Material use of lignite – an opportunity for the energy transition

Lignite can do more – as an indispensable source of carbon

Lignite plays an important role for power generation in Germany as a non-subsidised, domestic resource that will be available for many years to come thanks to extensive reserves. In addition, the benefit of lignite is that almost all of the added value derived from it remains here in Germany.

But lignite has even more to offer. As a source of carbon, it can also be put to material use. Basic materials for the petrochemical or chemical industry can be derived from it, such as naphtha, waxes, ammonia or methanol. Alternative fuels for diesel engines like DME (dimethyl ether) and OME (oxy methyl ether) can also be produced from it. They can significantly improve the exhaust gas behaviour of the most efficient combustion engine, in carbon emission terms, by substantially reducing the amount of NOx and soot emissions. Products made from lignite that in most cases would otherwise be produced from oil or natural gas lead at the further processing stage to higher yields of downstream products or ensure clean combustion. In addition, products made out of lignite are free of sulphur and have low levels of aromatic compounds.

Given the growing worldwide demand for raw materials and the fact that the supply of raw materials is frequently dictated by unpredictable political and economic developments, lignite can make a significant contribution to security of supply and diversification of raw material sources for the petro-chemical industry in Germany. The Enquete Commission II of the North Rhine-Westphalian Parliament came to the same conclusion in the spring of 2015 in its study entitled “The Future of the Chemical Industry in North Rhine-Westphalia”. In its “Final report with regard to sustainable resources, products and production processes” it ascribes an important role to lignite. The report concludes that material use of lignite would diversify the basis of supply for the chemical industry and reduce its dependence on imports (Press release).

From coal to fuel – how does it work?

The processes used to convert lignite to chemical and petrochemical feedstocks are referred to as "Coal-to-Liquids" or "Coal-to-Gas" techniques (CTL and CTG in short).

Zoom Synthesis gas © francis bonami, Fotolia.com;
Basic chemicals © Zsolnai Gergely, Fotolia.com 
Firstly lignite, which has a water content of about 50%, is dried until it has a residual water content of around 12%. Then, in the gasifier, it is converted at high temperatures and with a limited amount of oxygen so that, instead of the coal being burnt, a gas compound known as "synthesis gas" is formed. The main components of the gas compound are hydrogen and carbon monoxide as well as minimal amounts of hydrogen sulphide. Depending on the desired end product, aside from separation of the CO2 and H2S components, the next step of gas treatment involves adjustment of the hydrogen-carbon monoxide ratio. In the final step, known as synthesis, the desired end product, e.g. methanol, ammonia, naphtha (basic material for the production of plastics, for instance), or methane (main component of natural gas) is formed.

The use of Rhenish lignite in these CtL/CtG techniques is based on two key technologies: coal drying and coal gasification.

RWE has accumulated decades of experience in these two technologies from numerous projects, pilot and demonstration plants. The company has even developed a proprietary process: "fluidised-bed drying with internal waste heat utilisation", or WTA® for short.

Challenge 1: Optimising investment costs

Exploiting synergies: the 'annex' scheme

Commercial use of coal as a raw material supplier for the chemical and petrochemical industry is already happening in some countries, such as China, South Africa and the USA.
Whether this will be the case in Europe and specifically in Germany depends primarily on the development of oil and gas prices and CO2 allowance costs, but also on the capex for production facilities.

The route from coal to chemical product can be economically improved by so-called annex schemes. Existing industrial and power plant locations are vetted to determine whether it is possible to integrate, for instance, a synthesis gas plant into an existing site in order to make optimal use of infrastructure that is already in place.

Developing new processing concepts

There is further cost-reduction potential to be derived from systematic development of alternative plant concepts. RWE is currently researching a new scrubbing process for treating the raw gas that comes from the gasifier to the point where it is of a suitable quality for synthesis. It is anticipated that this new scrubbing concept will roughly halve the cost of investment in gas treatment plants.

