Optimising Lignin Extraction: Utilising Current Pulp Mill Resources

Interest in lignin and its applications has been significant for a long time. However, market development has progressed slowly, regarding extraction levels and the further processing and commercialisation of lignin-based products. One reason is that lignin is a complex material, and its extraction has significant consequences for pulp mills.

Currently, several techniques exist for extracting kraft lignin from black liquor, such as acid precipitation, electrolysis, and ultrafiltration. Acid precipitation is the most developed and the only industrially implemented technique. In this process, CO2 is typically used in the first step to acidify the black liquor and precipitate the lignin. In the subsequent step, the lignin is further acidified and washed with sulfuric acid (H2SO4).

Implementing lignin extraction generates consequences for the pulp mill

Pulp mills are balanced in terms of energy and chemicals. They have worked extensively on their sodium/sulphur (Na/S) balance to control sulphidity and monitor the costs of makeup chemicals. Implementing lignin extraction has significant consequences for the mill and its balances. Although the lignin extraction itself occurs from the black liquor, connected to the evaporation plant, it impacts several other areas at the mill, such as the energy supply system. To enable lignin extraction on a larger scale, these consequences must be managed efficiently and sustainably.

Unlocking lignin's full potential

The processes for lignin extraction are basically linear. Chemicals with a certain CO2 footprint are brought in from outside the mill and introduced into the mill's balance. Consequently, the outgoing flows from the mill increase, leading to higher emissions. The most common extraction techniques primarily use CO2 and sulphuric acid for acidification and washing. Depending on their production and origin, these chemicals have different environmental impacts. Although lignin is considered a sustainable and green product, there are still improvements to be made to make the extraction process itself more sustainable.

Maintaining the chemical balance

The vector diagram in Figure 3 illustrates a schematic example of how the Na/S balance could be affected by lignin extraction using sulphuric acid for acidification and washing. It shows that the amount of purged electrostatic precipitator (ESP) ash needs to be significantly increased to balance the Na/S ratio. As the amount of purged ash (primarily Na2SO4) increases, the need for Na makeup, shown as NaOH in the figure, also rises. It is crucial to maintain this Na/S balance at the desired level to prevent significant impacts on the mill's operation, such as increased SO2 emissions from the recovery boiler.

Kraft pulp mills are under increasing pressure to reduce sodium and sulphur emissions into wastewater for both economic and environmental reasons. Emissions of salts, such as ESP ash, are expected to be further restricted by stricter environmental regulations.

Reducing process linearity

One way to reduce linearity in lignin separation is to produce sulphuric acid internally at the mill, utilising existing residual streams. This internal production reduces the need to purge ESP ash to maintain the mill's Na/S balance. It is also an effective method to separate sodium and sulphur streams, benefiting the mill's Na/S balance and its control.

Sulphuric acid is one of the most widely used chemicals globally, with an annual production of over 250 million tons. It is primarily produced by burning elemental sulphur to generate SO2, but industrial waste gases like SO2, H2S, COS, and CS2 are also utilised. All production processes involve the catalytic reaction of SO2 with O2 to form SO3, followed by the reaction of SO3 with H2O to produce H2SO4.

The use of sodium sulphide (Na2S) in the kraft pulping process generates a significant amount of sulphur-containing gases, such as methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3), dimethyl disulfide (CH3SSCH3), and hydrogen sulphide (H2S). These gases, also known as strong gases (CNCG), are typically collected and burned for destruction. Instead, these sulphur-containing gases can be converted into sulphuric acid, reducing or eliminating the need for fresh sulphuric acid in lignin extraction. The amount of sulphur in the strong gases varies between mills, with an estimated range of 3 to 7 kg S per ton of pulp produced. This is sufficient to generate a relevant amount of sulphuric acid for lignin extraction, depending on the specific mill and the size of the lignin separation facility.

There are established techniques for producing sulphuric acid from strong gases, and several facilities are already in operation. Additionally, there are methods to increase the amount of generated strong gases, enabling higher internal production of sulphuric acid.

Internal CO2 capture at various scales

A significant portion of CO2 emissions comes from the process industry. In 2020, for example, 21 of the largest pulp and paper mills in Sweden generated over 21 million tons of biogenic CO2. CO2 emissions from a modern pulp mill are generally of biogenic origin and come from three main sources: the recovery boiler, the lime kiln, and the biomass boiler. There are several methods for CO2 capture, all facing the same challenge of reducing energy requirements and investment costs. These costs are high and pose a barrier to large-scale commercial implementation.

Internal CO2 capture could create a more sustainable process for lignin separation, while also being economically viable. The cost of purchasing CO2 is a significant part of the operating cost in lignin extraction, as CO2 is used to lower the pH and precipitate the lignin. A lignin separation plant with a capacity of 50,000 tons of lignin per year consumes approximately 8,000-13,000 tons of CO2 annually. The purchase cost of this CO2 is substantial.

However, the amount of CO2 required for lignin extraction is relatively small compared to the total amount available in the flue gases at the pulp mill. The recovery boiler generates the largest amounts of flue gas and CO2, but the flue gas has a relatively low CO2 content compared to the flue gases from the lime kiln. Therefore, the flue gas from the lime kiln is a more suitable source of CO2 for this application, as it has more than sufficient CO2 at a relatively high concentration. Flue gas with a higher CO2 concentration results in slightly lower investment costs and specific energy consumption.

Energy balance: A key factor for pulp mills

There are currently equipment and technologies for CO2 capture at various scales, from modular solutions suitable for lignin extraction to large facilities capable of capturing millions of tons of CO2 per year. Since the CO2 for lignin separation is used locally and does not require high purity, it does not need extensive purification or liquefaction. Interest and incentives for capturing large amounts of biogenic CO2 (BECCS) are increasing, both for storage and as a raw material for chemicals and fuels. If pulp mills focus on large-scale CO2 capture, for example from the recovery boiler, the amount of CO2 required for lignin separation would be relatively small and negligible. This solution, combining lignin separation and large-scale CO2 capture, would be interesting from an environmental and sustainability perspective, but may encounter problems related to the mill's energy balance. Both lignin extraction and CO2 capture will significantly impact the mill's energy balance. Lignin extraction reduces fuel for the recovery boiler, thus reducing steam production, while CO2 capture is energy-intensive, requiring steam and/or electricity

The mill's capacity to make energy available will be a limiting and crucial factor in how much CO2 can be captured. Therefore, it may be relevant to design a BECCS concept to capture a limited amount of CO2 so that the mill's energy balance can be maintained without extensive changes or investment in additional equipment.

Today, the technologies for kraft lignin extraction, sulphuric acid production utilising strong gases, and CO2 capture from flue gases are all commercialised and proven processes. These three process steps can be combined to extract kraft lignin in a more resource-efficient and sustainable way. Internal production of sulphuric acid and CO2 is also likely to positively impact operating costs (OPEX) since the cost of CO2 and sulphuric acid, combined with sodium makeup, is significant.

Why AFRY; how can we support?

AFRY has extensive experience in lignin-related projects, including both lignin extraction and further refining processes. Our deep knowledge of pulp mills and their process areas makes us well-suited to handle issues related to lignin extraction integration and its impact on the mill's balance. We are also active in developing lignin-based products and their market. In several projects, we have analysed how lignin can be refined into new products and applications.

AFRY also has strong competence and experience in CO2 capture, covering both technical and strategic business aspects. We offer services that range from conceptual development to implementation, leveraging the combined expertise of our management consultants and engineering teams.