1 Introduction

In the past years, fossil fuels have been the cheapest source of energy, justifying why it is the most used around the world. However, the price of fuel has reached records triggered by geopolitical conflicts, rising demand, and environmental concerns [1, 2]. In contrast to the present situation, where the fuels demand depends majorly on a single source, a flexible system composed of multiple possibilities is attractive for a long-term solution [1]. Therefore, the need of studying alternative energy sources has been expanded to replace fossil-based with green fuels.

In this context, biomass has been highlighted by several researchers as a potential solution for this issue. Biomass resources refer to all organic materials, such as wood and wood waste, energy crops, aquatic plants, crops, animal wastes, sawdust, manure, sewage sludge, and some industrial and household organic wastes that can be used for biofuels production [3,4,5]. In addition, another purpose for biomass also lies in the production of chemical products that can have a cheaper price than petroleum-based chemicals [1, 2]. The dependence on petroleum reduction is mainly stimulated by the low cost, which can vary by feedstock, conversion process, the scale of production, and region. Thereby, the objective is to produce inexpensive biomass that can be used to make a range of fuels, chemicals, and other materials that are cost-competitive with conventional commodities [6]. In addition, the environmental appeal is also taken into account, since the photosynthetic biomass consumes CO2, enabling it to reduce or neutralize greenhouse gas emission, and biofuel consumption also restricts SOx and NOx released into the atmosphere [1, 7, 8]. The algae biomass, especially on the micro-scale, has been extensively researched over the years for producing third-generation biofuels [9,10,11]. The microalgae use advantages are related to the non-competition with human food production, the possibility of using wastewater and gas flue as a source of nutrients and carbon, and also the higher photosynthetic efficiency, which leads to higher growth rates, biomass production, and CO2 mitigation [12,13,14]. However, economic analyses conclude that microalgae biofuels are not competitive compared to conventional fuels and have many challenges to achieve marketable low-cost [10, 13]. Consequently, further studies in the field are welcome to guide researchers to a future solution aiming to enable the process on an industrial scale.

Different thermal routes can be taken to convert biomass into biofuels, such as combustion, gasification, pyrolysis, and hydrothermal liquefaction (HTL) [7, 9]. The direct combustion of biomass releases hazardous products, such as ammonia and NOx [2], losing the advantages in the environmental aspects. Gasification is a process of partial oxidation in high temperatures to produce syngas. Pyrolysis involves the production of biocrude, or bio-oil, syngas, and biochar from dry biomass in the absence of air at medium to high temperatures [5, 12]. However, the HTL process has been widely studied as it is not necessary to dry the biomass as a pretreatment, saving investments, energy, and time in the operation, and also because it produces the biocrude as the main product [9, 12, 15]. HTL is a technique that works in the presence of a solvent at temperatures between 250 and 550°C and pressures 5–28 MPa with or without added catalyst during a residence time between 10 and 60 min. These severe operating conditions cause dehydration of the biomass components breaking into small molecules which are reactive and can re-polymerize into oily compounds, dismissing the need to pre-dry the matter [12, 16, 17]. The biocrude produced by the liquefaction process has an oxygen content of 10–20 wt.% and a heating value of about 30–36 MJ/kg, which can be further improved to liquids similar to diesel and jet fuel [3, 6, 7]. The biocrude and derivatives dissemination does not require larger modifications in the transportation infrastructure that is already established in most countries, nor in the internal combustion engines, since biocrude is a liquid fuel similar to currently most-used fuels. Additionally, the biorefinery concept would also be similar compared to the existing petrol-based facilities, which would aim to transform renewable biomass into fuels and chemicals [1].

The description of the current state of the art and future perspectives of HTL has been studied by several researchers over the years [11, 18,19,20]. However, only a few based their information on systematic bibliometric analysis [21, 22], and no study, until this moment, has worked focusing on the algae relevance in the HTL process aiming to obtain biocrude using multiple scientometrics tools of investigation and visualization. Therefore, the present work aimed to use the bibliometric analysis to evaluate the HTL of algae focused on biocrude production. The papers published, countries’ contributions, author's relevance, journals and institutions’ prominence, and keywords occurrences are also discussed in the present study to describe the current state of the art, hot spots, and bottlenecks of the technology in a biorefinery approach.

2 Methodology

The bibliometric study used scientific output data collected from Web of Science (WoS) (Clarivate Analytics®, Boston, USA) on May 24, 2022. Figure 1 shows the methodology flowchart.

