Speciﬁc and Cumulative Exhaust Gas Emissions in Micro-Scale Generators Fueled by Syngas from Biomass Gasiﬁcation

: Climate change, environmental degradation, and biodiversity loss are prompting production systems to shift from a fossil-based economy to a circular bio-based one. In this context, biomass gasiﬁcation is a promising alternative to fossil fuels that can contribute to power generation in rural communities and remote areas as well as provide a sustainable source of energy for developed countries. In this work, exhaust gas emissions (CO, NOx, and SO 2 ) of two syngas-fueled micro-scale generators were measured. The ﬁrst system is a commercial biomass gasiﬁer genset, whereas the second is composed of a laboratory-scale gasiﬁer prototype and a portable petrol generator. For this second facility, emissions were measured both running on gasoline and on syngas. The comparison was performed both on the pollutant concentration and on their cumulative amount. This comparison was made possible by calculating the exhaust gas ﬂow by knowing the combustion stoichiometry and fuel consumption. The results showed a much lower pollutant concentration running on syngas compared to gasoline. In particular, considering the best conﬁgurations, every cubic meter of exhaust gas released running on syngas contains about 20 times less CO and almost one-third less NOx compared to gasoline. Moreover, the cumulative amount of emissions released was also considerably lower due to the lower exhaust gas ﬂow (about 25%) released running on syngas.


Introduction
Climate change, along with degradation of the environment with consequent biodiversity loss, is prompting production systems to shift from a fossil-based economy to a circular bio-based one [1]. Biomass is a promising alternative to fossil fuels for electrical power generation [2]. Biomass is widely available, uniformly distributed, and less intermittent compared to other renewable sources like wind power and solar photovoltaic, which are not always available when needed [2]. Furthermore, the growth of bioenergy can increase food supply, economic growth, and female employment [3].
In particular, biomass gasification can contribute to power generation in rural communities and remote areas where access to electric power is obtained by using electric generators coupled to compression-ignition or spark-ignition engines [2]. Additionally, it can provide a sustainable source of energy for developed countries [4].
Gasification consists of the conversion of a carbonaceous material, such as biomass, into a synthetic gaseous fuel, called syngas [5], composed mostly of carbon monoxide (CO), carbon dioxide (CO 2 ), methane (CH 4 ), hydrogen (H 2 ), and nitrogen (N 2 ) [6] as well as char, ash, tar, and oils [7]. Syngas can be used in spark-ignition engines as an alternative to gasoline [5]. Fossil fuels are the leading cause of air pollution, greenhouse gas (GHG) emissions, and climate change [8].
It has been demonstrated that fossil fuel combustion is the major source of nitrogen oxide (NOx) and black smoke (suspended particulate matter less than 15 µm) emissions [9]. Another pollutant produced through combustion is CO [10], one of the main contributors to air pollution due to its high toxicity [11]. Furthermore, sulfur-containing fossil fuel combustion is responsible for most of the sulfur dioxide (SO 2 ) content in the atmosphere [12]. SO 2 is a toxic gas and one of the major components of acid rain [12].
CO and NOx concentrations in the exhaust gas of syngas fueled engines are significantly lower than gasoline operation [5]. For fuel-rich mixtures, CO concentration increases steadily with the fuel/air equivalence ratio [13] due to the higher chances of having incomplete combustion [14], while for near-stoichiometric combustion, NOx formation reaches its peak [13] along with the highest combustion temperature [14].
Another major global problem is CO 2 emissions. Even if CO 2 concentration in the exhaust gas of an engine is higher when gasoline is replaced with syngas [5], biomass gasification can contribute to a reduction in CO 2 emissions [15], because it is a carbon neutral energy source [16]. Furthermore, biomass gasification is considered a carbonnegative technology thanks to the co-production of biochar [17]. Biochar is a carbonaceous residue of gasification. It is highly recalcitrant, remaining stable for decades, and when used as soil amendment, can increase crop productivity [17], soil organic matter, microbial activity, and water retention while decreasing fertilizer needs and erosion [18].
Modern engines offer various strategies to reduce pollutants and CO 2 emissions such as exhaust gas recirculation or double and ducted fuel injection [19,20]. However, there are several applications where these kinds of approaches are not always possible, such as in the case of water pumps for irrigation, combined heat and power (CHP) systems based on old engine architectures, and portable and emergency generators. In these cases, a successful retrofitting of their performances could happen through a change of their fuel, from fossil-based to bio-based such as syngas from biomass gasification.
In this work, the exhaust gas emissions of two syngas-fueled micro-scale generators were measured. The first one was the commercial biomass gasifier genset system manufactured by All Power Labs (APL PP30). This genset was equipped with a three-way catalyst that was removed to compare the exhaust emissions from this study with those measured in a previous experimental campaign described in [21]. In the second syngas-fueled microscale generator, the engine emissions were measured both running on gasoline and on syngas. In this last configuration, the engine was coupled with a tabletop gasifier prototype to test a possible single system, extremely cheap and compact, made of a gas generation unit and a portable power unit.
During the gasoline operation, the fuel/air equivalence ratio was delivered by the carburetor; while running on syngas the fuel/air equivalence ratio was manually controlled through a gate valve [22]. Different fuel/air ratios were tested to evaluate the effect on the emissions for syngas combustion and to assess the best configuration. This is not the first comparison between gasoline and syngas in a spark-ignition engine [5], but this time, not only were the emission concentrations compared to each other but also their cumulative amount considering the exhaust volume flows. The cumulative amount is crucial because different fuels lead to different exhaust gas flows at the same power output and, therefore, comparing just the emission concentration does not provide enough information about the evaluation of the environmental impact. The exhaust gas flows were estimated knowing syngas and gasoline combustion stoichiometry.

