC gas chromatograph (manufactured by Varian, sold by Bruker in Portugal). This sophisticated instrumentation possesses the capability to both identify and quantify various gaseous elements contained within the samples. Among the elements detectable and measurable by the Varian 450GC, there are CO (carbon monoxide), O₂ (oxygen), H₂ (hydrogen), H₂S (hydrogen sulfide), CO₂ (carbon dioxide), N₂ (nitrogen), and CH₄ (methane), along with other shorter-chain hydrocarbons. This analysis provides precise insights into the composition of gases within the samples, which proves valuable for the characterization and assessment of syngas quality.

Considering the molar volume of any gas at standard temperature and pressure, which is precisely 0.0224 m³/mol, to ascertain the combustion enthalpy of each constituent within the syngas, calculations rely on the molar concentrations provided by the chromatograph. To obtain this information, the molar concentration must be divided by the molar volume (0.0224 m³/mol) and subsequently multiplied by the respective enthalpy of the syngas component.

3. Results

The results from the engine tests are displayed in

Table 4. Temperature of exhaust gases and consumption of fuel comparison.

Fuel	Texhaust (°C)	Consumption (mL/s)	Percentual of Variation (%)
Diesel only	175	0.105
Bio + diesel (50:50)	175	0.121	+15.24%
Biodiesel only	175	0.114	+8.57%
Biodiesel + syngas	375	0.083	−20.95%

. The fixed parameters are the torque, at the value of 4 N·m, and the engine rotation (3000 RPM). The first three tests relate to the engine operating with diesel only, 50:50 diesel–biodiesel blend, and biodiesel only. The last test given in Table 4 is related to the experiments performed with the gasification unit supplying syngas for the engine running with biodiesel which resulted in a gas–liquid fuel blend. The fuel consumption of the diesel-only configuration is assumed as the reference; therefore, the last column of Table 4 presents a percentual variation of each fuel configuration compared to the reference case.

The material characterization of samples from forest biomass pellets and the MSW material is presented in

Table 5. Proximate and ultimate analyses of the utilized materials.

			Laboratory Characterization		Estimate
Analysis	Parameters	Units	Forest Biomass	MSW	85:15 Mixture
Proximate	Moisture	%	9.8	3.1	8.6
	Volatile matter	%	59.3	82.7	61.0
	Fixed carbon	%	28.7	4.5	23.9
	Ashes	%	2.0	12.8	7.0
Ultimate	Nitrogen	%	0.6	0.0	0.5
	Carbon	%	49.7	42.8	48.7
	Hydrogen	%	7.5	5.9	7.3
	Sulfur	%	0.0	0.0	0.0
	Oxygen	%	26.7	35.4	28.0
HHV		MJ/kg	18.4	24.0	19.2

. To estimate the characterization of the feedstock mixture (85:15 biomass–MSW), a proportion based on the mixture mass was applied.

The results from the co-gasification of forest biomass and MSW at an 85:15 blending ratio for dual-fuel engine operation (syngas + biodiesel) are displayed in

Table 6. Main parameters of co-gasification of forest biomass and MSW with engine association.

		Forest Biomass 85% + MSW 15%
Parameters	Units	Initial	Engine	Final
CO2	%	12.81	4.16	10.37
C2H4	%	0.74	0.25	0.77
C2H6	%	0.24	0.06	0.16
C2H2	%	0.11	0.01	0.07
H2S	%	0.05	0.00	0.03
N2	%	46.75	77.73	48.72
CH4	%	3.69	1.41	3.43
CO	%	23.74	7.86	21.50
H2	%	11.87	8.52	14.95
LHV	MJ/m3	6.16	2.53	6.03
Trst	°C	794	797	796
Tred	°C	542	548	551
Pcomb	KPa	−7	−6	−8
PReact	KPa	−12	−10	−14
Pfilt	KPa	−38	−37	−40
Vair	m3/h	6.84	6.99	7.41
Tair	°C	18.70	18.65	18.81
Vtars	g/Nm3	81.64
Vchars	g/Nm3	58.80
ER	-	0.18	0.18	0.19
Cold gas efficiency	%	64.85	-	65.94
Vsyngas	m3/h	11.37	7.00	11.81
Sample collection	min	120	125	130
Fuel flow	kg/h	5.63
Experiment time	min	200

. In the “Initial” condition, the results refer to the first collection point of syngas produced, that is, before mixing with atmospheric air and entering the engine, thus consisting of pure syngas sample. The “Final” condition is similar, being the last collected sample, thus referring also to pure syngas. In the “Engine” condition, results refer to the association between the gasifier and ICE, and the sample collected consists of a syngas–air mixture that was thereafter injected into the engine air inlet.

