Chemical Recovery

For economic and environmental reasons, chemical recovery processes are used in chemical and semi-chemical pulp mills to reclaim spent cooking chemicals from the pulping process. At kraft and soda pulp mills, spent cooking liquor, referred to as “weak black liquor,” from the brown stock washers is routed to the chemical recovery area. The chemical recovery process involves concentrating weak black liquor, combusting organic compounds, reducing inorganic compounds, and reconstituting the cooking liquor.

In black liquor concentration, the weak black liquor from pulping processes – containing wood lignins, organic materials, oxidized inorganic compounds (Na2SO4, Na2CO3), and white liquor and having a solids concentration of about 12 to 15% – passes through a series of evaporators to increase solids content. Depending on the type of evaporators used in later stages (direct or indirected), an intermediate oxidation step may also be included. The solids content of the black liquor following the final evaporator/ concentrator typically averages 65 to 68% (US EPA, 2010).

Combustion of organic compounds takes place in a recovery furnace. The black liquor has a has an energy content of approximately 14 to 16 MJ/kg-dry solids. Consequently, steam for mill processes and/or electricity can be produced from this combustion. Recovery boilers typically have a thermal efficiency of around 65% and this efficiency can be increased with increased solids content in black liquor (around 2% increase in steam production per 5% increase in solids contents). During combusion, the inorganic process chemicals are reduced to a molten smelt, which is removed from the bottom of the boiler and is further refined in subsequent steps (Kramer et al., 2009. p.18).

in order to reconstitute the cooking solution (white liquor), the smelt from the recovery boiler is first mixed with weak white liquor solution, forming green liquor. Recausticization of green liquor then takes place with the addition of calcium hyrdroxide (Ca(OH)2). The calcium carbonate (CaCO3) formed during recausticizing is removed as lime mud leaving behind a white liquor that can be reused in the cooking process. The lime mud sent to a lime kiln to be calcined. The resulting lime (CaO) is dissovled in water to produce calcium hydroxide (Kramer et al., 2009. p.18-19).

Chemical RecoveryTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
Black Liquor Gasification

Theoretical balance calculations show that a black-liquor-based Integrated gasification with combined cycle (IGCC) technology may reach a power efficiency of about 30 % calculated on the heat value of the black liquor. This may be compared with 12-13 % for the conventional recovery boiler. In other words, IGCC can increase power production by about 900 kWh/ADt, while at the same time reducing heat production by 4 GJ/ADt - which is more than a typical surplus in a Kraft mill (BREF, 2010. p. 310). Fuel savings of 1.6 GJ/t pulp is estimated for a complete gasification and combined cycle (Martin et al., 2000. p. 37).

It is also expected that black liquor gasification will reduce emissions of SOx,NOx,CO, volatile organic compounds, particulate matter, CH4, etc.

A study estimated total investment costs for gasification systems for a 2 720 ton of black liquid solids per day system to be $234 million for low-temperature system and $194 million for high-temperature system (2002 dollars) (US EPA, 2010. p.45). The investment cost of the technology is expected to cost $320/t production. Operation and Maintenance cost is estimated to be $6.9/t pulp (Martin et al., 2000. p. 37).

Demonstration
Collection of Most SpillagesTo handle the collected spills, 5-10% more evaporation capacity will be needed. This could consume 5-10% more steam and electrical power. However the collected spills may result in energy and chemical recovery when incinerated in the recovery boiler. The investment cost for spill-liquor handling systems at a kraft mill producing 1500 ADt/d pulp mill is estimated to be EUR 0.8 - 1.5 million. The operating cost of the system is in the range EUR 100000 - 400000/year. Commercial
High Temperature Odor-Free Recovery Boiler

With the new equipment, the boiler efficiency increases by 20%. For a boiler capacity of 150 t/hr and annual operating time of 24 h/day and 330 d/y, effective steam generation rate increases by 30 t/hr (NEDO, 2008. p. 164).

Indian flag In an Indian plant, with the installation of a high efficiency boiler, black liquor concentration was increased from 65% to 75% and resulting steam generation has increased from 2.6 t-steam/t-black liquor to 3.4 t-steam/t-black liquor (CII, 2008. p. 97).

Equipment cost is estimated as ¥1,000 million. Construction costs are approximately ¥300 million. With an oil price of ¥1.81/4.14 MJ, the system has a pay-back time o f3.7 years (NEDO, 2008. p. 164).

Indian flag

 The installation cost of the high-efficiency bolier in the Indian plant (with 1100 tpd solids firing capacity) is estimated at Rs. 70 crores. The boiler is expected to generate Rs. 27 crores of additional revenue due to increased electricity production (CII, 2008. p. 97).

Commercial
Lime Kiln ModificationsOne published estimate suggests that newer high-performance refractory use can lead to lime kiln energy savings of up to 5%. Average energy savings by the combined application of described measures are estimated to be 0.50 GJ/t pulp (Kramer et al., 2009. p.93). For these measures, investment cost of about $2.5/t [for the year 2000]pulp has been assumed (Martin et al., 2000. p.27). Commercial
Borate AutocaustisizingThe technique reduces lime kiln load and energy requirement. Research
Improved Composite Tubes for Kraft Recovery Boilers

The technique increases thermal efficiency.

Commercial
Infrared Camera for Kraft Recovery BoilersThe technology reduces soot-blowing steam usage by 20%. It also improves heat transfer and therefore reduces fuel use. The technology reduces NOx emissions.The technology reduces tube maintenance costsCommercial
Combined Heat and Power (CHP) GenerationWhile in conventional power plants only less than 40% of the energy input is converted into electricity – and the rest of the energy is wasted –in cogeneration facilities 80-93% of the energy input is converted into 40-70% heat and 20-45% electricity.

The CHP can reduce CO2 release rate by 50% compared to conventional power generation systems (BREF, 2010. p. 542).

The specific investment cost for transforming existing steam back-pressure units into combined cycle cogeneration plants is estimated to be EUR 1000/kW. The achievable savings and the payback time depend mainly on the price of electricity and fuels within the country. Commercial
Quaternary Air InjectionFor every avoided boiler reheat cycle, around 10.6 GJ of energy can be saved (Kramer et al., 2009. p..95). Each boiler reheat costs around $50 000. Capital costs for this measure are estimated between $ 300 000 and 500 000 (Kramer et al., 2009. p.95)Commercial
Combined Heat and Power in Chemical RecoveryWhile in conventional power plants only less than 40% of the energy input is converted into electricity – and the rest of the energy is wasted –in cogeneration facilities 80-93% of the energy input is converted into 40-70% heat and 20-45% electricity.

The CHP can reduce CO2 release rate by 50% compared to conventional power generation systems (BREF, 2010. p. 542).

The specific investment cost for transforming existing steam back-pressure units into combined cycle cogeneration plants is estimated to be EUR 1000/kW. The achievable savings and the payback time depend mainly on the price of electricity and fuels within the country. Commercial

Chemical Recovery Publications

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