APL PP30
Exhaust emissions of the APL PP30 ( Figure 1) were measured and compared with the results obtained in a previous experimental campaign with the same power plant described in [21], with the difference being that in the previous trial a three-way catalyst was installed in the exhaust line. This device can simultaneously control CO, NOx, and hydrocarbon emissions [23]. This comparison was carried out to get an idea of how much a catalyst could improve the emission performances of an engine running on syngas. The biomass used during the test was vine pruning pellets. The gasifier was started up and the syngas was burnt in the flare until the proper temperatures inside the reactor were reached, then it was sent to the engine. An MRU Vario Plus Industrial gas analyzer was used to measure oxygen, carbon monoxide, nitrogen oxide, and sulfur dioxide contents in the engine exhaust gas. This analyzer complies with USEPA methods CTM-030 and CTM-034 and international ASTM D6522, and it is certified according to DIN EN 50379-1 and DIN EN 50379-2 [25]. Different fuel/air equivalence ratios were tested while keeping the electrical power output constant, and different power outputs were tested maintaining the same fuel/air equivalence ratio. The MRU analyzer was set to perform a measurement every 2 s, and for every condition at least 10 samples of operation were analyzed. The emissions analyses was compared with the ones obtained in the previous experimental campaign carried out using a three-way catalyst [21].