From the results of the gasification experiment, it is possible to apply Equation (1) for the energy balance, and

Table 7. Energy analysis results.

Parameter	Unit	Diesel (Reference)	Syngas + Biodiesel
Efuel	kW	4.68	5.71
${\dot{m}}_{syngas}$	kg/s	-	0.0011
${\dot{m}}_{biodiesel}$	kg/s	0.0001	0.0001
${\dot{m}}_{air}$	kg/s	0.0016	0.0016
${\dot{m}}_{eg}$	kg/s	0.0017	0.0029
$\dot{W}$	kW	1.26	1.26
Eeg	kW	0.82	1.98
Ejw	kW	0.0075	0.0075
L	kW	2.60	2.49
${\eta }_{ele}$	%	25.50	20.89

is elaborated considering the results from the ICE operating with diesel only (reference case) and with the syngas–biodiesel blend. The engine bench test was fixed with a torque of 4 N·m and 3000 RPM, resulting in the delivered power of 1.26 kW. The mass flow of liquid fuels was determined by dividing the volume consumed by the experiment time. In both cases, the volume in the bench test was fixed at 16 mL, and the diesel was fully consumed after 2.2 min, while the biodiesel (in the syngas–biodiesel configuration) was fully consumed after 2.75 min. It is important to note that a calorific value of air at 1.05 kJ/kg·K and an air inlet temperature of 298 K were considered, with an exhaust gases calorific value calculated as 1.10 kJ/kg·K. The temperature of exhaust gases was measured during the experiments, and then the heat flux of the exhaust gases (Eeg (kW)) was obtained. Furthermore, the specific mass of syngas was calculated based on its composition resulting in 1.08 g/L, and the specific mass of biodiesel is 0.88 g/mL. The LHV of syngas was calculated based on its composition, resulting in 2145 kJ/kg, and the LHV of biodiesel was obtained using a bomb calorimeter IKA C200 model resulting in 38,200 kJ/kg, approximately, and the LHV of diesel was considered as 43,800 kJ/kg.

4. Discussions

First, when examining the results related to gasification, it is important to observe the influence of key parameters on their potential impact on the quality of the syngas. For this reason, it is important to mention the influence of temperature on the composition of syngas and the influence of ER. The gasification temperature directly influences the rate and extent of chemical reactions that occur during the process. Normally, gasification is carried out at elevated temperatures, generally above 700 °C, to promote the breaking of chemical bonds of carbonaceous materials and the formation of synthesis gas. The composition of the synthesis gas produced in the gasification is determined by the operating temperature. Lower temperatures tend to produce synthesis gas with a lower concentration of hydrogen (H₂) and a higher concentration of methane (CH₄). In addition, temperature also affects the presence of other components, such as sulfur and nitrogen compounds [33]. Figure 8 shows the influence of the gasification temperature on the synthesis gas composition over time, namely, on the production of CO₂, H₂, CH₄, and CO.

Although there were no significant changes in temperatures during the gasification test, it is obvious that the stabilization of the system influenced the characteristics of the syngas. An increase in the gasification temperature will promote products in endothermic reactions, which will also favor an increase in H₂ formation while decreasing CO₂ and CnHm [34]. This phenomenon can be observed during the gasification process carried out.

In general, the increase in temperature resulted in an increase in the concentration of hydrogen in the synthesis gas, from 11.87 to 14.95%. On the other hand, the concentration of carbon monoxide in the produced synthesis gas tended to decrease with increasing temperature, from 23.74 to 21.50%. The high concentration of these two gases in the produced synthesis gas is essentially due to endothermic Boudouard reactions (C + CO₂ ↔ 2CO) and thermal cracking (C_nH_m + CO₂ ↔ 2CO + 2H₂) [35]. As the temperature increases, both reactions become dominant, which promotes higher concentrations of H₂ and higher concentrations of CO. However, the decrease in CO may be related to the decrease in standard Gibbs free energy of the Boudouard reaction (responsible for the production of CO), which means that the concentration of CO tends to remain stable [36]. The composition of methane and light hydrocarbons in the produced synthesis gas shows a constant trend of increasing hydrocarbon concentrations as the temperature increases. Normally, the hydrocarbons present in the synthesis gas are due to the cracking of the tar; this aspect may explain the non-constant level of hydrocarbon composition in the synthesis gas throughout the test.

