ReviewBiodesulfurization of diesel fuels – Past, present and future perspectives
Introduction
Sulfur is the third most abundant heteroatom in crude oil and can vary from 0.05% to 10% of the composition. The types of sulfur compounds vary greatly within a crude supply (Blumberg et al., 2003). In addition to elemental sulfur, sulfate, sulfite, thiosulfate and sulfide, together with more than 200 sulfur-containing organic compounds have been identified in crude oils (Ma, 2010). Sulfur-containing heterocyclic compounds are among the most potent environmental pollutants. Reducing sulfur levels in fuels can decrease harmful emissions in three ways: (i) directly reducing sulfur dioxide (SO2) and sulfate particulate matter (PM), (ii) achieving better performance from the emissions control systems, especially catalysts, and (iii) enabling the use of new emission control technologies such as diesel PM filters, NOx absorbers and selective catalyst reduction systems (Stanislaus et al., 2010).
Conventional HDS processes have been employed by refineries to remove organic sulfur from liquid fuels for several decades. This technology is economic in terms of removal of a number of classes of compounds containing sulfur, other than refractory organic sulfur compounds (Breysse et al., 2003). Deep desulfurization of diesel fuel has become an important research subject due to the upcoming legislative regulations to reduce sulfur content. However, to meet the challenges of producing ultraclean diesel fuels, especially with sulfur content lower than 15 ppm, both capital investment and operational costs would be high due to more stringent operating conditions. Consequently, several alternative approaches have been used, including selective adsorption, extraction by ionic liquid, oxidative desulfurization and BDS.
Microbial desulfurization of organosulfur pollutants is attracting more attention because of cost effectiveness and environmental friendliness. However, this technology is not yet available for large-scale applications, so future research must investigate modifications of this process for industrial applications (Xu et al., 2006). Several previous reviews outline progress in microbial desulfurization from the basic and practical point of view (McFarland et al., 1998, Monticello, 1998, McFarland, 1999, Ohshiro and Izumi, 1999, Tong et al., 2001, Acero et al., 2003, Gray et al., 2003, Gupta et al., 2005, Kilbane, 2006, Soleimani et al., 2007, Mohebali and Ball, 2008, Xu et al., 2009, Debabov, 2010, Nuhu, 2013, Boniek et al., 2015). A recent mini-review on the role of biotechnology in the petroleum industry (Bachmann et al., 2014) highlighted the potential significance of BDS although few details were presented. In this current review, attention is focused solely on the biodesulfurization of diesel fuels as an alternative technology, which has become an important research subject.
Section snippets
Sulfur in petroleum and its fractions
In crude oil, sulfur is present in soluble organic form, the principal generic groups being: (i) aliphatic and aromatic thiols and their oxidation products (disulfides); (ii) aliphatic, aromatic and mixed thioethers, and (iii) heterocyclics based on the thiophene ring: thiophene itself, benzothiophene (BT), dibenzothiophene (DBT), and their alkyl substituted derivatives (Oldfield et al., 1998). The most abundant form of sulfur in petroleum is usually the thiophenic form. Thiophenic sulfur often
Air pollution as a result of fuel combustion
A typical flue gas from the combustion of fossil fuels will contain quantities of NOX, SO2 and particulate matter (PM); these gases react in the atmosphere with water, oxygen and other chemicals to form a mild solution of sulfuric and nitric acids. The acid rain dissolves buildings, kills forests and poisons lakes as well as damaging agricultural areas located downwind of combustion facilities (Mohebali and Ball, 2008). Acid rain also damages the environment by upsetting the natural balance of
Legislative regulations
The quality of crude oil is changing; the American Petroleum Institute (API) gravity of oil is decreasing and sulfur content is increasing (Swaty, 2005). The sulfur is preferentially associated with the higher molecular weight components of crude oils and consequently, heavy crude oils typically have more sulfur than light crude oils (Kilbane and Le Borgne, 2004). Increasing sulfur concentrations in crude oil supplies results in an increase in the sulfur content of finished petroleum products.