Fig. 1
figure 1

The methodology adopted to conduct the bibliometric study

Aiming for a broad discussion on the topic, two search queries (SQ) were set up for the selection of documents as the first step of the bibliometric study. The first SQ was focused on the production of biocrude from liquefaction, according to the following search equation: TI=(liquefaction AND (biocrude$ OR crude$ OR oil$ OR biooil$)) OR AB=(liquefaction AND (biocrude$ OR crude$ OR biooil$ OR oil$ )) OR AK=(liquefaction AND (biocrude$ OR crude$ OR oil$ OR biooil$)) OR (TI=liquefaction AND AB=(biocrude$ OR crude$ OR oil$ OR biooil$)) OR (TI=(biocrude$ OR crude$ OR oil$ OR biooil$) AND AB=liquefaction), where the symbol “TI” means the title field, “AB” abstract field, and “AK” authors’ keywords field.

Secondly, verifying the high relevance of the algae (on the micro or macro scale) in the process, a second search was performed aiming to restrict the liquefaction process that produces biocrude using algae as a feedstock, according to the following the SQ: TI=(liquefaction AND *alga$ AND (biocrude$ OR crude$ OR biooil$ OR oil$)) OR AB=(liquefaction AND (microalga$ OR alga$ OR macroalga$ OR spirulina OR chlorella) AND (biocrude$ OR crude$ OR biooil$ OR oil$)) OR AK=(liquefaction AND (microalga$ OR alga$ OR macroalga$) AND (biocrude$ OR crude$ OR biooil$ OR oil$)) OR (TI=liquefaction AND AB=((microalga* OR alga* OR macroalga*) AND (biocrude$ OR crude$ OR biooil$ OR oil$))) OR (TI=( microalga* OR alga* OR macroalga*) AND AB=(liquefaction AND (biocrude$ OR crude$ OR biooil$ OR oil$))) OR (TI=(*crude OR *oil$) AND AB=(liquefaction AND (microalga* OR alga* OR macroalga*))).

The search queries were structured using strategic keywords that are related to the research field and a preliminary analysis of the WoS database was performed aiming to validate the SQ. For the first analysis related to the liquefaction process aimed to produce biocrude, the search equation included the keywords referring to the process, which was “liquefaction,” since it encompasses its variations, and to the product, such as “crude$” or “biocrude$” or “oil$” “biooil,” since the product has naming variations. We used the “$” to include plural variations that were mentioned by some authors. These words were considered to appear in the title, abstract, and the author’s keywords. Some WoS research categories were excluded from the database since the subject does not agree with the selected topic: Pharmacology pharmacy, Economics, Toxicology, Nutrition Dietetics, Public Environmental Occupational Health Acoustics, Automation Control, Systems, Chemistry Medicinal, Engineering Biomedical, Integrative complementary Medicine, Radiology, Nuclear Medicine & Medical Imaging, Entomology, Communication, Dermatology, Cell Biology, Physiology, Law, Biology, Optics, Management, Biodiversity Conservation, Oceanography, and Engineering Ocean. In addition, some other off-topics were also excluded from the topic field using the keywords “earthquake,” “soil liquefaction,” and “pipeline.”

Similarly, in the second search query related to the liquefaction using algae in the process, the keywords used were “liquefaction", and “crude$” or “biocrude$" or “oil$" “biooil". In addition, the algae-related keywords were added, such as “microalga*” or “alga*” or “macroalga*.” The search query was also refined to exclude off-topic papers, which had been analyzed manually one by one, and for their exclusion, a “not” function was added with the boolean operator “or” considering the following keywords in the title: “livestock,” “carpet,” “phenylalanine,” “urine,” “aliphatic,” “fungi,” “azolla,” “pyrolytic,” “hartbeespoort,” “cryptococcus,” “quinoline,” “minamisoma,” “atmosphere,” “soybean,” “turbocharged,” “hemin,” “asphalt,” “petroleum way,” “hydrophobic,” “glycerol fermentation,” “food waste using,” “hydrogen production in,” “pyrolysis of microalgae,” “supercritical carbon dioxide extraction,” “lemon-peel: parameter,” “Baltic,” and “isoprenoids.” Additionally, the Derwent Innovations Index (DII) database was evaluated to record the patents in the subject, according to the following the SQ: TI=(liquefaction AND *alga$ AND (biocrude$ OR crude$ OR biooil$ OR oil$)).

