Introduction

Energy acts as an important role in the economic, societal, and sustainable development of a country. In the present perspective, all over the world, fossil fuels bring about 80% of energy supplies (Sharma 2016). For several years, the fast escalation in the world populace has triggered the energy required to increase multiple folds, which has strained the conventional sources of energy all over the world (Nazar et al. 2021). Moreover, fossil fuel usage increases environmental problems such as climate change, air pollution, and discharge of greenhouse gases (Abdeshahian et al. 2016; Bariani et al. 2020). Current power production has a massive economic load on the state’s budget due to the import of oil to sustain the current energy mix, and the condition is increased by the fast reduction of gas resources. The severe energy crisis also causes adversative economic and social impacts. This directly affected industrial, commercial, and population activities (Naseem et al. 2020). Sustainable energy production and distribution of power to meet the existing and forthcoming industrial and local requirements in Pakistan will depend on maximum production from the diverse energy assets to create major inputs to the supply chain (Ahmad et al. 2015; Raheem et al. 2016).

Pakistan is placed seven in positions of coal comprising country extending from sub-bituminous to brown lignite coal (Malkani 2012). These assets contain around 185 billion tons; of that, 175 billion tons are situated in the Thar Desert of Sindh domain and are called Thar coal (Muneer and Asif 2007). Thar lignite coal can generate 100,000 MW of electricity every year for 200 years (EIU Report 2017). Biomass is a renewable and environmentally pleasant asset of energy that is mostly derivative from agronomic waste (Saini et al. 2015; Abaide et al. 2019). Biomass usage as an alternative energy asset has progressively gained attention because of its low cost and easy accessibility as a by-product (Shahbaz et al. 2020). Approximately 0.30 tons of biomass residue is annually generated in the country with an approximately energy capability of 166.72 TWh/year (Irfan et al. 2020).

All over the world, rice is the main crop that is made in rice husk through the rice milling practice in huge volume and comprises small contented of sulfur and other pollutants (Gautam et al. 2020). Rice husk comprises organic and inorganic substances. Paddy lands might come to be contaminated by the accumulation of numerous, i.e., fertilizers; releases from the rapidly spreading industrial lands; organic solid waste; and wastewater irrigation schemes (Akram et al. 2018). The use of biochar in the soil might play a vigorous part in decreasing the adversative effects of climate change and dangers to sustainable crop production (Tauqeer et al. 2022). Annually in the world, around 134 Mt of rice husk is formed and around 90% of residual is combusted outdoor or discarded into ponds and canals (Quispe et al. 2017). Due to its ease of transport and low cost, rice husk takes too much popularity as boiler fuel. Hence, rice husk is the most productive biomass asset in Pakistan (Hussain et al. 2015). Due to the constant escalation in oil prices, at present, the consumption of coal and other viable assets together with biomass as an energy source has revealed the potential for cost-effective and economic benefit (Xu 2013). Both coal and biomass are carbonaceous resources, created from floras and have similar major fundamental ingredients (Boerrigter and Rauch 2005). Coal blends are used for low-grade coals, to increase the coals ignition performance, attempt to enhance the flexibility of fuel type, and comply with the conditions of emission regulation (Arenillas, et al. 2004). Methods like combustion, gasification, and pyrolysis might be used to produce electricity from coal-biomass blends (Chieng and Kuan 2020). The combustion method is the utmost broadly used worldwide (Cardozo et al. 2016).

Co-combustion is an environmentally friendly practice for the use of low-grade coal to reduce CO2 discharges and collaborate to decrease NOx and SOx pollution (Siddiqi et al. 2018). The blending proportion of biomass up to 10 to 30% is best for the decrease of precarious pollutants (Zhong et al. 2006). The blending proportion of around 40% of biomass drops the temperature and decreases energy efficacy by around 8% (Suksankraisorn et al. 2004). Furthermore, the addition of biomass for power creation in combination with coal might make the procedure further sustainable (Kanwal et al. 2021). The United Nations Environment Sustainable Development Goal (SDG 7) also confirms admittance to reasonable, consistent, sustainable, and modern energy for all. The environmental effects of energy schemes contain local, regional, and worldwide emission pollution from the burning of fuels, climate change, and effects on the veracity and stability of diverse environments. Limited literature is existing that explained the emission characteristic of a furnace, co-combustion with Thar lignite coal,and the utmost significance for the usage of rice husk as a blended fuel in a furnace with emission properties. Current research explores in detail the coal-rice husk blending effects on coal configuration and emission gases and also helps to find out new thermo-chemical characteristics data of Thar coal and rice husk to deliver the solution of energy shortage by an operative and effective consumption of Thar coal reserves.

The study objective is to inspect the physicochemical characteristic of low-grade Thar lignite coal and their blends of rice husk biomass for burning. Characterization of coal and rice husk and their blends in several mixtures such as 90:10, 80:20, and 70:30 (Coal: RH) by weight % was correspondingly investigated and also to find out the influence of these combination proportions on discharge gases such as NOx, SO2, CO, and CO2. This work shows a significant part in saving our natural assets and meeting industrial energy supplies. This investigation is a guideline for co-firing of Thar coal and rice husk efficiently with reduced emissions and support to fulfill the requirement of the UN Environment SDG 7 goal.