The performed gasification performances were also investigated in terms of LHV and CGE throughout the trial, as shown in Figure 9.

The stabilization of the gasification system has a positive influence on the CGE due to the more dominant thermal cracking reactions and Boudouard reactions at higher gasification temperatures [35]. The stabilization of the system led to a decrease in the LHV. At higher temperatures, more CO and H₂ are produced, which in turn can result in increased LHV. However, the presence of high levels of hydrocarbons (high enthalpy of combustion) at low temperatures increases the calorific value of the produced synthesis gas [37] The increase in LHV is ultimately an improvement in CGE.

The ER is one of the most studied parameters when approaching a gasification process. Although gasification with the oxidizing agent air presents some gaps concerning syngas LHV, compared to gasifications where the oxidizing agent is steam or oxygen, this is due to the high concentration of N₂ present; the advantage is that this gasification agent reduces costs of the system [38]. The ER, despite not having been a parameter controlled in the study, being a consequence of the working temperature, was situated in the range between 0.18 and 0.19, values that are in accordance with those in the literature, which refer to a range between 0.2 and 0.4 for a similar gasification process [39] In an initial analysis, it is possible to observe that the increase in ER promotes the N₂ content. The increase in ER, when the oxidizing agent is air, causes more inert gas to enter the interior of the reactor, and the gas obtained will be more diluted in nitrogen, resulting in a synthesis gas with a lower calorific value. The increase in H₂ can also be explained by the increase in the reaction temperature, since as the flow of the oxidizing agent increases, it tends to favor exothermic reactions and, consecutively, thermal cracking reactions, which increase the concentrations of this element. Conversely, ER negatively affects the C_nH_m content, derived from thermal cracking reactions, leading to methane decomposition [40]. However, as there are other properties in the raw material that can influence the gasification process, these conclusions are not absolute.

The equivalence ratio for the engine (ɸ_premixed) is calculated as the volumetric O₂ flow required for the stoichiometry divided by the actual volumetric O₂ flow (Equation (10)). A global equivalence ratio (Ø_global) was also calculated as a function of the amount of fuel and the stoichiometric air/fuel mass ratio of the decane (15.03), as indicated in Equation (11). Decane (C₁₀H₂₂) was the fuel used as a surrogate for diesel fuel to facilitate the kinetics.

$${ɸ}_{premixed}=\frac{0.5 V_{CO}+0.5 V_{H_2}+2.0 V_{{CH}_4} }{0.21 V_{air}}$$

(10)

$${Ø}_{global}={ɸ}_{premixed}+\frac{15.03}{{ṁ}_{air}/{ṁ}_{pilot}}$$

(11)

Concerning the tests with the diesel engine, it was noticeable that with the addition of about 7% syngas, there was a 20% reduction in diesel consumption. The same observation was made by Aslam et al. [41], who reported that dual-fuel operation led to a decrease in diesel consumption. Due to the introduction of syngas and, consequently, an increase in hydrogen in the combustion chamber, it soon presents a higher flame speed leading to a better consumption of gases. This increase in the consumption of gases leads to a higher combustion temperature and, therefore, to more heat losses, thus depleting the thermal efficiency, as highlighted by other authors [42].

To study the effect of the syngas/air equivalence ratio, the injection duration was kept constant, as well as the amount of syngas in the admission charge. The premixed equivalence ratio did not change despite the change in syngas composition, remaining at 0.2 for the duration of the test, for which it induced very unstable combustion. According to Castro et al. [43], increasing this parameter leads to an increase in the synthesis gas mass flow but a decrease in the air mass flow, creating a lot of instability in the engine. The global equivalence ratio parameter is closely related to the fuel used and the gaseous emissions incorporated by the engine. This parameter showed a value of 0.22 throughout the test, but it was not possible to study the gaseous emissions. Still, according to the same author [43] it is noted that NOx emissions are closely related to the introduction of fuel and seem to reach a maximum when the global equivalence ratio tends to 1.