In both selected databases, the document type was limited to research articles, proceeding papers, book captures, and reviews published in the English language in the timespan from 2000 to 2022. The bibliometric parameters investigated in the study were: year of publication, countries, authors, journals, the affiliation of the authors of the documents, and author’s keywords. The number of citations of each paper is provided in the .txt document extracted from WoS, and in addition, the authors’ names were corrected manually in the .txt since some redundancy was found, in order of words, different names and abbreviations for the same author. Then, the data was exported to BibExcel [23], the software used to organize the information and calculate the author’s, journal’s, and institution’s local h-index. The local h-index score is a measure of the number of papers published in the selected field and the number of citations according to a formula where the h score is defined when the given author published at least h articles that were cited at least h times [24]. Although BibExcel has no visualization option, it allows users to easily link the data imported from WoS with other software, such as Excel, SPSS, and Gephi [25]. In Excel, some missing data, such as year of publication, institution, and country, were completed by accessing the paper and obtaining the information manually. In this context, PowerBI was used for interactive visualization [26].

The information was also compiled by VOSviewer 1.6.17 using default options to generate networks of the author's co-authorship and the keyword’s co-occurrence. In the networks, the size of the label and the circle of an item is determined by the weight of the item according to the number of publications or occurrences, the distance between two items approximately indicates its relatedness, and the lines between items represent links of co-authorships or the keyword's co-occurrence [27]. Aiming to better analyze the keywords, the network and table were settled excluding the words used in the search equation, and a thesaurus file was created to avoid redundancies between words (e.g., “microalgal” and “microalgae”) replacing all similar words with a representative one. Finally, the historic data was examined considering the process with and without algae, and future trend topics were also discussed.

3 Results and discussions

The search results for the liquefaction process that aims to produce biocrude are discussed in this topic considering 2393 papers published since 2000. The documents type of the papers in the selected database was analyzed and it was found that 85.29% of the papers are articles, 7.52% are reviews, and 7.15% are proceedings papers. “Biomass,” with 204 citations, is the most occurred word in the selected database, representing the main feedstock used in the process. Thus, some relevant reviews were published under this theme, such as Alonso et al. (2010) [1], Zhang et al. (2006) [8], and Toor et al. (2011) [9]. However, “biomass” is a general and non-specific term that englobes different types of materials and makes it difficult to distinguish which types of biomasses are being used. On the other hand, besides being the second most occurred keyword with 198 occurrences, “microalgae” specify the most used raw material. Moreover, the presence of the words “algae” and “macroalgae” is also noticed in the network, with 76 and 29 occurrences respectively. It is clear the eminent importance of this feedstock in the process since its sum is given in 303 occurrences, being more expressive than the keyword “biomass.” In addition, other feedstocks have been used in the HTL, such as lignin and lignocellulosic biomass, with 45 and 28 occurrences respectively [28,29,30,31,32], and sewage sludge, with 48 [16, 33,34,35,36]. Nevertheless, algal biomass still is the main feedstock used for biocrude production. For a better understanding, this paper analyzed the literature which focuses on algal biomass, and the results are presented below.

Given the restricted search to select articles relating to the liquefaction process using algal biomass as feedstock, 607 papers were found since 2000, in other words, 25.36% of the articles published regarding the liquefaction process deal with algal biomass. The database is composed of 87.97% of articles, 9.06% of reviews, and 97% proceedings papers. Figure 2 shows the publications from 2000 to 2021. The first study recorded was published in 2001 [37] and, until 2008, the scientific production in the area was constant and incipient, not exceeding two articles published per year. However, from 2009 started a new phase characterized by an accentuated rising, reaching 79 productions in 2017. Thereafter, the number of publications did not exceed the maximum reaching 77 papers published in 2017 and 75 papers published in 2021. Even so, there are still great expectations for researchers to work in the area given the observed rising behavior drawn in Fig. 2, showing that the technology is not concrete and mature in the literature yet.

Fig. 2
figure 2

The annual publications from 2000 until 2021 about the liquefaction process that aims to produce biocrude using algae as a feedstock

Some studies are highlighted by the number of citations, such as Biller and Ross [12], with 690 citations. The authors worked with the liquefaction of a range of biopolymers commonly found in microalgae at 350 °C for 1 h with a heating rate of 10 °C/min in the presence of 1 M base Na2CO3, 1 M of the organic acid HCOOH, or pure distilled water. Then, they estimated the biocrude yield based on the algae biochemical characterization. The modeling has been validated using Chlorella vulgaris, Nannochloropsis occulata, Porphyridium cruentum, and the cyanobacteria Spirulina under the same conditions. From these results, Biller and Ross (2011) understood the influence of the biochemical content of algae on liquefaction yields, which follow the trend of lipids > proteins > carbohydrates.