Portable Petrol Generator and Femto Gasifier
The generator used in the second part of this study was made of a single-cylinder, air-cooled, four-stroke spark-ignition engine coupled with a single-phase alternator manufactured by Zhejiang Anlu Cleaning Machinery Co., Ltd. Its technical data are summarized in Table 1. The gasification unit used to provide syngas to the generator was the "Femto Gasifier", a table-top downdraft gasifier designed and developed by the authors at the "Enzo Ferrari" Engineering Department of the University of Modena and Reggio Emilia Figure 2 [22]. During the current experimental trial, the Femto gasifier was operated with A1 ENplus wood pellets to have a stable gasification process, but it can also be fueled with small wood chips, hemp hurd, and vine pruning pellets [22]. This gasifier was designed to have a nominal biomass flow rate of 2 kg/h [6]. In previous experimental campaigns, it has shown a cold gas efficiency near 70% [22] with an equivalence ratio (the ratio between the air amount entering into the gasifier divided by air amount required for stoichiometric combustion [28]) of 0.24, and a chemical power output (syngas flow multiplied by its heating value) of around 5.7 kW [22]. Cold gas efficiency was calculated using Equation (1) [29].
where HHV indicates the high heating value; . V syn , the volumetric flow of syngas; and . m bio , the mass flow of biomass. It was chosen to measure the emissions with the engine only at idle to have more reproducibility without the risk of instability that can be caused by an excessive demand for gas at a high load. Furthermore, the emissions analysis carried out on the APL PP30 (both with and without the catalyst) did not show significant changes by varying the power output, while even small variations in the fuel/air ratio led to very different results. Firstly, the engine of the portable generator runs on gasoline at idle. The same emissions analyzer instrument described in the previous paragraph was used. The fuel tank was weighed before and after the runs to measure the fuel consumption. In this way, it was possible to calculate the chemical energy consumption (CEC) knowing the higher heating value of gasoline (HHV gasoline = 46.88 MJ/kg [30]).
Through the chemical formula of gasoline (C 6.97 H 14.02 [31]), it was possible to estimate the exhaust mass flow and its composition considering stoichiometric combustion and knowing the molar mass (M) of carbon (12.01 g/mol [32]), hydrogen (1.008 g/mol [32], oxygen (15.999 g/mol [32]), nitrogen (14.007 g/mol [32]), and argon (39.95 g/mol [32]). In this way, it was also possible to assess the absolute amount of CO 2 and other pollutants released in the atmosphere through gasoline combustion and not only their concentration in the exhaust flow. This analytical calculation is summarized as follows.
where mf indicates the mass fraction of the considered element in the gasoline. The CO 2 number of moles (n) obtained burning 1 kg of gasoline (gas) was calculated with Equation (6).
n CO 2 per kg of gas = 1000g·mf C in gas M C = 71.23 mol CO 2 kg gas (6) Therefore, the mass (m) of CO 2 released in the atmosphere burning 1 kg of gasoline can be obtained with Equation (7): m CO 2 per kg of gas = n CO 2 per kg of gas ·(M C + 2·M O ) 1000g/kg = 3.13 kg CO 2 kg gas (7) In a similar way, number of moles and mass of water (H 2 O) released together with CO 2 were obtained with Equations (8) and (9) The number of moles of oxygen necessary to completely burn 1 kg of gasoline were calculated with Equation (10): n O 2 per kg of gas = n CO 2 per kg of gas + n H 2 O per kg of gas 2 = 107.05 molO 2 /kg gas (10) Considering the air to be dry with 78.1% N 2 , 20.9% O 2 and 1% Ar (by volume), the amount of nitrogen and argon in the exhaust gas flow can be calculated as: m N 2 per kg of gas = n O 2 per kg of gas ·78.1%·M N 2 20.9%·1000g/kg = 11.209 kg N 2 kg gas (11) m Ar per kg of gas = n O 2 per kg of gas ·1%·M Ar 20.9%·1000g/kg = 0.205 kg Ar kg gas (12) The total amount of exhaust gas (exh) produced from burning 1 kg of gasoline was calculated as the sum of nitrogen and argon of the comburent air and CO 2 and water produced through the combustion of gasoline: m exh per kg of gas = m CO 2 per kg of gas +m H 2 O per kg of gas +m N 2 per kg of gas + m Ar per kg of gas = 15.839 kg exh kg gas (13) The volume (V) of the exhaust per kg of gasoline at a certain temperature can be calculated knowing the density (ρ) of the various components at that temperature ( Table 2). The temperature of 500 K for the exhaust was chosen for the comparison between syngas and gasoline combustion.  Knowing both the exhaust volume and the CO 2 volume released per kg of gasoline, the volumetric concentration (x) of CO 2 in the exhaust can be obtained with Equation (15).
x CO 2 , exh = m CO 2 per kg of gas /ρ CO 2 @500K V exh@500K per kg of gas (15) Once the emissions were measured, they were correlated to the absolute amount of exhaust gases released during the combustion in the engine knowing the fuel consumption. After that, the test with syngas was set up adapting the generator to be coupled with the gasifier as described in [22]. A wideband O 2 sensor was implemented on the exhaust pipe to manually adjust the fuel/air ratio of the mixture through a gate valve added at the intake manifold. The original air filter was removed and substituted with an Iveco air filter for trucks to preserve the engine from tars and particulate without excessive pressure drop. It is important to prevent an excessive amount of these contaminants from reaching the engine to maintain the operation and endurance of components such as valves, combustion chamber, and pistons [36]. Tar yield depends on the gasifier type [37] and the downdraft reactors produce syngas with a low tar content [38,39] and, therefore, was chosen to implement this very simple filtration stage. The fuel tank was removed from the generator for safety reasons. Four thermocouples were used to measure the following temperature: reactor grate, syngas downstream the reactor, exhaust gas flow, and ambient air. A G4-grade gas totalizer was used to measure the airflow entering the gasifier as a gasifying agent. A differential manometer was used to measure the pressure drop across the reactor. The scheme of the gasifier engine system is depicted in Figure 3. The gasifier was started up and the syngas was burnt in the flare until the proper conditions of temperature and pressure drop were reached, then the instrument was placed at the outlet of the exhaust pipe of the generator and the engine was turned on, operating on syngas. Different fuel/air ratios were tested and monitored with the emissions analyzer and 10 or more samples were analyzed for each configuration, always at idle. Knowing the airflow entering the gasifier, it was possible to estimate the syngas flow going to the engine with Equation (16) [40]: The following assumptions were made: • Air was considered dry with 78.1% N 2 , 20.9% O 2 , and 1% Ar; • Syngas was considered dry and without molecular oxygen.
The considered syngas composition is shown in Table 3. The composition previously obtained in [22] was used in this work. All the gas analyses performed on syngas produced with the Femto gasifier running on A1-grade pellets showed a quite stable gas composition, with typical values of downdraft gasifier [41][42][43] and, therefore, using this composition was considered quite reliable.
Through Equation (2), it was also possible to calculate the chemical energy consumption running on syngas. As previously done in the test with gasoline, the exhaust flow was analytically calculated knowing syngas flow and composition with the following equations, considering again a stoichiometric combustion. In this way, it is possible to make a comparison of the cumulative emissions released during the combustion (XY indicates the generic component). .