Methodology

Sample selection and preparation

Lignite coal gained from Thar Block-II and rice husk samples collected from locally Sindh province Pakistan were tested. Samples were pulverized to 60 meshes particle size by a local ball mill. Sample assortment, preparation, and investigative procedures were accompanied according to the ASTM techniques. The samples were determined for air-dry loss in an air-drying oven (ASTM D-3302) as well as crushed and pulverized to 60 meshes (ASTM D-2013).

Coal and rice husk biomass analysis

The research data on As-determined (Ad) basis was changed into As-received (AR) basis (ASTM D-3180). Proximate analyses of coal, rice husk, and their blends were conceded out rendering to the ASTM standard procedure (D-3172–5). In proximate analysis, muffle furnace Volcan USA (Model A-550) was used for volatile matter, moisture, ash contents, and fixed carbon. The sample moisture was measured in a dehydrating oven at a temperature 105–110 °C by the sample weight loss. For volatile matters, cover crucible comprising the residue after moisture test then heat at a constant temperature 950 ± 20 °C for accurately 10 min in a muffle furnace. Ash contents were deliberate at 750 °C by the remains left after heating the sample until the persistent weight was attained. The fixed carbon value is intended by deducting the sum of the percentages of moisture, volatile matter, and ash from 100.

Ultimate analysis of coal, rice husk, and their mixtures were conceded out by ASTM procedure (D-3176, D-5373). The ultimate analysis comprises hydrogen (H), carbon (C), nitrogen (N), sulfur (S), and oxygen (O2) substances. For the examination of C, H, N, and S substances, the Elementar CHNS analyzer is used. The machine is assembled upon gas chromatographic column and thermal conductivity detector. The analyzer is completely computer companionable and fixed with an auto-sampler. Oxygen is measured by subtracting from 100 the sum of the other elements of the ultimate analysis. Heating/calorific values of coal, rice husk, and their mixtures have been determined rendering to ASTM standard procedure (D-2015) bomb calorimeters.

Coal and rice husk biomass blending

Coal-rice husk blend samples of several proportions were prepared. Rice husk was assorted with coal at fraction of 10%, 20%, and 30% correspondingly. In this method, three altered samples of mixtures were made, viz., CRh-1 comprises 90% coal and 10% rice husk, CRh-2 comprises 80% coal and 20% rice husk, and CRh-3 comprises 70% coal and 30% rice husk. Coal samples were assorted in diverse proportions shown in Tables 1 and 2.

Table 1 Proximate analysis of Thar lignite coal, rice husk, and their blends
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Table 2 Ultimate analysis of Thar lignite coal, rice husk, and their blends
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Combustion and emission analysis

Harmful emissions such as CO, CO2, NOx, and SO2 were examined by co-firing in muffle furnace Volcan USA (Model A-550) and Testo flue gas analyzer (Model-350) for emissions monitoring. At furnace temperature around 850 °C coal and rice husk blend, samples were heated. By co-firing, emission was discharged from the stack of the muffle furnace. The probe of the gas analyzer was introduced into the furnace exit. Emissions were analyzed using a flue gas analyzer with the capability to analyze CO, CO2, NOx, and SO2 emissions as per USEPA Standard methods (CTM-034 and 6C).

Results and discussion

Rice is cultured generally in Punjab and the Sindh provinces of Pakistan. The production of rice husk in Sindh is around 23110 tones/year (Iqbal et al. 2018). Lignite coal was obtained from Thar Block-II (Fig. 1).

Fig. 1
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Flow chart showing sequence of coal and rice husk sample preparation and analysis

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Coal and rice husk proximate analysis

As displayed in Table 1 and Fig. 2, the coal comprises higher moisture content (47.30%), fixed carbon (18.12%), and calorific value (3687.60 kcal/Kg) while lower the volatile matter (29.13%) and ash (5.45%). However, rice husk is higher in the volatile matter (61.98%), lower in fixed carbon (10.57%), moisture (12.76%), and higher ash (14.69%).

Fig. 2
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Proximate analysis of Thar lignite coal, rice husk, and their blends

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Coal and rice husk ultimate analysis

Ultimate analysis of the coal sample gave higher carbon content of 36.73%, hydrogen at 7.53%, and sulfur at 1.51% and lower nitrogen at 0.58%, as shown in Table 2 and Fig. 3. The rice husk sample offered a lower 34.23% of carbon, 5.78% of hydrogen, 0.18% of sulfur, and higher 1.31% nitrogen. The basic configuration of the rice husk fluctuates depending on its source and environmental circumstances.

Fig. 3
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Ultimate analysis of Thar lignite coal, rice husk, and their blends

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Coal-rice husk blended samples proximate and ultimate analysis

The diverse samples of coal-rice husk blend CRh-1, CRh-2, and CRh-3 were evaluated. CRh-1 gave higher moisture content of 43.57%, fixed carbon of 16.20%, and lower ash of 7.0%. CRh-3 gave a higher volatile matter content of 45.02% and lower moisture of 34.11% as shown in Table 1 and Fig. 2. Coal-rice husk blends ultimate analysis was conceded out, and CRh-1 gave a higher amount of carbon content 36.23% and a lower amount of hydrogen 6.36%, nitrogen 0.63%, and sulfur 1.39%. It was observed that the overall sulfur percentage declined in sample CRh-3, 0.99%, whereas hydrogen and nitrogen content is increased as revealed in Table 1 and Fig. 3.