Brown et al. (2010) [38] is also one of the most cited authors, with 512 citations. They converted, for the first time, Nannochloropsis sp. into biofuel via hydrothermal liquefaction (from 200 to 500 °C for 1 h). The authors also focused on the analysis of the oil and gas phases. As result, a moderate temperature of 350 °C led to the highest biocrude yield of 43 wt %, the major biocrude constituents include phenol and its alkylated derivatives, while in the gas phase, CO2 and H2 were the most abundant component. While these most-cited papers are related to experiments involving microalgae, some recent studies focus on understanding how the conversion can be optimized using other devices for biofuel production with better quality. For example, Wang et al. (2021) [40] used the macroalgae Enteromorpha clathrata to compare a two-step reaction method with a single-step reaction. The two-step is characterized by the reaction of the raw material with a solvent at a lower temperature (200 °C) and, after 30 min, the mixture obtained undergoes a second reaction for 30 min at 300 °C. Results revealed that the two-step method can ensure a high biocrude yield, while preventing the occurrence of side reactions caused by long-term high-temperature reactions, produce a low solid residue rate, and improve biocrude quality. Also, the authors analyzed the relevant possible reaction paths for the chemical structures aiming to understand how the conversion occurred.

3.1 Countries’ contributions

The country’s analysis also shows the high influence of China and the USA on the topic. As shown in Fig. 3a, China is the country with the most contributions, with 201 publications (33.11%) and local h-index of 46. The USA is the second most relevant country in this field, with 197 (32.45%) with local h-index of 59 and the next country in the ranking is India with 59 contributions (9.72%) and an h-index of 21.

Fig. 3
figure 3

a 10 most relevant countries and b Chinese and American publications considering the liquefaction process that aims to produce biocrude from 2000 to 2022

The USA and China had the first contribution in 2009 [41, 42] as shown in Fig. 3b. The USA had an earlier and rapid increase in contributions when compared to China until reaching the peak in 2017 with 27 publications. However, in the past four years, China has exceeded the number of publications, reaching a peak in 2020 with 28 publications. In 2021, the USA and China presented almost the same contribution, with 22 American and 23 Chinese papers published, respectively. Brown et al. (2010) [38] and Vardon et al. (2011) [15], which study focuses on the HTL of Spirulina algae, swine manure, and digested sludge at 300 °C for 30 min, are the most-cited American articles published, with 512 and 416 citations respectively. On the other hand, Zhou et al. (2010) [43] and Shuping et al., (2010) [44] are the most-cited Chinese articles with 380 and 304 citations respectively. Zhou et al. [43] worked with the marine macroalgae Enteromorpha prolifera to convert it to biocrude by HTL in a batch reactor at temperatures of 220–320 °C. Effects of the temperature, reaction time, and Na2CO3 catalysts on the yields of liquefaction products were investigated. As result, the conditions with the highest biocrude yield were at 300 °C with 5 wt % Na2CO3 and a reaction time of 30 min, which led to the biocrude yield of 23.0 wt %. Similarly, Shuping et al. (2010) [44] investigated the HTL of microalgae Dunaliella tertiolecta under various temperatures, reaction times, and catalyst dosages. A maximum biocrude yield of 25.8% is obtained at a reaction temperature of 360 °C during 50 min using 5% Na2CO3 as a catalyst.

Figure 4 shows the clustered co-authorship considering 32 countries and the minimum of four documents of a country. The network also presents the co-authorship between other countries, and it is clear the formation of six main clusters and one isolated represented by Turkey. The USA is linked with other five clusters expressed in green, red, yellow, and purple, and China is linked only with four clusters colored with cyan, yellow and green.

Fig. 4
figure 4

Co-authorship of countries that works with the liquefaction process that aims to produce biocrude using algae as feedstock

The metric co-authorship of countries engaged in the investigation of the liquefaction process to produce biocrude using algae as feedstock helps to understand how countries are committed to the joint development of this topic. As shown in Fig. 4, China and the USA are in separate clusters. This means that although there is a collaboration between the countries, they work more independently in the development of technology. Nevertheless, China and the USA are the largest economies in the world [45]. Therefore, the fact that these two economic powers are interested in the development of microalgae liquefaction to produce bio-oil reinforces the worldwide relevance of this topic.

According to Glanzel [46], populous countries such as China and the USA have a larger scientific community and tend to have less motivation for international collaboration when compared to smaller countries such as Italy, Germany, the Netherlands, and Poland, which can be found together in the same cluster (red). In addition to political and economic factors, geographical factors also influence international cooperation. For this reason, India, Taiwan, South Korea, and Thailand, which are closer, are part of the same cluster (dark blue).

As shown in Table 1, a total of 14 patents were identified in Derwent Innovations Index (DII) database. There was a growth in the number of patent filings from 2018, with 57.15% of documents published after 2018. It reinforces that the researched area is a current and developing technology with new application possibilities.