Results and Discussion
The format of emission results is the following: • (Average result ± Standard deviation) if the result is a percentage. • (Average result ± Relative standard deviation) for the others.
All pollutant concentrations are presented in mg/m 3 and normalized at 13% oxygen content in the exhaust gas (mg/m 3 @ 13% O 2 ).
The percentage of oxygen contained in the exhaust shown in the second column is a qualitative indicator of the fuel/air ratio, the higher the oxygen content is, the lower the ratio.
It is possible to see that the fuel/air ratio has a great influence on the emission contrary to the power output. As expected, by decreasing the fuel/air ratio, the carbon monoxide decreases as well, while the nitrogen oxides increase. For all the tests, sulfur dioxide was not detected in the exhaust gas. In Table 6, the two best cases, number 6 for the CO content and number 7 for the NOx content, were compared to the two best cases described in [21] with the same PP30, but with the catalyst. Emissions with the implementation of a three-way catalyst are between 2 and 3 orders of magnitude lower. Table 7 shows the results obtained through the analytical calculation of mass and volume of the exhaust and its CO 2 content considering the gasoline and the syngas consumption measured during the experimental tests at idle. The first outcome is that the energetic consumption running on syngas is considerably lower compared to gasoline (all the tests were carried out at idle). This is in accordance with the higher efficiency measured by [5].

