Biofuels from Kraft Black Liquor : Pilot-Scale Gasification Development and Techno-Economic Evaluation of Industrially Relevant Biofuel Production Pathways
Abstract: Fuel combustion for transport was responsible for 24% of EU-28 greenhouse gas (GHG) emissions in 2016. Member states are expected to ensure that the share of renewable energy in the transport sector is at least 14% by 2030. Some countries, such as Sweden, have more ambitious targets. By 2030, GHG emissions from domestic transport are to be reduced by 70% relative to 2010 levels. A national emission reduction scheme launched in 2018 obliges suppliers of diesel and petrol to reduce their GHG footprints annually by a pre-determined amount, which could further stimulate demand for drop-in biofuels, and potentially even high blend alternatives. However, recent changes to Swedish domestic policy and the EU-wide RED sustainability criteria mean that biodiesel from some common crop-based biomaterials may no longer qualify for certain important tax benefits or count towards the 2030 renewables target. Given Sweden’s extensive forest products industry, woody residue and by-products, such as branches, toppings and black liquor (BL) are viewed as strategically important feedstock for the production of RED-compliant biofuels. Upgrading kraft BL to biofuels can also potentially help pulp mills expand their product base. Two conversion routes are seen as industrially relevant in the short-to-medium term: (a) gasification-catalytic synthesis, (b) liquefaction-hydrotreatment.The production of methanol and dimethyl ether from kraft BL has been demonstrated extensively in a pilot facility, although a complete analysis of technical performance based on experimental measurements has not been performed previously. Since BL is not transportable, the biofuel production potential of a mill is effectively set by its pulp throughput, which limits potential economies-of-scale. BL is rich in catalytically active sodium compounds and blending it with other woody residue-based feedstock, such as pyrolysis oil (PO) before gasification allows the partial decoupling of biofuel production capacity from available black liquor volumes. Several commercial actors are currently testing the technical viability of using liquefaction-hydrotreatment for the production of renewable petrol and diesel blendstock from kraft lignin. The first step in both cases is co-located at a pulp mill. Depending on the desired end product and production route, subsequent upgrading and finishing may be carried out at a pulp mill or a crude oil refinery, with a resulting impact on both production economics and energetic performance that is complex and little studied. The main aim of this thesis is to evaluate the technical and economic viability of kraft liquor-based biofuel production pathways that are adjudged to be commercially relevant, that is they are seen as commercially deployable in the short-to-medium term at the present time. The catalytic co-gasification of PO and BL was studied in pilot-scale experiments, and resulting data were used to calculate key performance indicators. Blending BL with the more energy-rich PO led to an increase in cold gas efficiency without adversely affecting organic carbon conversion, which remained nearly complete at blend ratios of up to 20 wt.% PO. There were no signs of an increase in either soot or tar formation. Analyses of gas condensate samples found traces of unknown uncharged forms of sulfur and while sulfur reduction efficiencies were generally high, balance closures could not consistently be obtained owing to analytical difficulties. Further study appears warranted. Pilot-scale entrained-flow gasification experiments are expensive and time consuming. An alternative is thermodynamic equilibrium calculations, which were found to predict the flows of major syngas and slag constituents with a high degree of accuracy, subject to expected deviations in methane formation and sulfur distribution.An analysis of production economics found nth-of-a-kind investments in gasification-based methanol and lignin liquefaction and hydrotreatment-based drop-in biofuels to be profitable for a range of sizes, although the latter pathway, which is currently prioritized for development in the short-run was at a lower level of technology maturity. In assessing the overall energetic performance of a pulp mill-integrated kraft BL-based biofuel production, explicit consideration of mill energy balance is needed to avoid misestimation. Modern market pulp mills with a large energy surplus, normally exported as electricity, are at an advantage. There are indications that relatively small gasification units of around 100 MW could potentially also be profitable, which means biofuel production facilities based on both principle conversion tracks could be used to debottleneck pulp production by relieving recovery boiler capacity constraints, although the viability of such an approach is mill specific and requires detailed investigation. While co-gasifying black liquor with pyrolysis oil or blending syngas from unblended black liquor with electrolysis hydrogen offers better carbon utilization, larger biofuel production potentials and a more diverse feedstock base, gasification of unblended black liquor comes out as the lowest cost option for producing drop-in biofuels from part-streams.
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