1 Introduction

Nigeria is one of the major oil-producing nations in the world and the largest in sub-Saharan Africa [1] but still suffers shortage and price-hike of oil products, especially gasoline, diesel and dual-purpose kerosene (DPK). Long queues are a common sight at service stations and the situation forces consumers to resort to purchasing from artisanal refiners who vandalise oil pipelines, steal the oil and refine using crude technology [2]. The unpleasant position of this sector of the economy is exacerbated because Nigeria exports its crude oil for sale rather than refining, making the commodity available for theft and its finished products unaffordable.

Although the government assumed it was doing its best to proscribe artisanal activities in Nigeria and ensure illegal products do not get into the society, it is unfortunate that a reasonable portion of the gasoline products consumed by the populace, especially in the South-South and South-Eastern part of the country are procured from the artisanal camps. The fuel is used for domestic and industrial stationary combustion purposes including cooking and electricity supply through generating sets. Consumers go through this route because of the insufficiency of fuels supplied by the Nigerian National Petroleum Commission (NNPC) and the few independent marketers licensed to import fuel. There have been indications of likelihood of product blending in which imported fuel is mixed with indigenously refined products by some unfaithful oil marketers in the country.

Studies have shown that energy generation is a major contributor to air pollution globally [3,4,5,6] and fuel combustion is the oldest and currently the primary method of creating energy. Combustion of fossil fuels is associated with the emissions of gaseous pollutants, namely carbon monoxide (CO), Hydrocarbons (HC), oxides of nitrogen (NOx), and sulphur dioxide (SO2) [7,8,9,10]. The presence of these pollutants in ambient air beyond regulated concentrations poses some health risks including respiratory complications, heart diseases, and cancer-related problems [4, 10,11,12]. The pollutants also act as precursors to destructive phenomena in the environment including acid rain, weakening and defacing of monuments and global warming, among others [13, 14].

During fuel combustion, the measure of emissions generated depends on certain factors including the type of fuel, the efficiency of the combustion unit at achieving adequate fuel–air mixing ratio, and sufficient residence time [15]. Furthermore, a major factor that determines the various levels of polluting potentials of different types of fuels is their composition [7, 16]. Natural gas is considered the cleanest of the three commonly used fossil fuels because of its lower hydrocarbon content [8, 16]. Nevertheless, the composition of a fossil fuel is determined by not only its origin but also the fuel production and pre-treatment methods [17,18,19]. The consideration of pollutants potential of a fuel arising from its production methods begs an answer to the question regarding the possible effects of the utilisation of artisanal-refined fuel in Nigeria, with emphasis on the environmental implications. Since these refiners use crude technology coupled with the fact that no mechanism is in place neither for the complete removal of environmentally unsafe components in their products nor for its reduction, fuels from their camps pose a great threat to sustainable air quality.

The artisanal technology involves a simple distillation process in which crude oil is heated in distilleries by open fire generating gasoline, kerosene and diesel. Locally available materials including drilling machines, drums, hose lines, boats, pipes, firewood, crude oil, pumping machines, rubber hose, dried wood and storage facilities, are employed for the operation [20]. Some studies have been targeted at the economic and social implications of the activities of artisanal refiners in Nigeria [21,22,23]. Nwankwoala et al. [24], Yabrade and Tanee [25], Asimiea and Omokhua [26] have reported the impacts of artisanal oil refining activities on vegetation, soil, and water quality around the Niger Delta. Experimental studies on emissions from artisanal-refined fuel products are relatively rare because of it very challenging nature of getting research materials i.e. fuel samples from reliable sources. This is because because of its proscribed nature, investigating artisanal refining and getting reliable fuel samples for experimental purposes is highly risky for many researchers as most of these artisanal refiners’ activities and locations are highly shrouded in secrecy and frequently involve armed conflicts with the military that are trying to protect oil installations in the region. Moreover, artisanal refining operations are mainly limited to the Niger Delta region.