Initially, the first parameter clearly observed was the consumption of fuel in the engine when associated with the gasification unit. It is observed that when the combustion mixture is enriched with syngas from biomass–MSW blend, the instantaneous consequence was the rise of exhaust gas temperature, reaching almost 400 °C, which is almost 2.3 times higher than its temperature level when running with diesel-only. Meanwhile, the addition of syngas in the engine reduced the consumption of biodiesel by almost 20% when compared to the situation without the injection of syngas into the mixture. Some technical aspects of the percentage of syngas injection should be studied, since the excess injection of syngas may damage the engine because of abnormal combustion and, consequently, cause it to overheat.

The co-gasification of MSW and forest biomass has proved to be feasible and with good results regarding the energy efficiency and the composition of produced syngas. The content of hydrogen from the produced syngas resulted from the gasification of forest biomass and MSW at a blending ratio of 85:15 was observed as around 15% in volume while presenting a lower heating value (LHV) of around 6 MJ/m³.

The use of commercial pellets ensures energy supply, providing reliability to the system. Moreover, utilizing a portion of waste in the feedstock material makes it more environmentally friendly, reducing its aggressiveness. This approach considers environmental benefits, including greater energy efficiency and proper waste disposal, which may lead to the generation of carbon credits.

In addition to what has been mentioned, polymeric residues decompose before the gasification reaction, unlike residues of lignocellulosic origin, and may cause bridges and backings, which cause clogging of the system [44]. Another important issue is that these residues have high amounts of chlorine and sulfur, which in thermochemical processes such as gasification, are transformed into HCl and H₂S, which are corrosive reagents for both the metallic components of the gasifier and the engine. Hence, in downdraft gasification with an air-oxidizing agent, it is not usual to exceed 30% of residu [45].

When considering the viability of gasification, it is crucial to account for the actual operating costs involved. By doing so, we can evaluate its economic feasibility and compare it to alternative waste-to-energy technologies. Gasification is known for its high energy conversion efficiency. The process involves converting carbon-based materials, such as biomass or waste, into a synthetic gas known as syngas. This syngas can be utilized in various applications, including power generation, heat production, or as a raw material to produce chemicals and fuels. The efficient conversion of waste into valuable energy resources minimizes energy wastage and maximizes resource utilization and, at the same time, offers a higher degree of flexibility compared to other waste-to-energy technologies. It can handle a wide range of feedstocks, including municipal solid waste, biomass, agricultural residues, and even hazardous waste. This adaptability makes gasification suitable for diverse waste streams, enabling effective waste management in different sectors. Gasification has the potential to reduce environmental impacts compared to traditional waste disposal methods. The process operates at high temperatures and in an oxygen-starved environment, which significantly minimizes the formation of harmful byproducts, such as dioxins and furans. Additionally, the syngas produced from gasification can be cleaned and processed to remove contaminants, resulting in cleaner emissions. The syngas produced through gasification has multiple applications, enhancing its economic value. It can be combusted in gas turbines or engines to generate electricity or used as a substitute for natural gas in various industrial processes. Furthermore, syngas can be further refined through processes like Fischer–Tropsch synthesis to produce valuable chemicals and transportation fuels, providing additional revenue streams. A very important point in terms of costs and feasibility is that gasification offers an opportunity to promote energy security by utilizing local waste resources. Rather than relying solely on traditional energy sources, communities can leverage gasification to convert their waste into a reliable and locally available energy source. This decentralization of energy production can enhance resilience and reduce dependence on external energy supplies.

5. Conclusions

Gasification is a waste-to-energy technology that offers several advantages over other methods and is gaining attention as a promising solution for waste management. For example, when comparing gasification to combustion, gasification offers several advantages when it comes to waste-to-energy conversion. Gasification typically achieves higher energy conversion efficiencies that convert the waste into syngas composed of hydrogen, carbon monoxide, and other combustible gases.

In this study, synthesis gas was generated by combining forestry biomass and municipal solid waste. The resulting syngas was then blended with biodiesel in specific proportions for experimentation on an engine test bench. The ICE operating with the dual-fuel configuration showed an electric efficiency of nearly 21%, and when compared to the diesel reference case, the temperature of exhaust gases increased by almost 200 ºC. This extreme temperature increase is a result of adding syngas in the combustion chamber of the ICE, which caused a reduction in the consumption of biodiesel, while increasing the heat flux of exhaust gases which could be used in a heat exchanger for better efficiency yields.

The analysis revealed relevant syngas LHV produced from waste materials, along with a high hydrogen content. The test results on the bench demonstrated a noteworthy reduction of 20% in specific consumption at lower loads. The produced syngas presented an LHV of around 6 MJ/m³ and 15% of H₂ in volume.