## 1 Introduction
Thereby, NS is available at a low cost and is used as a potential raw material for biodiesel production, in which NS-based biodiesel has been used as a potential replacement fuel for compression ignition engines [26; 28]. NS is a non-edible oil seed (constituting \(\sim\)37-50% oil content) grown largely in Ethiopia and India, and on a low scale in different regions in Asia, America, and Africa [29; 30]. Unless the seeds are plucked from plants, they are currently used forbird feeding in many countries [31]. The plants are grown in a low acidic, wide range of less fertile (waste area) soil conditions, roadsides, coastal areas, and so on [32]. NS oil possesses medicinal value, which acts as a partial replacement for olive and sesame seed oil utilized in pharmaceutical industries [33]. Furthermore, the byproducts are used as personal care products (soaps, perfume, etc.). NS contains higher oil content that could offer better protein and nutrition [32]. Niger plants offer flowers in July and collectible seeds in August. Niger seeds yield \(\sim\)200-300 kg/hectare from plants. Niger seed contains higher fatty acids, possessing 75-80% linoleic acid and 7-8% stearic and palmitic acids [34]. Indian varieties had 25% oleic acid and 55% linoleic acid. The niger-seed-based feedstock contains free fatty acid (FFA), whose composition and acid value are estimated before applying the transesterification process. For biodiesel production, the acid or alkali catalyst transesterification process is often advantageous, wherein an oil containing triglycerides reacts faster with alcohol at a low cost without a high amount of heat requirement [35]. Higher values of free fatty acids and acids result in saponification, which deactivates the catalyst, leading to soap formation and thereby reducing the biodiesel yield [36]; therefore, the determination of free fatty acid and an acid value of oil is indeed essential before the initiation of the transesterification process. It was observed from the above literature review that not much research effort is being made to maximize the biodiesel yield from niger seed oil via the transesterification process. The attempt to optimize the biodiesel yield for various raw materials processed through the transesterification process is presented in Table 1. Response surface methodology (RSM) based on central composite design and BBD were applied to maximize biodiesel yield after determining the optimal set of transesterification variables. It should be noted that the use of used cooking oil reduces the process costs by 60-70% [37]. The authors [37; 38] applied the central composite rotatable design (CCRD) for experimentation and analysis unless practical constraints dictate that the rotatable design should not be used for experimentation and analysis [39]. Catalyst concentration was determined to be the dominant factor in the transesterification process towards conversion to biodiesel yield [40; 41; 42]. The reaction temperature showed a significant impact on biodiesel production [35; 43; 44]. The methanol ratio was identified as an important factor contributing to the conversion of biodiesel yield [45; 46]. It has been observed that the KOH catalyst has led to a higher conversion of biodiesel than that obtained for the CaO catalyst [47; 48]. Note that the catalyst concentration is the most important contributing factor for the KOH catalyst, whereas the most important contributing factor was reaction time for the CaO catalyst during the conversion of waste cotton-seed cooking oil to biodiesel [47]. From the analysis of the above literature, it was observed that, although many experiments and optimization of transesterification parameters were performed, the optimal conditions and influence of the parameters on biodiesel yield are different from each other. This is due to the existing collective differences in the raw material and their properties.