To investigate the air pollution caused by artisanal refiners’ activities, Onakpohor et al. [20] quantified gaseous emissions generated from indigenous processes employed in the artisans’ camps in the Niger Delta and reported significant pollutant load across the unit operations involved in the fuel processing, without any measure taken by the illegal operators to prevent the eventual release of the pollutants into the environment. Gaseous pollutants reported were HC, CO, NOx, H2S and SO2, and each emission was way higher than standard set by the set by the Environmental Guidelines and Standard for Petroleum Industry in Nigeria (EGASPIN) [27]. Michael and Joepen [28] also carried out an air sampling in the Port Harcourt area of the region using experimental model to show level of concentrations at different locations and Gaussian dispersion model to predict pollution concentration at various distances to ascertain the level of suspended particulate matters and criteria air pollutants responsible for pollution within the research study area. However, the pollution issue arising from Onakpohor et al. [20] and Michael and Joepen [28] has a localized and/or regional effect, with the operators and dwellers of communities within the vicinities of the production sites being the major population exposed to poisonous gases. The dispersion and impact of the emitted pollutants affect larger sections of the country when the artisanal-refined gasoline, which is a widespread commodity, are marketed by the artisans across and beyond the region. As a result, the effects and benefits of the artisanal-refined gasoline are felt beyond the region and across the country, especially at a time when government oil subsidy has been removed and fuel products are unaffordable for many Nigerians [29]. In this regard, it is therefore considered necessary to characterize the emissions from these artisanal-refined gasoline products from Bayelsa and Rivers States in the Niger Delta area of Nigeria to determine the criteria air pollutants associated with their combustion products; this is the impetus for this study.

2 Materials and methods

2.1 The study area

The study area locations are in Bayelsa and Rivers States in the Niger Delta region of Nigeria as shown in Fig. 1. The Niger Delta region covers an area of 20,000 km2 within wetlands of 70,000 km2 formed mainly by sediment deposition and is the largest wetland with the third-largest drainage basin in Africa [30, 31]. Home to 20 million people and 40 different ethnic groups, this floodplain is up to 7.5% of Nigeria's total land mass [30]. With different network of swamps, streams, creeks and rivers, the Niger Delta is home to proven oil reserves and production in the country [32]. It is in these various creeks of the Niger Delta that the illegal artisanal refiners carry out their proscribed operations. Bayelsa and Rivers States were selected for the artisanal sites because these are the two major oil producing states in the region where the artisanal refiners sited most of their refineries due to the proximity to crude oil pipes installed in the remote areas of the two states [33]. This makes it hard to police the areas and gives artisanal refiners advantage to steal high amount of crude oil for refining [34].

Fig. 1
figure 1

Map of Nigeria showing the Oil Producing States of Rivers and Bayelsa States in Niger Delta region [20]

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2.2 Sampling

Gasoline, the majorly used fuel in Nigeria was randomly collected in air-tight plastic containers from thirty (30) artisanal refiner’s camps in Bayelsa and Rivers States. The plastic sample containers were rinsed with the gasoline sample to be collected, then stored in the air-tight plastic containers to avoid contamination and to maintain samples quality. The containers were leak-proof to prevent the escape of gases including hydrocarbon [35].

2.3 Apparatus

The combination of equipment used in this study are air compressor coupled with zero air generator, pitot-tube anemometer, combustion chamber, Combustion emission analyser, lighting gun, and measuring cylinder.

Zero air generator with compressor: a precision air compressor coupled with zero air generator was used purposely to supply pure air necessary to combust the fuel during the experiment. Zero air was used for quality assurance purpose to eliminate air impurities to get accurate emissions reading during the experiment. The maximum output pressure of the compressor is 120 psi. Pitot-tube anemometer was used to monitor the velocity and flow rate of the air supply into combustion chamber.

The fuel to be combusted was placed in the combustion cavity; air was received from the compressor through the outlet duct; and flame and gaseous product of combustion escaped through the outlet duct [36].