Gasoline and Syngas Analytical Comparison
The second outcome is the higher mass and volume flow of the exhaust gas running on gasoline. Therefore, even hypothesizing that the emission concentrations in the exhaust gas were the same, the impact on the environment would be lower running on syngas.
The third outcome is that, as measured by [5], the CO 2 concentration in the exhaust gas is more than 30% higher running on syngas. However, considering the total amount of exhaust gas released by the engine, less than 1% more CO 2 is released in the atmosphere. This fact, together with the carbon negativity of the biomass gasification process, makes the higher CO 2 concentration irrelevant. Table 8 summarizes the emissions results of the analyses performed on the generator running on gasoline and running on syngas. The oxygen content shown in the second column can be used as a qualitative indicator of the different fuel/air ratios tested running on syngas. The percentages of oxygen are quite higher compared to the ones measured with the APL PP30, probably due to the different placement of the O 2 sensor in the two engines. The CO emissions released running on syngas are similar to the ones of the APL PP30 considering the best configurations, while the nitrogen oxides are always around one order of magnitude lower compared to the APL PP30. This can be due to a lower compression ratio [44] of the portable generator (8.5:1 [27]) compared to the Ashok Leyland engine [24].

Gasoline and Syngas Emissions Comparison
Concerning syngas and gasoline comparison, the emissions released running on syngas are much lower than the ones running on gasoline for all the tested fuel/air ratios. In particular, considering the best performances, the CO content is about 20 times lower, while the nitrogen oxides are almost one third running on syngas when compared to gasoline.
Very similar ratios between gasoline and syngas emissions were measured by [5] during their test at the lowest power output (about 740 W).
Even with the syngas from Femto gasifier, no sulfur dioxides were detected unlike the operation with gasoline.
Results concerning only CO and NOx are also depicted in Figure 4. The error bars in the graph show a great uncertainty, calculated as standard deviation, when the syngas emissions are higher. This can be due to combustion instability, while when the emissions are low, they are also quite stable.
Gasoline emissions, even if they are much higher than syngas, are also quite stable. Furthermore, considering the analytical calculation, not only does the exhaust gas running on syngas contain a much lower amount of pollutants, but it is also about one-third less. Therefore, the difference between the cumulative amounts of pollutants released in the atmosphere is even higher than the difference shown by the emission concentrations.

Conclusions
In this study, the emissions of two engines running on syngas were analyzed compared with the emissions obtained running on gasoline and the effect of a three-way catalyst was assessed. Emissions are usually expressed as a concentration. Therefore, for a better evaluation, the total amount of exhaust gas released with the two different fuels was calculated. The results showed a much lower pollutant concentration using syngas compared to gasoline. In particular, considering the best configurations, every cubic meter of exhaust gas released running on syngas contains about 20 times less CO and almost one-third of nitrogen oxides compared to gasoline. Furthermore, no sulfur dioxides were detected running on syngas as opposed to gasoline. In addition, both volume and mass flow of the exhaust gas was about 25% lower with syngas, further increasing the gap between the pollutant cumulative amounts released in the atmosphere running on syngas and gasoline.
Finally, to obtain a very clean exhaust gas, a three-way catalyst can be used. The tests showed that it can allow a reduction between 2 and 3 orders of magnitude of the pollutant's concentration. These results, together with the carbon negativity of the biomass gasification process, make the replacement of gasoline with syngas from biomass gasification in sparkignition engines very attractive. Running on syngas can significantly and immediately improve the GHG and pollutant emissions of various applications like irrigation, CHP systems based on old engine architectures, and portable and emergency generators.
The next step would be to measure the engine airflow in order to assess the fuel/air ratio that guarantees the best performances and evaluate it at different electrical power outputs as well as analyze the in-cylinder pressure for a better understanding of the combustion process [45].

Conflicts of Interest:
The authors declare no conflict of interest.