Black liquor is the spent cooking liquor from the kraft process

Kraft process

The kraft process describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers...

 when digesting pulpwood

Pulpwood

Pulpwood refers to timber with the principal use of making wood pulp for paper production.-Applications:* Trees raised specifically for pulp production account for 16% of world pulp production, old growth forests 9% and second- and third- and more generation forests account for the balance...

 into paper pulp

Pulp (paper)

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...

 removing lignin

Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

, hemicellulose

Hemicellulose

A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...

s and other extractives from the wood to free the cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 fibers.

The equivalent spent cooking liquor in the sulfite process

Sulfite process

The sulfite process produces wood pulp which is almost pure cellulose fibers by using various salts of sulfurous acid to extract the lignin from wood chips in large pressure vessels called digesters. The salts used in the pulping process are either sulfites , or bisulfites , depending on the pH...

 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.

The black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals

Inorganic compound

Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 used in the process. The black liquor comprises 15% solids by weight of which 10% are inorganic and 5% are organic. Normally the organics in black liquor are 40-45% soaps, 35-45% lignin and 10-15% other organics.

The organic matter

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 in the black liquor is made up of water/alkali soluble degradation components from the wood. Lignin

Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

 is degraded to shorter fragments with sulphur content at 1-2% and sodium content at about 6% of the dry solids. Cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 and hemicellulose

Hemicellulose

A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...

 is degraded to aliphatic carboxylic acid soaps and hemicellulose fragments. The extractives gives tall oil

Tall oil

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"...

 soap and crude turpentine

Turpentine

Turpentine is a fluid obtained by the distillation of resin obtained from trees, mainly pine trees. It is composed of terpenes, mainly the monoterpenes alpha-pinene and beta-pinene...

. 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

Recovery boiler

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 or in making electricity,...

 by G.H. Tomlinson in the early 1930s, was a milestone in the advancement of the kraft process.

By 2000, the better kraft mills recovered 99.5% or more of the black liquor, and purified the remainder in biological treatment plants, reducing the environmental impact of the waste waters below the level of scientific significance, except perhaps in very small streams. 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, still recovered under 98% of the black liquor in 2007, which can cause some environmental issues, even when biologically treated. The general trend is for such obsolete mills to modernize or shut down.

Usage

The black liquor contains more than half of the energy content of the wood fed into the digester

Digester

A digester is a huge vessel where chemical or biological reactions are carried out. These are used in different types of process industries.Digester may also refer to :* Digester-Processes where digesters are used:* Anaerobic digestion...

 of a kraft pulp mill. It is normally concentrated to 65 - 80% by multi-effect evaporator

Evaporator

An evaporator is a device used to turn the liquid form of some chemical into its gaseous form. For example, an evaporator is used in an air conditioning system to allow the compressed cooling chemical to evaporate from liquid to gas, absorbing heat in the process.-Uses:As stated above, an...

s and burned in a recovery boiler

Recovery boiler

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 or in making electricity,...

 to produce energy and recover the cooking chemicals. The viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 increases as the concentration goes up. At about 50 - 55 % solids the salt solubility limit is reached. Tall oil

Tall oil

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"...

 is an important byproduct separated from the black liquor with skimming

Skimmer (machine)

An oil skimmer is a machine that separates a liquid from particles floating on it or from another liquid. A common application is removing oil floating on water. These technologies are commonly used for oil spill remediation but are also commonly found in industry...

 before it goes to the evaporators or after the first evaporator stage.

Energy source for the pulp mill

Pulp mill

Pulp mill

A pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fibre board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods...

s have used black liquor as an energy source since at least the 1930s.
Most kraft pulp mills use recovery boiler

Recovery boiler

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 or in making electricity,...

s to recover and burn much of the black liquor they produce, generating steam and recovering the cooking chemicals (sodium hydroxide and sodium sulfide

Sodium sulfide

Sodium sulfide is the name used to refer to the chemical compound Na2S, but more commonly it refers to the hydrate Na2S·9H2O. Both are colorless water-soluble salts that give strongly alkaline solutions...

