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.

Composition

Approximately 7 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.

The residual lignin components currently serve for hydrolytic or pyrolytic conversion or just burning only. Hemicellulosis may undergo fermentation processes, alternatively.

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. [5]

By the 1990s, most kraft mills were consuming nearly all of their black liquor byproduct, [6] 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 ]

Usage

The black liquor contains more than half of the energy content of the wood fed into the digester of a kraft pulp mill. [7] 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. [8] 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. [6] 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.

In the United States, paper companies have consumed nearly all of the black liquor they produce since the 1990s. [6] 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.

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. 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. 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 under operation in a 3 MW pilot plant at Chemrec's [9] test facility in Piteå, Sweden. The DME synthesis step will be added in 2011 in the "BioDME" project, supported by the European Commission's Seventh Framework Programme (FP7) and the Swedish Energy Agency. [10]

Used for biofuels production, the black liquor gasification route has been shown to have very high conversion efficiency and greenhouse gas reduction potential. [11]

Hydrothermal liquefaction

Hydrothermal liquefaction is suitable for converting black liquor to advanced biofuels due to the process's ability to handle high moisture inputs. [12]

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.

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. 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. [6] [13] Weyerhaeuser announced in May 2009 that it was also pursuing the tax credit. [14] Some paper industry analysts [15] 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.

Related Research Articles

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

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemical 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">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.

Cellulosic ethanol is ethanol produced from cellulose rather than from the plant's seeds or fruit. It can be produced from grasses, wood, algae, or other plants. It is generally discussed for use as a biofuel. The carbon dioxide that plants absorb as they grow offsets some of the carbon dioxide emitted when ethanol made from them is burned, so cellulosic ethanol fuel has the potential to have a lower carbon footprint than fossil fuels.

<span class="mw-page-title-main">Biorefinery</span>

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>

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.

<span class="mw-page-title-main">Biomass to liquid</span>

Biomass to liquid is a multi-step process of producing synthetic hydrocarbon fuels made from biomass via a thermochemical route.

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>

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.

Treethanol is an ethanol fuel made from trees.

<span class="mw-page-title-main">Recovery boiler</span>

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.

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.

Dissolving pulp, also called dissolving cellulose, is bleached wood pulp or cotton linters that has a high cellulose content. It has special properties including a high level of brightness and uniform molecular-weight distribution. This pulp is manufactured for uses that require a high chemical purity, and particularly low hemicellulose content, since the chemically similar hemicellulose can interfere with subsequent processes. Dissolving pulp is so named because it is not made into paper, but dissolved either in a solvent or by derivatization into a homogeneous solution, which makes it completely chemically accessible and removes any remaining fibrous structure. Once dissolved, it can be spun into textile fibers, or chemically reacted to produce derivatized celluloses, such cellulose triacetate, a plastic-like material formed into fibers or films, or cellulose ethers such as methyl cellulose, used as a thickener.

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.

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.

<span class="mw-page-title-main">Alholmens Kraft Power Station</span>

The Alholmens Kraft Power Station is a biomass power station in Alholmen, Jakobstad in Ostrobothnia region, Finland. It is the largest biomass cogeneration power station in the world.

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.

Cellulosic sugars are derived from non-food biomass (e.g. wood, agricultural residues, municipal solid waste). The biomass is primarily composed of carbohydrate polymers cellulose, hemicellulose, and an aromatic polymer (lignin). The hemicellulose is a polymer of mainly five-carbon sugars C5H10O5 (xylose). and the cellulose is a polymer of six-carbon sugar C6H12O6 (glucose). Cellulose fibers are considered to be a plant’s structural building blocks and are tightly bound to lignin, but the biomass can be deconstructed using Acid hydrolysis, enzymatic hydrolysis, organosolv dissolution, autohydrolysis or supercritical hydrolysis. A more recent mechanical method offers hope that at last, a more economic and waste free method has been found although it is still to scale and is not yet commercial.

Mechanical pulping is the process in which wood is separated or defibrated mechanically into pulp for the paper industry.

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. ↑ E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press.
  6. 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.
  7. ↑ The Center for Paper Business and Industry Studies (CPBIS)
  8. ↑ Empie, H. Jeff (2009). Fundamentals of the craft recovery process. USA: Tappi Press. p. 7. ISBN   978-1-59510-186-0.
  9. ↑ Chemrec Archived 2010-08-11 at the Wayback Machine website
  10. ↑ BioDME Project website
  11. ↑ 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
  12. ↑ 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.
  13. ↑ "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.
  14. ↑ "Weyerhaeuser says applied to become qualified as alternative fuel mixer". CNBC. May 5, 2009. Retrieved 2009-06-03.[ dead link ]
  15. ↑ Dizikes, Cynthia (April 24, 2009). "Paper firms reap billions from tax credit — but should they?". MinnPost . Retrieved September 19, 2021.
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