Black liquor

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A black liquor sample Black Liquor (cropped).jpg
A black liquor sample

In industrial chemistry, black liquor is the by-product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers. [1]

Contents

The equivalent material in the sulfite process is usually called brown liquor, but the terms red liquor, thick liquor and sulfite liquor are also used.[ citation needed ]

Composition

Approximately seven tonnes of black liquor are produced in the manufacture of one tonne of pulp. [2]

The black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process. The black liquor comprises 15% solids by weight of which two thirds are organic chemicals and the remainder are inorganic. [3] Normally the organics in black liquor are 40-45% soaps, [4] 35-45% lignin and 10-15% other organics.

The organic matter in the black liquor is made up of water/alkali soluble degradation components from the wood. [1] Lignin is degraded to shorter fragments with sulphur content at 1-2% and sodium content at about 6% of the dry solids. Cellulose and hemicellulose is degraded to aliphatic carboxylic acid soaps and hemicellulose fragments. The extractives gives tall oil soap and crude turpentine. The soaps contain about 20% sodium.[ citation needed ]

The residual lignin components currently serve for hydrolytic or pyrolytic conversion or just burning only. Hemicelluloses may undergo fermentation processes, alternatively.[ citation needed ]

Black liquor creates chemical oxygen demand and biological oxygen demand, and is highly alkaline, dark colored and may be warm. [5] These properties contribute to the toxicity of plant effluent spills due in part to rapid depletion of dissolved oxygen and changes in water pH, [6] with additional possible long-term effects on ecosystems from the presence of adsorbable organic halides generated by chlorine bleaching processes. The composition of black liquor is also influenced by the type of material used to create the paper, such as softwood. [5]

History

Early kraft pulp mills discharged black liquor to watercourses. Black liquor is quite toxic to aquatic life, and causes a very dark caramel color in the water. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process. [7]

By the 1990s, most kraft mills were consuming nearly all of their black liquor byproduct, [8] and purifying the remainder in biological treatment plants, reducing the environmental effect of the waste waters below the level of scientific significance, except perhaps in very small streams.[ citation needed ] Even in the 21st century, some small kraft mills remained (producing at most a few tons of pulp per day) that discharged all black liquor. However, these are rapidly disappearing. Some kraft mills, particularly in North America,[ citation needed ] still recovered under 98% of the black liquor in 2007, which can cause some environmental contamination, even when biologically treated. The general trend is for such obsolete mills to modernize or shut down.[ citation needed ]

In August 2011, a fish and mollusk kill occurred on the Pearl River when a neighboring Louisiana paper mill accidentally discharged black liquor due to a problem in its waste water treatment system. [9] [10]

Usage

The black liquor contains more than half of the energy content of the wood fed into the digester of a kraft pulp mill. [11] It is normally concentrated to 65 - 80% by multi-effect evaporators and burned in a recovery boiler to produce energy and recover the cooking chemicals. The viscosity increases as the concentration goes up. At about 50 - 55% solids the salt solubility limit is reached. [12] Tall oil is an important byproduct separated from the black liquor with skimming before it goes to the evaporators or after the first evaporator stage.

Energy source for the pulp mill

Pulp mills have used black liquor as an energy source since at least the 1930s. [8] Most kraft pulp mills use recovery boilers to recover and burn much of the black liquor they produce, generating steam and recovering the cooking chemicals (sodium hydroxide and sodium sulfide used to separate lignin from the cellulose fibres needed for papermaking). This has helped paper mills reduce problems with water emissions, reduce their use of chemicals by recovery and reuse, and become nearly energy self-sufficient by producing, on average, 66 percent of their own electricity needs on-site.[ citation needed ]

In the United States, paper companies have consumed nearly all of the black liquor they produce since the 1990s. [8] As a result, the forest products industry has become one of the United States' leading generators of carbon-neutral renewable energy, producing approximately 28.5 terawatt hours of electricity annually.[ citation needed ]

Use as biofuel feedstock

Gasification

Black liquor as used for gasification Svartlut 76.jpg
Black liquor as used for gasification