The analyser employed in this study was the E8500 portable industrial flue gas and emissions analyser operating EGAS™ Windows software, manufactured by E Instruments International LLC. E8500 analyser identifies the various components of the air emissions and measures their concentration in situ. It is effective in monitoring emissions from stationary combustion equipment and has a detection range up to 22,900 mg/m3 for CO, 750 mg/m3 for NOx and SO2, and 0–5% Pellistor-type sensor for HC and 99.9% sensing ability for CO2 [35, 36]. Lighting gun was utilized for igniting the fuel samples under study within the combustion chamber.

2.4 Experimental procedure

Fifty (50 ml) of fuel sample was transferred into the combustion chamber with the aid of a measuring cylinder. Air was supplied into the chamber at a constant flow rate to achieve a rich air–fuel mixture before the mixture was ignited using a lightning gun through the ignition hole provided on the combustion chamber. As the fuel began to burn, the probe of the emission analyser was inserted into the combustion zone at about 1.5 cm above the outlet duct [37]. The concentration, in mg/m3 of each of the gases present in the detected air emissions was displayed on the instrument’s screen from which it was reported [35, 36]. The air supplied was at a constant flow rate which was achieved at the maximum output pressure of the compressor [35]. The air compressor was powered by electricity so that no additional combustion emission was detected by the analyser. The experimental setup is as shown in the flowchart of Fig. 2. The experiment was conducted for each of the PMS samples collected across the thirty (30) refiners’ camps across the Bayelsa and Rivers States. Three experiment trials were conducted for each sample and the average value is reported in the results section. To ensure quality assurance of the experimental procedure, the following steps were taken:

  1. (a)

    Zero air was used for the combustion to prevent impurities from ambient air, including those from earlier experiments from adding to the reported emission levels for each sample.

  2. (b)

    The probe of the analyser was inserted close to the outlet of the combustion chamber to further reduce the likelihood of the analyser reporting additional pollutants in the ambient air to a negligible value.

  3. (c)

    The experiment for each sample was repeated thrice to replicate the emissions data and accommodate the possibility of measurement error. The triplicate emission data of each pollutant was reported and the average value with the deviation was computed [35, 36].

Fig. 2
figure 2

Flowchart of the experimental setup

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2.5 Comparison with emissions standard and reference laboratory

The measured average concentration of emissions from the experiment were compared to the limits on emissions from stationary sources published by the Federal Environmental Protection Agency FEPA [38], Environmental Guidelines and Standard for Petroleum Industry in Nigeria (EGASPIN) [27], World Health Organization (WHO) [39] and World Bank [40] in other to determine compliance with standards. Furthermore, the measured emission concentrations were compared with similar emission concentrations determined from the combustion of gasoline refined by a government-managed refinery i.e. Kaduna Refinery in Kaduna State as shown in Fig. 3 and reported by Osunnusi et al. [36]. The comparison was made because both studies were conducted with the same experimental design, utilizing the same equipment, and under similar environmental conditions.

Fig. 3
figure 3

Map of Nigeria showing the Kaduna refinery in Kaduna State [13]

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2.6 Estimation of the average daily concentration of pollutants

To assess the extent of atmospheric pollution associated with the use of artisanal gasoline in Nigeria, the daily concentrations of each pollutant resulting from the consumption of artisanal-refined fuel in Nigeria were also estimated by utilising data obtained from the Nigerian Midstream and Downstream Petroleum Regulatory Authority (NMDPRA) statistics on national daily PMS evacuation. NMDPRA [41] reported that an average of 56 million litres of PMS is consumed daily in Nigeria and a safe assumption made that an additional 10% of the official data on fuel consumption is procured from the artisanal refiners and consumed daily. Utilising the emission factor model shown in Eq. 1, the empirical emission rates obtained from this study were multiplied with the activity (\(5.6 \times 10^6 \,{\text{l}}\)) to quantify the average daily artisanal fuel-related emissions [42,43,44].