Emission analysis coal-rice husk blends

An emission study is a portion of a burning system conceded out for enhancing the fuel budget, decrease of adverse emission discharges and protecting the fuel-burning device. Burning inspection initiates with the quantity of exhaust gas concentrations. Flue gas discharges were conceded out at the laboratory in a muffle furnace for combustion and emissions, flue gas analyzer was used for O2, CO2, CO, SO2, and NOx as shown in Table 3 and Fig. 4. Different gases are discharged in flue gases, which show the utilization of O2 in the oxidation reactions throughout the burning, which caused the carbon bonds breakdown and CO2 production. The sulfur contented in coal and biomass act with O2 to produce SO2, whereas the nitrogen contented generally acts with O2 to produce NOx (NO, NO2) (Kanwal et al. 2021).

Table 3 Emission analysis of coal-rice husk different blends
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Fig. 4
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Coal-rice husk blends emission

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Oxides of carbon emissions

The coal and rice husk emissions analysis from the co-combustion is shown in Table 3 and Fig. 4. The co-burning of 90:10% (CRh-1) coal and rice husk biomass showed a CO and CO2 value of 1031 ppm and 6.49% respectively, whereas 70:30% (CRh-3) coal and rice husk ratio made CO 789 ppm and CO2 9.96%. Once the CO value was high, the CO2 value was low. The reduction in CO is because of the transformation of CO to CO2 with a temperature rise. Also, biomass combination caused less carbon type resulting in the CO discharges being decreased with a rise in blending fraction and increase in furnace temperature. Furthermore, the occurrence of rice husk oxygenates would have helped the conversion of CO2 from CO; hence, CO discharges might be relatively lesser (Akhtar et al. 2018

Oxides of nitrogen emissions

The maximum NOx level was reported in CRh-1 (89 ppm) and the minimum in CRh-3 (34 ppm) as shown in Table 3 and Fig. 4. Nitrogen oxides (NOx) are the most significant emissions resultant from the burning of fuels. Characteristically main NOx discharges from co-firing are NO and NO2. NOx discharges rely on the occurrence of nitrogen in the coal, air (O2), and co-firing course circumstances (Kumar 2017). Furthermore, a 90:10% (CRh-1) blend of coal and rice husk formed more NOx discharges as compared to 70:30% (CRh-3). Hence, the blending of coal-rice husk would decrease NOx production from vent gas (Munir 2011).

Oxides of sulfur emissions

SO2 discharge relies upon the occurrence of sulfur content (Mittal et al. 2012). It is evaluated that all the existing coal sulfur is transformed into SO2 discharges in the course of co-firing. Coal-rice husk blends discharged SO2 in CRh-1 (828 ppm), CRh-2 (649 ppm), and CRh-3 (169 ppm) respectively as shown in Table 3 and Fig. 4. Overall, the SO2 discharges were considerably reduced by increasing the rice husk biomass ratio. This reduction in SO2 discharges according to the blending proportion might be accredited to the dilution influence of rice husk in coal. Additionally, the rice husk ash comprises a substantial amount of CaO that might be stimulating to fix SO2 as CaSO4 (Akhtar et al. 2018).

Conclusion

Thar Coal Block II comprises higher content of moisture and sulfur, while lower in the volatile matter and ash. However, rice husk is higher in volatile matter, and lower in moisture and sulfur with higher ash. By adding biomass rice husk in Thar coal, its basic configuration changed and provided lesser sulfur and other content as related to the coal. This study explores the coal-rice husk blending effects on coal configuration and emission gases. It was observed that the blending fraction facilitated decreased discharges of SO2 and NOx. A decline in SO2 and NOx discharges was found by increasing the blending ratio. NOx and SOx emissions reduced from 89 ppm (CRh-1) to 34 ppm (CRh-3) and 828 ppm (CRh-1) to 169 ppm (CRh-3) respectively by increasing the blending percentage from 10 to 30%. Samples of coal and rice husk blend 70:30% (CRh-3) were found to be feasible for the power production point of opinion for the reason of low emissions gases, high calorific value, and volatile substances and low moisture and ash contents as related to other blended samples. In addition to that, low sulfur was observed in this blend, which marks it a possible contestant for energy production in environmentally friendly conditions. Coal-rice husk blends provide effective burning and decrease NOx and SOx discharge to resolve the environmental problem of coal-related emissions. Hence, the usage of rice husk might be a beneficial selection to decrease discharges of SO2 and NOx throughout burning and support fulfilling the requirement of the UN Environment SDG 7 clean energy goal. The utmost advantageous and effective tool for co-combustion energy creation is circulating fluidized bed combustor (CFBC) as they allow cleaner and further effective coal utilization to decrease emissions.