New waste-to-energy methods to recover and utilize the energy in the black liquor have been developed.[ citation needed ] The use of black liquor gasification has the potential to achieve higher overall energy efficiency than the conventional recovery boiler, while generating an energy-rich syngas from the liquor.[ citation needed ] The syngas can be burnt in a gas turbine combined cycle to produce electricity (usually called BLGCC for Black Liquor Gasification Combined Cycle; similar to IGCC) or converted through catalytic processes into chemicals or fuels such as methanol, dimethyl ether (DME), or F-T diesel (usually called BLGMF for Black Liquor Gasification for Motor Fuels). This gasification technology is currently[ when? ] under operation in a 3 MW pilot plant at Chemrec's [13] test facility in Piteå, Sweden. The DME synthesis step will be added in 2011[ needs update ] in the "BioDME" project, supported by the European Commission's Seventh Framework Programme (FP7) and the Swedish Energy Agency. [14]

Used for biofuels production, the black liquor gasification route has been shown to have very high conversion efficiency and greenhouse gas reduction potential. [15]

Hydrothermal liquefaction

Hydrothermal liquefaction is suitable for converting black liquor to advanced biofuels due to the process's ability to handle high moisture inputs. [16]

Extraction of lignin

Where recovery boiler capacity is limited and a bottleneck in the pulp mill, the lignin in the black liquor may be extracted, then exported or used as fuel in the mill's lime kiln. This often replaces fossil-based fuel with biofuel.[ citation needed ]

U.S. tax credit 2007–2010

A tax credit created by the U.S. Congress in 2005 as part of the 2005 Highway Bill to reward and support the use of liquid alternative fuel derived from hydrocarbons in the transportation sector was expanded in 2007 to include non-mobile uses of liquid alternative fuel derived from biomass.[ citation needed ] This change meant that, in addition to fish processors, animal renderers and meat packers, kraft pulp producers became eligible for the tax credit as a result of their generation and use of black liquor to make energy. For one large company (International Paper) this could amount to as much as $3.7 billion in benefits. [8] [17] Weyerhaeuser announced in May 2009 that it was also pursuing the tax credit. [18] Some paper industry analysts [19] criticized the paper industry's eligibility for the alternative fuel mix tax credit on the grounds that it increased fossil fuel use, but the industry countered that adding a fossil fuel was a requirement of the law and that, regardless, this did not result in a net increase in fossil fuel use since companies were merely replacing the existing fossil fuel they already mixed with black liquor—natural gas—with one of the three fuels specified by the law: gasoline, kerosene or diesel. The bio-fuel credit for black liquor ended on January 1, 2010.[ citation needed ]

Related Research Articles

<span class="mw-page-title-main">Lignin</span> Structural phenolic polymer in plant cell walls

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.

<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically or mechanically producing cellulosic fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

<span class="mw-page-title-main">Pulpwood</span> Timber intended for processing into wood pulp for paper production

Pulpwood can be defined as timber that is ground and processed into a fibrous pulp. It is a versatile natural resource commonly used for paper-making but also made into low-grade wood and used for chips, energy, pellets, and engineered products.

<span class="mw-page-title-main">Bagasse</span> Residue of sugar cane after juice extraction

Bagasse is the dry pulpy fibrous material that remains after crushing sugarcane or sorghum stalks to extract their juice. It is used as a biofuel for the production of heat, energy, and electricity, and in the manufacture of pulp and building materials. Agave bagasse is similar, but is the material remnants after extracting blue agave sap.

<span class="mw-page-title-main">Kraft process</span> Process of converting wood into wood pulp

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes.

<span class="mw-page-title-main">Paper recycling</span> Process by which waste paper is turned into new paper products

The recycling of paper is the process by which waste paper is turned into new paper products. It has a number of important benefits: It saves waste paper from occupying homes of people and producing methane as it breaks down. Because paper fibre contains carbon, recycling keeps the carbon locked up for longer and out of the atmosphere. Around two-thirds of all paper products in the US are now recovered and recycled, although it does not all become new paper. After repeated processing the fibres become too short for the production of new paper, which is why virgin fibre is frequently added to the pulp recipe.

