Black Liquor Gasification

Outline

Black liquor is a biomass feedstock with unique properties suitable for gasification (Andersson and Harvey, 2004; Ådahl et al., 2004; Berglin et al., 2002; Dahlquist, 2003; Dahlquist and Jones, 2005; Dahlquist et al., 2009; Ekbom et al., 2003; Harvey and Facchini, 2004; Möllersten et al., 2003a,b, 2004; Maunsbach et al., 2001; Waldner and Vogel, 2005; Sricharoenchaikul, 2009; Bajpai, 2008, 2013; Dance, 2005; Marklund, 2006; Grigoray, 2009; Landälv, 2010; Salomonsson, 2013). First of all, it is available at existing industrial sites in large quantities. Second, it is a liquid. This makes it possibly to easily feed it by pumping into the pressurized gasifier. With biomass in solid or pulverized form this becomes significantly more difficult. The liquid state also makes the black liquor easy to atomize into a fine mist that reacts very fast in the gasifier. Third, the gasification of black liquor char is more rapid than for any other feedstock as the inherently high sodium and potassium content of black liquor acts as a catalyst. These properties make it possible to apply the high temperature, entrained flow gasification principle to black liquor. This type of gasification process provides many advantages over alternative gasification technologies: it is a very rapid, single-stage gasification process with low reactor volume; it minimizes the need for raw syngas cleanup as the Chemrec process directly provides a raw syngas of excellent quality with complete carbon conversion; no tar formation; and low methane content.

These properties make the gasification of black liquor easier and more rapid than for any other biomass feedstock. BLG should be an integral part of an Integrated Forest Products Biorefinery (IFBR) because its process heat may be used in the sugar conversion unit operations, and the synthesis gas may be used to replace fossil fuels, in particular oil in the lime kiln. The gasification synthesis gas may be used as feedstock to produce transportation fluids such as Fisher–Tropsch (FT) liquid hydrocarbons, methanol, and mixtures of higher alcohols (Bajpai, 2013). The key requirement for implementation of BLG is to demonstrate the reliability and efficiency of the technology at commercial scale while regenerating the pulping chemicals.

BLG is one of several biorefinery options for the kraft pulp industry. It enables production of several value-added products such as electricity, district heating, biofuels, or lignin in addition to pulp. Investment in biorefineries is a possible way for the industry to remain competitive with increased energy and raw material prices. Some mills, especially energy-efficient market kraft pulp mills, have the possibility to become major net exporters of electricity or lignin without purchasing external wood fuel (Pettersson, 2011). Nevertheless, in order for integrated pulp and paper mills, even those with a high degree of energy efficiency, to become major exporters of lignin or for any type of mill to become a major exporter of biofuels, external wood fuel is required. In such cases, the usage of biomass should be compared with other possible ways to use the biomass resource. Increasing the degree of heat integration could decrease the need for external process heating and thereby decrease the need for external wood fuel. If the technology becomes commercially available, implementation of carbon capture and storage (CCS) could significantly influence both the climate impact and economic performance of the studied systems. In BLG, and other gasification processes, relatively large amounts of CO2 could be captured at relatively low costs.

Large amounts of CO2 could be separated from the flue gases of the recovery boiler or other mill power boilers. But the separation costs are generally very much higher compared to implementation in the gasification processes. In order for mills to consider implementation of full-scale BLG plants, the recovery boiler has to be close to the end of its technical lifetime. But mills with a steam surplus, or mills planning to increase their production capacity (assuming that the recovery boiler is running at maximum capacity), could consider investment in a smaller BLG plant as a way to take advantage of a potential steam surplus or to achieve debottlenecking of the recovery boiler.

BLG is being considered primarily as an option for production of biofuels in recent years due to the focus on the transport sector’s high oil dependence and climate impact (Pettersson, 2011). The technology is included in several studies comparing climate and economic benefits of alternative ways to produce motor fuels. But there is a tendency to present BLG without consideration of the special implementation characteristics of each specific case. Some studies discuss how other characteristics, such as integration with another type of mill, would affect the results.

According to Pettersson (2011), estimating the climate impact and economic performance of possible future technologies is not straightforward. Uncertainties about future energy prices and policy instruments make the results highly variable. Also, when it comes to estimation of climate impact, a number of different approaches can be considered with significant variation of results. Gasification of black liquor is an alternative recovery technology that has gone through a stepwise development since its early predecessor was developed in the 1960s. The currently most commercially advanced BLG technology is the Chemrec® technology (Chemrec, 2013), which is based on entrained flow gasification of the black liquor at temperatures above the melting point of the inorganic chemicals. In a BLG system, the recovery boiler is replaced with a gasification plant. The evaporated black liquor is gasified in a pressurized reactor under reducing conditions. The generated gas is separated from the inorganic smelt and ash. The gas and smelt are cooled and separated in the quench zone below the gasifier. The smelt falls into the quench bath where it dissolves to form green liquor in a manner similar to the dissolving tank of a recovery boiler. The raw fuel gas exits the quench and is further cooled in a countercurrent condenser (CCC). Water vapor in the fuel gas is condensed, and this heat release is used to generate steam. Hydrogen sulfide is removed from the cool, dry fuel gas in a pressurized absorption stage. The resulting gas is a nearly sulfur-free synthesis gas (syngas) consisting of mostly carbon monoxide, hydrogen, and carbon dioxide.

The two main technologies under development are...