Table 1 Patents identified in the Derwent Innovations Index over the last 22 years (2000–2022), using the search query: TI=(liquefaction AND *alga$ AND (biocrude$ OR crude$ OR biooil$ OR oil$))

Patent ID202000521(U1) (2022) refers to a mini hydrothermal liquefaction plant to convert wet microalgae into biocrude oil. The design and prototype of this tool have a reactor volume capacity of 1 liter, which operates at a maximum pressure of 400 bar and a maximum temperature of 315 °C. On the other hand, patent CN113061454(A) (2021) refers to the production of biocrude from the hydrothermal liquefaction of microalgae (Spirulina platensis) with waste masks. In the context of the pandemic (COVID-19), this was an alternative to reduce the problem of environmental pollution caused by discarded masks.

The main patents depositing country is China, which is also the country with the highest number of articles published on the topic. One of the factors that explains China’s important role in the study of microalgal biofuels is the massive investment in research and development. In this context, the “973 Plan,” for example, is a program launched in 2011 by the Chinese government. This initiative aimed to reduce the high cost of large-scale algal-biodiesel production, advancing the technology readiness levels (TRLs) of the studied processes [47, 48].

Finally, it is noteworthy that the University of Jiangsu is a reference in the development of patents related to bio-oil production from microalgae via hydrothermal liquefaction. As discussed by Chen [48], institutions located in the eastern and southeastern coastal Chinese areas, such as Jiangsu, Zhejiang, and Guangdong Provinces, are leading in technology layout related to microalgal biofuels.

3.2 Author’s relevance

Table 2 shows the 13 relevant authors in the studied area according to their local h-index, which is shown in decreasing order with the number equal to or higher than 10. In addition, the total citations, publications, the number of citations of the most-cited paper, and references are also expressed in the table. Savage P. E., Zhang Y. H., and Duan P. G. are the most relevant authors occupying the first three colocations with the highest h-index. Savage P. E. and Duan P. G.’s most-cited article was already described above as the second most-cited in the whole database as they worked in co-authorship [38]. Moreover, both authors are strongly related due to other papers elaborated in co-authorship [49,50,51,52] as shown in Fig. 5, where the network considers 28 authors with a minimum of ten documents each. In one of these four articles published in 2011, the authors directed the research carrying out catalytic HTL using the microalgae Nannochloropsis sp. as feedstock. Duan and Savage (2011) [49] focused on the HTL in liquid water at 350 °C in the presence of six different heterogeneous catalysts under inert (helium) and high-pressure reducing (hydrogen) conditions. And as result, the influence of each catalyst in the presence or absence of H2 was analyzed as well as the biocrude yield and quality were determined. However, they also studied the catalytic treatment of its biocrude as the continuation of the research line [50,51,52]. Zhang Y. H.’s most-cited paper was also described above as the second American article most-cited in the database [15]. In addition, Zhang Y. H. worked on a review paper aiming to provide state-of-the-art HTL technologies from a perspective of algal biorefinery, focusing on feedstock types, process parameters, and HTL product distribution [17]. Zhang Y. H. shares authorship mainly with Liu Z. D. [17, 53,54,55] and Chen W. T. [56,57,58,59], as shown in Fig. 5. When the authors are arranged in citations descending order, Ross A. B. occupies the third position with an h-index of ten and ten publications, since the author has contributed to the field with significant contributions [12, 60, 61].

Table 2 Authors, local h-index, total citations, total publications, citations of the most-cited paper, and most-cited reference which regards the liquefaction process that aims to produce biocrude using algae as feedstock
Fig. 5
figure 5

Co-authorship of authors that works with the liquefaction process that aims to produce biocrude using algae as feedstock

3.3 Journals and Institutions

Bioresource Technology is the journal with the highest number of published documents (87 articles,14.33%) as shown in Fig. 6a, and also with the highest local h-index in the sector of 42. This journal has contributed over the years since 2011 and it reached the peak of 16 papers published in 2017. It is noticeable the relation that the older the article is, the more citations it has, with an emphasis on Biller and Ross (2011) [12], Vardon et al. (2011) [15], Anastasakis and Ross (2011) [61], and Jena et al. (2011) [67], which are the oldest articles from Bioresource Technology and are in the top 11 of the most-cited articles in the whole database, with 690, 416, 338, and 324 citations respectively. From 2018 to 2020, the number decreased until reaching three publications, and, in 2021, five papers were published [39, 68,69,70,71]. The following journals are Algal Research-Biomass Biofuels and Bioproducts, with 60 publications (9.88%) and an h-index of 29; and Fuel with 44 articles (7.25%) and an h-index of 19 (Fig. 6a). The Algal Research-Biomass Biofuels and Bioproducts journal has had publications since 2012 [72], but the maximum annual production was in 2015 with twelve papers. Afterward, the number has varied between four and nine publications. The first publication in the selected database in Fuel journal was in 2010 [60, 73], and later, the number of annual contributions in the field has increased, with a maximum of eight papers in 2020.