Hydrodesulfurization (HDS)
In a refinery, hydrotreating refers to a variety of hydrogenation processes which saturate unsaturated hydrocarbons and remove S, O, N and metals from different petroleum streams. The main aim of hydrotreating is to diminish air pollution emissions, to avoid poisoning of catalysts and to improve fuel quality. HDS also improves the characteristics of the material making it easier to crack (Swaty, 2005). HDS is a catalytic process converting organic sulfur to hydrogen sulfide gas by reacting
Deep desulfurization
Deep (ultra-deep) desulfurization refers to processes to remove sulfur to below 15 ppm for diesel fuels (Song, 2003). Deep reduction of diesel sulfur is dictated largely by the least reactive sulfur compounds, refractory sulfur compounds (Pawelec et al., 2011). The reactivity of sulfur compounds in HDS follows the order: thiophene (TH)> alkylated TH > BT > alkylated BT > DBT and alkylated DBT without substituents at the 4 and 6 positions > alkylated DBT with alkyl substituents at the 4 and 6
BDS as a complementary technology
While other alternatives to HDS are under development to desulfurize various refinery products, BDS would be a complete breakthrough in process development. BDS can potentially offer a low-cost alternative to HDS, reducing capital and operating costs (Worrell and Galitsky, 2004). With respect to the Kyoto Accord the interest in reducing greenhouse gas emissions led to calculations showing that CO2 emissions and energy requirements are reduced if BDS is used instead of HDS (Linguist and Pacheco,
Biodesulfurization in environment
In natural systems bacteria assimilate sulfur in very small amounts for their maintenance and growth. The sulfur present in both agricultural and uncultivated soils is largely in the form of organic-bound sulfur either as sulfonates and sulfate esters and not as free as bioavailable inorganic sulfate (Singh and Schwan, 2011); bacteria which are able to transform sulfur-containing compounds for utilization of either the sulfur or the carbon skeleton are widespread in nature (Le Borgne and Ayala,
Model sulfur compounds in BDS studies
Although there is no common model compound that can be used for all the various crude oil fractions, DBT and its derivatives have been reported to account for as much as 70% (w/w) of total sulfur content of West Texas crude oil and up to 40% (w/w) of the total sulfur content of some Middle East crude oils (Monticello and Finnerty, 1985). In brief, DBT as a model compound represents a reasonable choice on the basis that: (i) DBT and its derivatives represent a major proportion of thiophenic
Susceptibility of DBT to microbial attack
Many bacteria can degrade DBT aerobically following three major pathways (Fig. 1) (Díaz and García, 2010): in the first type, the carbon skeleton of DBT is partially oxidised, with the C–S bond remaining intact (Kodama pathway). In the second type, DBT is utilised as the sole source of carbon, sulfur, and energy. In the third type, DBT is desulfurized and the carbon skeleton remains intact (the 4S pathway). In another classification, two main types of pathways have been reported:
Ring-destructive pathways
Until now two ring-destructive pathways for metabolism of DBT have been recognised. The most common pathway of DBT degradation, known as the “Kodama pathway” is analogous to that of naphthalene (Kodama et al., 1973). There are several reports showing that DBT can be utilised via this pathway by several bacterial genera including Pseudomonas (Kodama, 1977, Hou et al., 2005), Beijerinkia (Labord and Gibson, 1977) and Rhizobium (Frassinetti et al., 1998). In this pathway initial dioxygenation is
Anaerobic sulfur-specific pathway
Sulphate-reducing bacteria (SRB) have been reported to desulfurize model compounds and fossil fuels (Kim et al., 1995, Lizama et al., 1995). This assimilatory desulfurization route produces H2S. Desulfovibrio desulfuricans M6 can anaerobically reduce DBT to biphenyl and H2S (Kim et al., 1995). Desulfomicrobium escambium and Desulfovibrio longreachii have been reported to desulfurize DBT following a pathway in which biphenyl was not the end-product (Díaz and García, 2010). In anaerobic
Desulfurizing microorganisms at a glance
Maliyantz (1935) reported bacterial desulfurization of petroleum oil with the accumulation of hydrogen sulfide (Yamada et al., 1968). Strawinski (1950, 1951) and Zobell (1953) issued patents concerned with the procedures for microbial desulfurization (Yamada et al., 1968). Isbister and Koblynski (1985) described a Pseudomonas sp. strain CB-1 that could accomplish sulfur specific metabolism of DBT. The intermediates were DBTO, DBTO2 and the end product was dihydroxybiphenyl. Unfortunately this
Non-cellular desulfurizing biocatalysts
Besides cells of wild type and genetically modified bacteria, BDS studies using other isolated biocatalysts such as monooxygenases, oxidases and peroxidases have been carried out. The use of monooxygenases was determined not useful as they required, besides O2, the high-cost flavin mononucleotide co-factor (Stachyra et al., 1996). Better results have been obtained with oxidases and peroxidases (Madeira et al., 2008). Eibes et al. (2006) evaluated fungal peroxidases, including manganese
Dsz enzymes
As mentioned earlier, the 4S pathway involves sequential oxidation of the sulfur moiety and cleaving of the C-S bonds (Fig. 1). In the sequential oxidation, 4 key enzymes are involved, two monooxygenases, one desulphinase and one NADH:FMN oxidoreductase. The latter supplies the two monooxygenases with reduced flavin. In strain IGTS8 each of the key Dsz pathway enzymes has been purified and characterised. In strain IGTS8 the pathway proceeds via two monooxygenases (DszC and DszA) supported by a
Sulfur substrate specificity
The broad substrate range of the Dsz system is one of the driving forces for the development of BDS as a commercial process. Despite the obvious chemical similarity of DBT and BT, the two desulfurization pathways are mutually exclusive. Thus BT cannot be desulfurized via the DBT-specific pathway and vice versa (Gilbert et al., 1998). Therefore these pathways are complementary in terms of their potential roles in development of a fuel BDS technology. Substituted BTs and DBTs remain in diesel oil
Sulfur substrate and end product inhibition
The growth kinetics of Rhodococcus sp. strain JUBT1 has been examined using DBT, alkylated DBT and diesel as limiting substrates (Guchhait et al., 2005a, Guchhait et al., 2005b). The substrate inhibited growth followed Haldane type kinetics. Kinetic parameters were highly affected by the increase in the extent of alkylation of DBT and linear correlations have been observed between the functionality of the parameters with the number of alkylation.