 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. 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 million megawatt hours of electricity annually—more than the solar, wind and geothermal industries combined.

Gasification

New waste-to-energy

Waste-to-energy

Waste-to-energy or energy-from-waste is the process of creating energy in the form of electricity or heat from the incineration of waste source. WtE is a form of energy recovery...

 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

Energy conversion efficiency

Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

 than the conventional recovery boiler while generating an energy-rich syngas

Syngas

Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...

 from the liquor. The syngas can be burnt in a gas turbine

Gas turbine

A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 combined cycle

Combined cycle

In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...

 to produce electricity (usually called BLGCC for Black Liquor Gasification Combined Cycle; similar to IGCC) or converted through catalytic processes

Catalysis

Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 into chemicals or fuels such as methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

, dimethyl ether

Dimethyl ether

Dimethyl ether , also known as methoxymethane, is the organic compound with the formula . The simplest ether, it is a colourless gas that is a useful precursor to other organic compounds and an aerosol propellant. When combusted, DME produces minimal soot and CO, though HC and NOx formation is...

 (DME), or F-T

Fischer-Tropsch process

The Fischer–Tropsch process is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a petroleum substitute, typically from coal, natural gas, or biomass for use as synthetic...

 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

Chemrec

Chemrec is a Stockholm, Sweden corporation that develops technology for entrained flow gasification of black liquor and certain types brown liquor for production of biofuels from the resulting syngas....

 test facility in Piteå

Piteå

Piteå is a locality and the seat of Piteå Municipality in Norrbotten County, Sweden. The town has 22,650 inhabitants out of a municipal total of 41,000 in 2008.- Geography :...

, Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

. The DME synthesis step will be added in 2011 in the "BioDME" project, supported by the European Commission

European Commission

The European Commission is the executive body of the European Union. The body is responsible for proposing legislation, implementing decisions, upholding the Union's treaties and the general day-to-day running of the Union....

's Seventh Framework Programme (FP7) and the Swedish Energy Agency

Ministry of Enterprise, Energy and Communications (Sweden)

The Ministry of Enterprise, Energy and Communications is a ministry within the government of Sweden. Its areas of responsibility include business development and competition, electronic communications, energy, forestry, information technology , postal communications and cashier service, primary...

.

Used for biofuels production the black liquor gasification route has been shown to have very high conversion efficiency

Energy conversion efficiency

Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

 and greenhouse gas

Greenhouse gas

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 reduction potential.

Extraction of lignin

Where recovery boiler

Recovery boiler

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 or in making electricity,...

 capacity is limited and a bottleneck in the pulp mill

Pulp mill

A pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fibre 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 lignin

Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

 in the black liquor may be extracted and exported or used as fuel in the mill's lime kiln, thereby often replacing 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

Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users

The Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users is a funding and authorization bill that governs United States federal surface transportation spending. It was signed into law by President George W. Bush on August 10, 2005, and expired as of September 30, 2009...

 to reward and support the use of liquid alternative fuel

Alternative fuel

Alternative fuels, known as non-conventional or advanced fuels, are any materials or substances that can be used as fuels, other than conventional fuels...

 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

International Paper

International Paper Company is an American pulp and paper company, the largest such company in the world. It has approximately 59,500 employees, and it is headquartered in Memphis, Tennessee.-History:...

) this could amount to as much as $3.7 billion in benefits. Weyerhaeuser

Weyerhaeuser

Weyerhaeuser is one of the largest pulp and paper companies in the world. It is the world's largest private sector owner of softwood timberland; and the second largest owner of United States timberland, behind Plum Creek Timber...

announced in May 2009 that it was also pursuing the tax credit. While some have criticized the paper industry's eligibility for the alternative fuel mix tax credit on the grounds that it is increasing fossil fuel use, the industry has countered that adding a fossil fuel is actually a requirement of the law and that, regardless, this does not result in a net increase in fossil fuel use since companies are merely replacing the existing fossil fuel they already mix 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 Jan 1, 2010.