Fig. 6
figure 6

a Top 5 journals and b top 5 institutions considering the liquefaction process that aims to produce biocrude from 2000 to 2022

The University of Illinois is the most relevant for the sector considering the number of publications, which has about 48 papers published (7.91%) and a local h-index of 25 (Figure 6b). The first publications were in 2011 [15, 74] and the maximum annual production was in 2014 with nine contributions. In the following years, the university has decreased its number, varying between one and seven papers, but two were already published in 2022 [75, 76]. The University of Michigan is the second organization with the highest number of publications, with about 33 (5.44%) and its local h-index is 25. This institution publishes articles in the field since 2010 [38] and has decreased its contribution since its peak, in 2014 with six publications. Next, the New Mexico State University has 28 studies in the field (4.61%), with an h-index of 18, and it started publishing in 2013 [77, 78]. Later, annual contribution increased, with a maximum of five publications in 2019 and 2020 equally. It is noticeable the presence of American institutions in the top three positions even though the number of Chinese publications has exceeded in the past four years. On the other hand, the fourth and fifth positions are from Chinese institutions, Xi'an Jiaotong University and China Agricultural University with 22 and 20 publications and a local h-index of twelve and ten, respectively, as shown in Fig. 6b.

3.4 Keyword analysis

Figure 7 shows 37 authors’ keywords considering a minimum of six occurrences. The keywords with recent average years and a small number of occurrences might suggest they are being selected in current research and possibly being a trend. In contrast, the keywords with bigger darker nodes indicate a solid topic in the studied field, such as “biofuel” (70 occurrences) and “biomass” (27 occurrences), which turn visible the high concern about the replacement of fossil fuels with alternative energy sources. The use of biofuels has benefits over traditional fuels, such as higher energy security, reduced environmental impact and socioeconomic issues, and foreign exchange savings [79]. Allied with this, many authors reinforce the importance of algal biomass in the sector as being the most important biofuel source in the near future for its flexibility in producing different biofuels (e.g. bioethanol, biodiesel, bio-oil, bio-syngas, bio-hydrogen) through different routes [17, 18, 79]. Converging to it, “biorefinery” (21 occurrences), “biodiesel” (20), “algal biofuel” (10), “biochar” (10), and “bioenergy” (10) are also described in the network. “Biochar” and “biorefinery” are represented by lighted nodes, showing recent scientific interest in such terms as alternative sub-products. Some studies focused on biochar as HTL’s sub-product [10, 80, 81], but others investigated its use in other applications, such as anaerobic digestion improvements [82], nutrient recovery [83], and catalyst substitutes [40, 84].

Fig. 7
figure 7

Keywords co-occurrences analysis about the liquefaction process that aims to produce biocrude using algae as feedstock

Moreover, biorefinery is a relatively recent term used by academics to describe the concept of HTL for algal biomass in order to reuse and recycle nutrients, wastewater, and chemicals [17] in the process of producing biofuel along with other high-end products [85]. Nutrients, wastewater, and chemicals could be directed to support the next cycle of algae growth since a study indicated the integration of HTL wastewater algae growth could lead to an increase of biomass up to ten times. The integrated system was called “environment-enhancing energy” since it can simultaneously provide environmental protection with wastewater treatment to provide bioenergy [86]. The keywords “nutrient recycling” (eight occurrences) and “energy recovery” (seven occurrences) demonstrate a similar concern about achieving environmental and economic benefits for the HTL process. For example, 70–75% of nitrogen and 35–80% of phosphorus—with amounts of other inorganic nutrients that remained in the aqueous phase—could be inserted into the algae cultivation system with the gaseous product, which contains about 75% of CO2 [17], and according to Das et al. [90], recovery of 59% of energy from the HTL process was possible using municipal sewage sludge. Practical biorefinery approaches have shown successful results from a techno-economical point of view since algae production has been reported as the main bottleneck that makes the process challenging from a sustainability perspective [85]. Different approaches are also discussed in the recent scientific literature and aim to improve the economic issues, such as using the co-HTL process to cover algal reduced productivity in colder seasons [87, 88] or algal cultivation costs [89] and previously extracting valuable compounds from the algae and then converting the residual biomass into biofuel [90,91,92].