$$E = A \times EF$$
(1)

where A is activity data in m3/s, EF is the emission factor in g/m3

3 Results and discussion

3.1 Quantified emissions from the use of artisanal gasoline

The concentrations of the gaseous pollutants emitted from combusting artisanal-refined fuel are presented and discussed in this section. The pollutants identified and measured are carbon monoxide (CO), oxides of nitrogen (NOx), sulphur dioxide (SO2), carbon dioxide (CO2) and hydrocarbon (HC) as shown in Table 1. The concentration of CO emitted for the gasoline samples across the randomly selected camps ranged between 78.14 ± 78.10 and 339.70 ± 78.10 mg/m3, with a mean value of 211.50 ± 78.10 mg/m3. The NOx emissions ranged from 2.84 ± 2.51 to 12.63 ± 2.51 mg/m3 with an average of 7.18 ± 2.51 mg/m3. Furthermore, the SO2 emissions ranged from 4.49 ± 8.53 to 31.34 ± 8.53 mg/m3 given an average of 17.51 ± 8.53 mg/m3; CO2 emissions are also as high as 5460 ± 2169.15 mg/m3 for some samples, with mean value of 3011.33 ± 2169.15 mg/m3. For HC concentrations, 111.84 ± 319.40–1286.86 ± 319.40 mg/m3 were measured with an average value of 500.39 ± 319.40 mg/m3. The presence of high CO and HC emissions in each of the artisanal gasoline fuels indicate improper distillation of the crude oil, resulting in the inclusion of large molecular mass hydrocarbon components in the fuel samples [45, 46]. The higher components are less volatile and difficult to combust at low temperatures and within a short combustion period [45]. Some of the heavy compounds that were not completely oxidized are emitted as carbon (II) oxide and other components which escape burning are released as hydrocarbon, either in the form of volatile organics or as fly ash (i.e. particulates) [47].

Table 1 Average concentrations of emissions from the combustion of gasoline (mg/m3)
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3.2 Emission levels compared to air emission standards.

The measured emissions were compared to the limits on emissions from stationary sources published by the Federal Environmental Protection Agency (FEPA), World Health Organisation (WHO), World Bank and Environmental Guidelines and Standards for the Petroleum Industry in Nigeria (EGASPIN) daily limits in other to ascertain the level of compliance. As presented in Table 2, the NOx and SO2 emission levels released during the combustion of artisanal fuel were low and passed the minimum FEPA standard but breached WHO, World Bank and EGASPIN limits. FEPA recommended a maximum value of 1000 mg/m3 and 3000 mg/m3 for NOx and SO2 emissions respectively, which is far beyond the highest NOx and SO2 concentrations measured in this study. The measured NOx and SO2 concentrations are indicators that nitrogen and sulphur contents of the artisanal-refined fuel are minimal and that the artisans’ refining methods are not completely ineffective. The low SO2 concentrations is also as a result of the crude oil grade used for refining by the artisans; crude oil explored in Rivers and Bayelsa States are classified as sweet because it contains low amount of sulphur [48, 49]. The carbon monoxide and hydrocarbon emissions from the combustion of artisanal fuel were quite substantial, with the least emissions being 78.14 and 111.84 mg/m3, respectively. These values are much greater than limits stipulated by FEPA, WHO, World Bank and EGASPIN limits. FEPA did not specify the limit for CO2 because CO2 is a major combustion product of fuel. A low CO2 emission level indicates that the fuel is not properly combusted and explains the reason for high CO and HC emissions which are products of incomplete combustion of fuel [50].