Zhang et al. (2013) investigated interactions
Genetics of DBT desulfurization
The genes involved in DBT metabolism have been called bds, dsz, tds, mds, and sox. Because several other unrelated genes have been labelled sox, the sox designation has been generally rejected. Bds, Dsz, Tds, and Mds have all been accepted as gene products (Mohebali and Ball, 2008).
R. erythropolis IGTS8 (Piddington et al., 1995, Gray et al., 1996, Li et al., 1996, Oldfield et al., 1997) and Rhodococcus sp. strain X309 (Denis-Larose et al., 1997, Denis-Larose et al., 1998) were among the first
Dsz genes regulation and repression
Several investigators reported that the Dsz activity in various bacteria was completely repressed by sulphate or other readily bioavailable sulfur sources including methionine, cysteine, taurine, methanesulfonic acid and casamino acids. In rhodococci, these sulfur compounds repress the promoter of the dsz gene sequence (Li et al., 1996) or enzyme synthesis at the transcription level (Monticello, 1998), but are not inhibitors of the Dsz enzymes.
It has been reported that Dsz activity of strain
Desulfurization of alkylated DBTs
Much of the residual post-HDS organic sulfur in intermediates and combustible products is thiophenic sulfur. The majority of this residual sulfur is present in DBT and derivatives thereof having one or more alkyl or aryl groups attached to one or more carbon atoms present in one or both flanking benzylic rings. The alkyl side chains have been shown to significantly affect the relative reactivity of the thiophenic molecules with chemical or biological catalysts. The most refractory Cx-DBTs have
Biodesulfurization of diesel oil fraction
Research work is currently being undertaken around the world to bring about deeper desulfurization of middle distillate oil fractions. Biodesulfurization of the fractions has been reported (Table 3).
BDS process
In order to make a BDS process competitive with deep HDS a five-step process is needed: (i) production of active resting cells (biocatalysts) with a high specific activity; (ii) preparation of a biphasic system containing oil fraction, aqueous phase and biocatalyst; (iii) biodesulfurization of a wide range of organic sulfur compounds at a suitable rate; (iv) separation of desulfurized oil fraction, recovery of the biocatalyst and its return to the bioreactor; and (v) efficient wastewater
Bioreactors for BDS studies
Two liquid–liquid bioreactors, a stirred-tank and a novel electrostatic-dispersion system have been used to investigate biodesulfurization of oil by sulfate reducing bacteria (SRB) (Tsouris et al., 1996). Biodesulfurization of diesel has been studied in airlift bioreactors (Nandi, 2010, Irani et al., 2011). Amin (2011) used an integrated two-stage process for biodesulfurization of a model oil using a vertical rotating immobilized cell reactor with the bacterium R. erythropolis.
Separation of biocatalyst from reaction mixture
Post-reaction separation of cells is another critical step in the BDS process. With hydrophobic biocatalysts, the separation of cells from the organic phase is a problem; with increasing catalyst concentrations, particle-stabilized emulsions are formed (Pacheco, 1999, Mohebali et al., 2007). It has been suggested that the formation of a stable water/oil emulsion could be avoided in order to facilitate oil recovery (Ayala and Vazquez-Duhalt, 2004). A BDS process using immobilized cells has been
Oxygen mass transfer and uptake rates
All the microorganisms employed for BDS are strictly aerobic, as oxygen is essential for cell growth and Dsz activity. In many cultures, dissolved oxygen concentration becomes the limiting factor and therefore, oxygen mass transfer from the gas to the liquid phase is important not only for growth but also for the BDS capability of the cells.
Oxygen uptake rate during growth of R. erythropolis IGTS8 has been measured using two experimental methods (dynamic and process methods) for the same set of
Improvement of BDS performance using nanotechnology
Nanotechnology has attracted great interest in recent years due to its impact on many scientific areas. Looking at the nanoscale has stimulated the development and use of novel and cost-effective technologies for catalytic degradation (Zhao et al., 2011). The present progress of nanobiocatalysis suggests that nanobiocatalytic approaches offer significant potential for the future (Kim et al., 2008). Guobin et al. (2005a) assembled γ-Al2O3 nanoparticles onto biodesulfurizing cells of Pseudomonas
Refinery challenges
With the new USEPA Tier II regulations, refineries are facing major challenges to meet the fuel sulfur specification along with the required reduction of aromatics (Song and Ma, 2007). Although the new environmental regulations limiting the sulfur levels of diesel and other transportation fuels are beneficial from an environmental point of view, meeting the required stringent specifications represent a major operational and economic challenge for the petroleum refining industry.
The problem
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