Another basic theme expressed in a big darker node presented in the network is “catalysis,” with 31 occurrences. The catalysts can be added to the HTL to control rates and select chemical reactions. Both alkali salts and metals can be employed as catalysts and they are called homogenous and heterogeneous catalysts, respectively [93]. Homogenous catalysts have been more used for HTL than heterogeneous catalysts, even though homogeneous catalysts are often more difficult to separate from the reaction products [94]. This occurs because homogeneous catalysts are generally more active due to diffusion properties [95]. Sodium carbonate (Na2CO3) is the most commonly used homogenous catalyst for microalgae liquefaction [93, 94] and has been applied in several studies to validate its efficiency [96, 97] or to compare with other homogeneous catalysts [98, 99]. Besides the homogeneous catalysts being more used, an increase in studies using heterogeneous catalysts has been noticed. The use of the “heterogeneous catalysts” keyword appears in the network with nine occurrences [100, 101] however this number is underestimated since multiple studies using heterogeneous catalysts in their process were found without being specified in the author’s keywords. In general, catalysts are used in order to increase biocrude yield or quality. However, some studies are focusing on the use of catalysts to upgrade the final biocrude product as the latest step of the process, as proved by the presence of the keywords “upgrading” (13 occurrences) [102, 103] and “catalytic upgrading” (six occurrences) [104, 105] in the network.

Also, “techno-economic analysis” (TEA), with 13 occurrences, is also a recently discussed topic among academics to overcome the challenges that remain as a barrier to industrial-scale dissemination, such as unreliable cultivation methods, large nutrient requirements, low energy return on investment, high capital and operating costs, the complexity of bio-oil composition, and challenges in product separation and production of high-value co-products [91, 106]. TEA is considered the most useful tool to determine the commercial feasibility of the process [107]. Additionally, the environmental aspects are also taken into account in “life cycle analysis” (LCA), a keyword expressed in the network with twelve occurrences. LCA is the most useful and accepted method to determine and quantify environmental impacts. However, these studies have shown different results because of the various algae systems, species, technology routes, product distributions, and handling of the utilization of the by-products [107, 108]. Masoumi and Dalai (2021) [107] performed a TEA and LCA of algal biofuel production through HTL with the best conditions of 275 °C and 11.5 MPa for a biocrude yield of 57.8 wt%. The authors compared two scenarios considering different biochar utilization (activation and combustion) for the analysis. Aspen plus simulation was applied to develop the TEA model where the results showed a minimum fuel selling price (MFSP), as an economic parameter, of $2.4/L for combustion and a lower MFSP of $2.2/L for activation. The LCA was used to determine greenhouse gas (GHG) emissions and environmental impacts which resulted in 5.2 g CO2-eq/MJ for combustion and 50 g CO2-eq/MJ for the activation route. Li et al. [109] focused on stochastic TEA of a continuous-flow wet waste HTL using a process reduced-order model (ROM) coupled with an economic model that was built in Microsoft Excel® instead of using the Aspen Plus model. Venkata Subhash et al. [110] worked with TEA related to microalgae cultivation, harvesting, and downstream processes required for biocrude production. Other studies also focus on TEA and LCA for different scenarios to conciliate biorefinery approaches and to improve the viability of algae to biofuel production in terms of environment, economics, and technology aspects [91, 106, 108, 111].

For the HTL development and optimization aiming at scaling up the process, is important to know mathematical models which describe the process based on the feedstock chemistry [112]. In this sense, the keyword “kinetic model” (with seven occurrences) indicates the concern about understanding how feedstock composition affects the HTL kinetic. Valdez et al. (2014) [112] demonstrated that microalgae rich in lipids or proteins produce higher biocrude yield than microalgae rich in carbohydrates. They also proposed a kinetic model based on lipids, protein, and carbohydrates present in Nannochloropsis sp. to correlate the effects of HTL parameters process (time and temperature) on biocrude yields. A new approach for HTL kinetic modeling was proposed by Hietala and Savage [113] and considered a second-order Maillard reaction pathway between amino acids and saccharides from biomass used in HTL to provide a useful robust kinetic model.

Since various parameters affect the HTL process and influence the biocrude yield, the conventional one-factor-at-a-time approach is an ineffective experimental methodology and needs to be replaced by another method. Thus, the “response surface methodology” (the keyword with six occurrences) has been used as a mathematical and statistical tool to evaluate the main factors and interactions of the HTL in a more precise way. Moreover, this methodology is the anticipation of optimum parameter conditions [114, 115].

Regarding the current interest in HTL to biocrude production, researchers have been focusing their efforts on achieving the economic viability of the process, making it necessary to find alternatives that minimize operating costs and increase HTL productivity. In this way, “techno-economic analysis,” “life cycle analysis,” “energy recovery,” and “nutrient recycling” are becoming hot research topics. Some ways to improve the economic viability process include optimizing HTL operational conditions using “catalyst,” “robust kinetics model,” and “response surface methodology.”