<span class="mw-page-title-main">Biorefinery</span> Refinery that converts biomass to energy and other beneficial byproducts

A biorefinery is a refinery that converts biomass to energy and other beneficial byproducts. The International Energy Agency Bioenergy Task 42 defined biorefining as "the sustainable processing of biomass into a spectrum of bio-based products and bioenergy ". As refineries, biorefineries can provide multiple chemicals by fractioning an initial raw material (biomass) into multiple intermediates that can be further converted into value-added products. Each refining phase is also referred to as a "cascading phase". The use of biomass as feedstock can provide a benefit by reducing the impacts on the environment, as lower pollutants emissions and reduction in the emissions of hazard products. In addition, biorefineries are intended to achieve the following goals:

  1. Supply the current fuels and chemical building blocks
  2. Supply new building blocks for the production of novel materials with disruptive characteristics
  3. Creation of new jobs, including rural areas
  4. Valorization of waste
  5. Achieve the ultimate goal of reducing GHG emissions
<span class="mw-page-title-main">Pulp mill</span> Facility which pulps wood or plant fibre

A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fully chemical methods. The finished product may be either bleached or non-bleached, depending on the customer requirements.

Tall oil, also called liquid rosin or tallol, is a viscous yellow-black odorous liquid obtained as a by-product of the kraft process of wood pulp manufacture when pulping mainly coniferous trees. The name originated as an anglicization of the Swedish tallolja. Tall oil is the third largest chemical by-product in a kraft mill after lignin and hemicellulose; the yield of crude tall oil from the process is in the range of 30–50 kg / ton pulp. It may contribute to 1.0–1.5% of the mill's revenue if not used internally.

<span class="mw-page-title-main">Lignocellulosic biomass</span> Plant dry matter

Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of biofuels. It is composed of two kinds of carbohydrate polymers, cellulose and hemicellulose, and an aromatic-rich polymer called lignin. Any biomass rich in cellulose, hemicelluloses, and lignin are commonly referred to as lignocellulosic biomass. Each component has a distinct chemical behavior. Being a composite of three very different components makes the processing of lignocellulose challenging. The evolved resistance to degradation or even separation is referred to as recalcitrance. Overcoming this recalcitrance to produce useful, high value products requires a combination of heat, chemicals, enzymes, and microorganisms. These carbohydrate-containing polymers contain different sugar monomers and they are covalently bound to lignin.

<span class="mw-page-title-main">Recovery boiler</span> Industrial equipment used in pulping

Recovery boiler is the part of kraft process of pulping where chemicals for white liquor are recovered and reformed from black liquor, which contains lignin from previously processed wood. The black liquor is burned, generating heat, which is usually used in the process of making electricity, much as in a conventional steam power plant. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process.

Second-generation biofuels, also known as advanced biofuels, are fuels that can be manufactured from various types of non-food biomass. Biomass in this context means plant materials and animal waste used especially as a source of fuel.

Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness is an important characteristic. These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

Soda pulping is a chemical process for making wood pulp with sodium hydroxide as the cooking chemical. In the Soda-AQ process, anthraquinone (AQ) may be used as a pulping additive to decrease the carbohydrate degradation. The soda process gives pulp with lower tear strength than other chemical pulping processes, but has still limited use for easily pulped materials like straw and some hardwoods.

<span class="mw-page-title-main">Environmental impact of paper</span>

The environmental impact of paper are significant, which has led to changes in industry and behaviour at both business and personal levels. With the use of modern technology such as the printing press and the highly mechanized harvesting of wood, disposable paper became a relatively cheap commodity, which led to a high level of consumption and waste. The rise in global environmental issues such as air and water pollution, climate change, overflowing landfills and clearcutting have all lead to increased government regulations. There is now a trend towards sustainability in the pulp and paper industry as it moves to reduce clear cutting, water use, greenhouse gas emissions, fossil fuel consumption and clean up its influence on local water supplies and air pollution.

Molten salt oxidation is a non-flame, thermal process that destroys all organic materials while simultaneously retaining inorganic and hazardous components in the melt. It is used as either hazardous waste treatment (with air) or energy harvesting similar to coal and wood gasification (with steam). The molten salt of choice has been sodium carbonate (m.p 851°C), but other salts can be used. Sulfur, halogens, phosphorus and similar volatile pollutants are oxidized and retained in the melt. Most organic carbon content leaves as relatively pure CO/CO2/H2/H2O gas (depending on the feed conditions, whether steam or air is used), and the effluent only requires a cold trap and a mild aqueous wash (except mercury-containing wastes). It has been used for safe biological and chemical weapons destruction, and processing waste such as scrap tires where direct incineration/effluent treatment is difficult. The major downside of the process compared to direct incineration is the eventual saturation of the melt by contaminants, and needing reprocessing/replacement.

White liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is used in the first stage of the Kraft process in which lignin and hemicellulose are separated from cellulose fiber for the production of pulp. The white liquor breaks the bonds between lignin and cellulose. It is called white liquor due to its white opaque colour.

Chemrec AB (Chemrec) is a Stockholm, Sweden-based company with comprehensive experience of pioneering the development of black liquor gasification (BLG) technology for energy and chemicals recovery at pulp mills.

In industrial paper-making processes, organosolv is a pulping technique that uses an organic solvent to solubilise lignin and hemicellulose. It has been considered in the context of both pulp and paper manufacture and biorefining for subsequent conversion of cellulose to fuel ethanol. The process was invented by Theodor Kleinert in 1968 as an environmentally benign alternative to kraft pulping.

References

  1. 1 2 Stenius, Per, ed. (2000). "2". Forest Products Chemistry. Papermaking Science and Technology. Vol. 3. Helsinki, Finland: Fapet OY. pp. 62–78. ISBN   978-952-5216-03-5.
  2. ↑ Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking . San Diego: Academic Press, Inc. ISBN   978-0-12-097360-6.
  3. ↑ Handbook of Pulping and Papermaking SE, 1996, ISBN   0-12-097362-6
  4. ↑ Speight, James G. (2019-01-01). "Upgrading by Gasification". Heavy Oil Recovery and Upgrading. Gulf Professional Publishing. pp. 559–614. doi:10.1016/b978-0-12-813025-4.00013-1. ISBN   978-0-12-813025-4. S2CID   186809412.
  5. 1 2 Ali, Muna; Sreekrishnan, T.R. (2001). "Aquatic toxicity from pulp and paper mill effluents: a review". Advances in Environmental Research. 5 (2): 175–196. doi:10.1016/S1093-0191(00)00055-1.
  6. ↑ Díaz, Ana Isabel; Laca, Adriana; Lima, Nelson; Díaz, Mario (2022). "Treatment of kraft black liquor using basidiomycete and ascomycete fungi". Process Safety and Environmental Protection. 168: 67–76. Bibcode:2022PSEP..168...67D. doi: 10.1016/j.psep.2022.09.065 .
  7. ↑ E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press.
  8. 1 2 3 4 Mufson, Steven (March 28, 2009). "Papermakers Dig Deep in Highway Bill To Hit Gold". The Washington Post . Retrieved 2009-06-03.
  9. ↑ "TIN Pearl River Spill - LA". U.S. Environmental Protection Agency On-Scene Coordinator. Retrieved 29 August 2024.
  10. ↑ Pearl River Fish Kill Post Incident Monitoring Report 2012 - 2014 (Report). Louisiana Department of Wildlife and Fisheries. Retrieved 29 August 2024– via Environmental Conservation Online System.
  11. ↑ The Center for Paper Business and Industry Studies (CPBIS)
  12. ↑ Empie, H. Jeff (2009). Fundamentals of the craft recovery process. USA: Tappi Press. p. 7. ISBN   978-1-59510-186-0.
  13. ↑ Chemrec Archived 2010-08-11 at the Wayback Machine website
  14. ↑ BioDME Project website
  15. ↑ Well-to-Wheels analysis of future automotive fuels and powertrains in the European context Archived 2011-03-04 at the Wayback Machine EUCAR / Concawe /JRC Well-to-Wheels Report Version 2c, March 2007
  16. ↑ Kosinkova, Jana; Ramirez, Jerome; Nguyen, Jenny; Ristovski, Zoran; Brown, Richard; Lin, Carol; Rainey, Thomas (10 Sep 2015). "Hydrothermal liquefaction of bagasse using ethanol and black liquor as solvents" (PDF). Biofuels, Bioproducts and Biorefining. 9 (6): 630–638. doi:10.1002/bbb.1578. S2CID   85838194.
  17. ↑ "International Paper Provides Update on Alternative Fuel Credits". International Paper. March 24, 2009. Archived from the original on July 14, 2012. Retrieved 2009-06-03.
  18. ↑ "Weyerhaeuser says applied to become qualified as alternative fuel mixer". CNBC. May 5, 2009. Retrieved 2009-06-03.[ dead link ]
  19. ↑ Dizikes, Cynthia (April 24, 2009). "Paper firms reap billions from tax credit — but should they?". MinnPost . Retrieved September 19, 2021.
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