Table 2 Comparison of measured emissions with FEPA, WHO, World Bank and EGASPIN daily standards
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The CO and HC concentrations were correlated across the camps as shown in Table 3 and the results showed the correlation coefficient r between the emission concentrations of CO and HC from the gasoline combusted in this study. CO and HC were the only parameters used to determine r because they are the pollutants of interest, having breached all the emission standards (FEPA, WHO, World Bank and EGASPIN) used in this study. As can be seen in Table 3, seventeen (17) of the selected camps showed positive correlation coefficient between CO and HC while thirteen (13) showed negative correlation between CO and HC. This could be because of the non-standard and unregulated refining techniques and storage systems being used by the artisanal refiners. The average time for complete combustion of 50 ml each of the PMS sampled across 30 camps, as presented in Table 4, is 13 min having a standard deviation of 1 min 15 s, and the mean temperature of combustion is 380.3 °C with a standard deviation of 17.16. The emission of NOx reported in this study is associated majorly with the elemental nitrogen contents of the fuel and partly to thermal formation because the highest combustion temperature for the experiment is 410 °C. According to Habib et al. [51], thermal NOx is formed from atmospheric nitrogen at a combustion temperature of 1300 °C. The emission of SO2 is also associated with the sulphur content of refined fuel products.

Table 3 Correlation coefficient r between HC and CO from the combustion of gasoline
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Table 4 Resident time and average temperature during combustion
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3.3 Emission levels compared to that emitted by fuel products from a licensed refinery

Having discovered that the artisanal fuels met two of the four emissions parameters specified by FEPA, its performance was compared to gasoline samples obtained from the NNPC-operated refinery in Kaduna as presented in Table 5 and shown in Fig. 4. The fuel was combusted under similar conditions as the primary samples of this study [29] reported the average measured concentration for the NNPC-refined gasoline to be in the range of 146.00–264.00 mg/m3 for CO, 767.00–864.00 mg/m3 for NOx, 3.00–4.00 mg/m3 for SO2, 72.03–91.67 mg/m3 for HC and 5400.00–7200.00 mg/m3 for CO2. The mean were 212.67 ± 49.40, 801.33 ± 44.40, 3.67 ± 0.47, 87.31 ± 11.13 and 6600.00 ± 848.50 for CO, NOx, SO2, HC and CO2 respectively. Larger concentrations of the emissions were measured from the NNPC-refined gasoline.

Table 5 Comparison of measured emissions with NNPC-refined emissions (mg/m3)
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Fig. 4
figure 4

Concentrations of the pollutants emitted from artisanal fuel compared with that of NNPC fuel

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for two of the four pollutants considered, namely CO and HC, and much lower concentrations were measured for SO2 and NOx. The NNPC-refined gasoline conformed with FEPA’s NOx and SO2 emissions requirement while CO and HC exceeded emissions limit [29].

NNPC-refined fuel has better combustion performance than the artisanal-refined fuel because the carbon dioxide emissions were higher, and the HC emissions were lower indicating that a larger portion of the hydrocarbon components of the fuel was completely oxidised in the process. The reason for this observation is attributable to more effective refining methods and instrumentation employed by the government-owned facility to separate and purify the petroleum components.

3.4 Air quality index of measured pollutants

The air quality index (AQI) for pollutants was calculated by dividing the measured concentrations by the national standard i.e. FEPA standard and multiplied by 100 to get the AQI as shown in Eq. 2. The AQI obtained for the measured concentrations are tabulated in Table 6 and its classification shown in Table 7. As shown in the table, the AQI of CO concentrations was calculated as 2115.00 and HC concentrations calculated as 500.39 based on FEPA limits. These values are above the 301 value which is considered as hazardous. Therefore, exposure to such levels of concentrations during consumption is considered hazardous for public health. Furthermore, AQI of NOx and SO2 concentrations were also calculated based on the FEPA standard and found to be 2.05–0.72 and 58.37–0.58 respectively. AQI of NOx is within 0–50 which is classified as good for public health while AQI of SO2 which is within 51–100 is classified as moderate for public health [52].

$${\text{AQI}} = \frac{{\text{pollutant concentration}}}{{{\text{standard}}}} \times 100$$
(2)
Table 6 Air quality index of measured pollutants
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Table 7 Air quality index classification [52]
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3.5 Air and health quality implications of combusting artisanal fuel