4 Research progress and prospects

HTL process dates from the 1960s to 1980s using lignocellulosic wood and sludge as feedstock for biocrude production. However, the research on the field and the importance of HTL has increased in the last decades due to the high price of petroleum since 2010. In this context, the US Department of Energy added HTL as one of the major technologies for biomass conversion and microalgae had become the most promissory candidate feedstock for biocrude production by since then. Several advantages of microalgae as feedstock can be highlighted such as there is no competition for water resources and land for farming compared with terrestrial plants; wastewater can be used for algal cultivations; great potential for biofixation of CO2 due to high photosynthetic efficiency; total solids of harvested algae are usually 10-25% being unnecessary drying process of feedstock and improving the overall thermal efficiency of the HTL process; and all kind of algae is suitable, including low-lipid content algae [17].

The research in the field has been progressing mainly about algae screening, study, and evaluation of HTL conditions such as temperature, pressure, and time reaction, use of catalysts, analysis of HTL products, biocrude oil upgrade, and, understanding of reaction pathways. Although research has intensified in the last ten years and advanced mainly in the technical-operational aspects on a bench scale, studies of the process scale-up and techno-economic and life-cycle assessments still lack in the literature [54, 116]. In addition, some bottlenecks pointed out by some authors are the pumpability of feedstock slurry with high solids content and the high capital investment. Furthermore, the important advantage of using wet algal biomass in HTL can also be a problem if a drying unit is required, due to the energy expenditures and, in addition, the high reactor volumes may increase both capital and operating costs. Therefore, bench scale studies to optimize the solids content and set a dewatering pretreatment are recommended [117]. Biocrude quality is another important issue to the technology because of its acid nature, high-water content, high viscosity, and high heteroatom content, which can hamper its industrial application. It’s because the biocrude may cause corrosion in equipment and the direct combustion may lead to high NOx emissions, due to the high amounts of nitrogen contained in microalgae cells [93, 117, 118]. These aspects reinforce the need to upgrade the biocrude which increases economic costs. On the other hand, biocrude quality can be improved by blending different biomass (co-HTL) or using catalysts. However, there are few studies on understanding the pathway for the co-HTL at a molecule level and the catalyst type, quantity, price, bottlenecks in recovery, and the deactivation process after a certain period in a continuous operation are also difficulties appointed in the literature [116,117,118]. Besides the algae and catalyst use, technical aspects are also highlighted as challenges in the field, such as finding an efficient technology for algae harvesting [119], downstream process, and a continuous operation HTL system with improved heat transfer [117]. These challenging aspects are still a gap in the literature.

Despite the challenges that limit the industrial application of algal HTL, many researchers have proposed a promising alternative to overcome these drawbacks under the E2E concept (presented in topic 3.4), which is based on energy and nutrient recovery mainly to achieve techno-economic feasibility. For this reason, researchers have been recently focusing on algal cultivation integrated with wastewater treatment and recycling of aqueous phase from HTL to reduce the consumption of water and nutrients [72, 120]. LCA (life cycle assessment) studies are also increasingly important to evaluate the environmental impacts related to the processes studied [106, 121, 122]. The main difficulties in conducting LCA studies for bio-oil production from the liquefaction process are the lack of data due to the low technology readiness level of the processes. However, this can be minimized through early-stage LCAs, which help to identify the main hurdles and hotspots in the development of the technology [123]. Finally, the use of biochar from microalgae is a promising application, since the production of multiple high-value products can improve overall techno-commercial feasibility.

5 Conclusions

All of the papers available in the WoS database related to the hydrothermal liquefaction (HTL) process aiming to obtain biocrude were briefly analyzed in the present study. After identifying algal biomass as the most-used raw material in the process, 607 papers were found since 2000 in the WoS database for discussion. The most relevant authors were shown regarding their number of citations, publications, and their local h-index factor. A majority of the contributions in terms of publications are from China (24.28%) and the USA (23.79%), where scientific production started to be published in 2009 in both countries, while the University of Illinois (3.75%) and the University of Michigan (2.58%) are the institutions with more published papers. In addition, the WoS categories and the author's keywords analyses suggest some recent related topics, such as techno-economic analysis, energy and nutrient recovery, and heterogeneous catalysis. These words reinforce the importance of improving the technology and explain the occurrence of new studies focusing not only on operational parameters but also on optimization analysis aiming to shorten the path toward industrial applications. Even though, some bottlenecks that prevent the diffusion of the technology are the developing of operation methodology for harvesting algae, performing HTL in an industrial scale, and producing high-quality and multiple products in order to favor the process technically and economically. Nevertheless, the considerable increase in publications for the last twelve years on this topic reflects the concern of academic experts on overcoming these barriers and finding a sustainable source of energy that is feasible.