Normally the Rectisol process is used in CtL/CtG plants for removing sulphur components and CO2. Since ice-cold methanol is used as the scrubbing agent, the necessary refrigeration systems make it a capital-intensive process. The alternative gas treatment process designed by an RWE project team combines other tried and true process steps. As a first step, most of the sulphur and CO2 components that are unsuitable for synthesis gas are removed under pressure with the aid of an amine detergent. Then in the ensuing regeneration phase, the laden amine detergent is freed of those components. The mixture of CO2 and hydrogen sulphide released as a result of this process is scrubbed in an H2O2 scrubbing chamber (hydrogen peroxide), where hydrogen sulphide is oxidised to sulphur in solid form as is continually separated from the surrounding scrubbing agent. The end result is that carbon dioxide in its purest form leaves the chamber and the amine-treated synthesis gas is further refined in an alkaline scrubbing chamber to produce ultra-clean synthesis gas. The used alkaline solution is then treated with atmospheric oxygen and steam until it is safe to be disposed of as wastewater.

Challenge 2: Reducing CO2 emissions

Compared to lignite power generation, material use of coal produces considerably less CO2, since about 50% of the carbon atoms are bound within the products themselves. By using renewables, any remaining CO2 emissions can be reduced to zero and the use of renewables is flexible, depending on the extent to which they are currently available. Material use of lignite is therefore an efficient and simple vehicle for increased use of renewables in the supply of raw materials for the chemical industry.

What options are available?

Biomass as a carbon source is one option for material use of lignite. During the gasification process, some of the lignite can be replaced by biomass. This is a way of creating a carbon cycle for the supply of raw materials to the chemical industry. The extent to which biomass can be used for this purpose largely depends on its availability.

Another option is the use of surplus power generated by photovoltaic panels and wind turbines. From our point of view, three alternatives have particular merit. Hydrogen can be produced from electricity via electrolysis. If instead of hydrogen an internal shift reaction within the process can be used, the formation of CO2 in the shift can be reduced to zero and, at the same time, the product yield is almost doubled.

Another option has the aim of reducing the amount of CO2 generated by the gasification process. With gasification, the amount of oxygen required to turn solid carbon into gaseous carbon monoxide has to be added to the gasifier. A further condition for the gasification process is that it has to occur at a certain minimum temperature. In practice, more oxygen is therefore used to generate heat than is actually required to convert the solid carbon into CO – the process by which carbon dioxide is formed. The idea of the RWE researchers is not to provide the required heat by burning coal but by applying electricity. The gasification plant itself could be heated by the electricity generated from surplus renewables. Preheating of the materials that usually flow into the gasifier at low temperatures (fuel, oxygen, fluidisation and transport media) is also conceivable. Calculations show that up to 20% of the usual carbon dioxide emissions can be eliminated in this way.

A third option uses unavoidable CO2 from industrial processes (e.g. cement and raw iron) as the carbon source for the process. In combination with renewable hydrogen, conventional synthesis gas can be produced from CO2 by the reverse shift reaction process, which can then be used to produce the above-mentioned products. This option is naturally dependent on economically competitive hydrogen being produced from “green” power. It could create a complete carbon cycle in the raw material industry, but is currently the least economically efficient option. The reason for this is the very high investment cost of electrolysers and insufficient quantities of economically viable surplus electricity. Until the economic efficiency of this form of CO2 utilisation becomes a reality, lignite is the appropriate vehicle for testing the necessary technologies of a large-scale option with good future prospects.

Current activities

In talks with potential customers in Germany and neighbouring countries, RWE is currently investigating certain chemical and petrochemical markets and analysing potentially suitable locations. The goal is an economically attractive product portfolio which includes the necessary infrastructure for material use of lignite.

In addition, the first phase of plant and systems planning at a technical processing level is already underway.

As part of the BMWi-sponsored Fabiene project, RWE will be working until 2020 with partners TU Darmstadt and ThyssenKrupp Industrial Solutions from Dortmund on technical trials of the key components of CtL/CtG processes. Applied research by our Coal Innovation Centre at the Niederaussem power plant will underpin the work of this project.

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