Some criteria air pollutants, namely CO, NOx, SO2 and HC were identified from the combustion emissions of artisanal-refined gasoline and the concentrations of CO and HC breached FEPA permissible limits while NOx and SO2 are within FEPA permissible limits. All the pollutants concentrations breached EGASPIN, WHO and World Bank permissible limits. As shown in Fig. 5, the estimated daily concentration of the pollutants added to the atmosphere from the consumption of the illegally refined gasoline was determined by assuming a safe assumption that 10% of the officially reported daily consumption rate of commercially available gasoline in Nigeria is procured from the artisanal refiners and calculated using Eq. 1. The lower and upper band in Fig. 5 are the camps whose gasoline fuel give the lowest and highest concentrations of emissions respectively. On average, 1.91 kg, 30.6 kg and 7.2 kg of CO, CO2 and HC emissions respectively, associated with the use of gasoline from each artisan’s camp are added to the atmosphere daily. Ikezam et al. [34] claimed that each state across the Niger Delta region has dozens of these illegal refiner camps operating within its geographical space. The estimated concentrations of CO and HC indicate that the availability of artisanal fuel could pose a major threat to ambient air quality, especially in locations where the fuel is continuously used. The presence of carbon monoxide in the atmosphere triggers certain chemical reactions resulting in the formation of ozone, a compound that contributes to climate change [53]. CO also contributes to acid rain which destroys the ecosystem: plants, animals and the built environment [54]. Montero‐Montoya et al. [55] further highlighted that hydrocarbon emissions in the atmosphere are known to add to the concentrations of volatile organic compounds (VOCs) in air. VOCs are also known to deplete the ozone layer and increasing global warming intensity as reported by Soni et al. [56]. Hydrocarbons restrict the ability of plants to make their food through photosynthesis, and negatively affect the growth of the plants [57].

Fig. 5
figure 5

Estimated average daily concentration of pollutants generated from consuming artisanal fuel

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Moreover, added to the aforementioned air quality impacts resulting from the emissions of CO and HC emissions is the global warming effect of carbon dioxide, the largest gaseous emission associated with fossil fuel combustion. Carbon dioxide also contributes significantly to the formation of smog, which impact visibility and human respiratory health [58]. Regarding health implications, CO reduces oxygen delivery to the body organs and tissues by poisoning breathable air which when inhaled forms complex with the haemoglobin component of the blood resulting in death on most occasions [59]. Other human health conditions associated with CO presence in air include chest pain, heart diseases and other cardiovascular problems [60]. Furthermore, human exposure to elevated levels of hydrocarbons emissions, both in the form of volatile organics and particulate matter, through inhalation can increase the risk of respiratory complications and cancer-related diseases [61].

The availability of artisanal refined fuel makes gasoline accessible to people in rural areas who often utilise vehicles and devices with poor combustion technologies, increasing the concentrations of the pollutants emitted as the fuel is used. This deficiency further aggravates the likely air quality and human health impacts of the associated emissions.

4 Conclusion

This study quantified the pollutants generated from the combustion of artisanal refined petroleum products in the Niger Delta region of Nigeria and reported elevated concentrations of all major gaseous emissions. Compared with all the set limits for emissions from stationary sources, HC and CO were above the allowable limits; although SO2 and NOx emissions were found within FEPA standard but they also breached WHO, World Bank and EGASPIN limits. The HC and CO emissions are considered hazardous and could pose a major threat to the ambient air quality in the region where artisanal-refined fuel is predominantly utilized and consequently several health implications. The study concluded that the continuous consumption of artisanal refined gasoline in its current form could pose great danger to the environment and worsening the global warming issue. Improvement in methods and technology employed by the artisans is highly encouraged while the deployment of emission control equipment, not only in the industrial sectors but also in homes where fuel is combusted for heating and lighting purposes is a necessity.