Biorefinery lignosulfonates as a dispersant for coal water slurry

The concept of biorefinery is to mimic petroleum refinery to produce multi-products such fuels, chemicals, polymers from a lignocellulosic feedstock to diversify product portfolio, avoid market saturation, and maximize resource utilization. The sugar platform as a major lignocellulosic biomass conversion pathway relies on the conversion of carbohydrates to sugars for subsequent processing to fuels and chemicals. While it is very attractive because sugars are flexible building blocks for producing a variety of chemicals and products [1], valorization of the lignin fraction is the key to commercial success because lignin is the second most abundant fraction in lignocelluloses of approximately 15–30 % [2]. Current technologies for the sugar platform rely on a pretreatment (or fractionation) step followed by enzymatic saccharification of the pretreated solids [3]. Depending on the pretreatment process employed, lignin is often fractionated into a soluble fraction in the pretreatment spent liquor and a fraction retained in solids. The current utilization of these two lignin fractions—biorefinery lignin—remains as a low value boiler fuel as practiced in pulp mills, despite substantial research and development efforts have been made in bioconversion of lignocelluloses [4].

Here, we demonstrate a biorefinery lignin, i.e., the water soluble lignin fraction from Sulfite Pretreatment to Overcome the Recalcitrance of Lignocelluloses (SPORL) [5] of softwoods—lignosulfonate (LS), as a dispersant of coal water slurry (CWS) without further processing. Coal is an important energy source. Approximately 39 % of the electricity was produced from coal in the U.S. (US Energy Information Administration). CWS was developed in the 1920s in Russia. Due to the shortage of oil supply in the 70s, CWS technologies was further developed as an alternative to liquid fuel in a variety of applications. CWS is a clean technology compared with coal itself which can alleviate many concerns of coal combustion [6, 7]. For example, it can produce high combustion efficiency, low discharge of ash, and lower NOx and SOx air emissions [8, 9]. Typical CWS contains 60–75 % small suspended coal particles in 25–40 % water, and 1 % chemical dispersants. CWS can be directly burned without dewatering [10]. Dispersants play an important role to reduce CWS viscosity and stabilize rheological properties for good atomization and efficient combustion [11, 12]. To meet the potential demands for CWS, several dispersants such as naphthalene sulfonate formaldehyde condensate [13], sulfonated acetone-formaldehyde [14], carboxylate type copolymer [15], cardanol formaldehyde sulfonate [7], sodium polystyrene sulfonate [16], sodium dodecyl benzenesulfonate [17] have been studied. However, lignin based dispersants attracted great attention [18, 19].

The practical significance of this study is the existence of a mature commercial market for CWS dispersant and the valorization of LS from wood biorefinery as a co-product. Furthermore, with the gradual closing of sulfite pulp mills in the last 40 years around the world, there is a shortage of commercial LS products. Some regions rely on a low quality LS derived from sulfonation of kraft lignin—from kraft pulping [20] to meet market demand. Therefore, LS from SPORL can be a commercially and economically viable co-product for biorefinery.