Technologies, challenges and practical examples

An overview of pyrolysis processes

The pyrolysis of biomass can be divided into different processes, which differ primarily in terms of temperature, residence time and heating rate [1]. In so-called slow pyrolysis, which is particularly widely used, the focus is on moderate temperatures between 300 and 600 °C and longer residence times. The aim here is to maximise the yield of biochar. Rapid pyrolysis, on the other hand, heats the biomass very quickly to temperatures between 450 and 550 °C, with pyrolysis oil being the main product. A special form is flash pyrolysis with short residence times, although this is mainly used in laboratory settings. New technologies such as microwave pyrolysis or catalytic pyrolysis – in which catalysts are used to specifically influence the products – also play a role, but are still under development. In practice, slow pyrolysis in particular has become established, as it produces large quantities of biochar and thus makes a significant contribution to carbon sequestration.

The carbon cycle

The carbon cycle illustrates why pyrolysis is such an effective process. During photosynthesis, plants absorb carbon dioxide from the atmosphere and store the carbon in their biomass. If this biomass were to decompose naturally or be burnt, the carbon would be released back into the air in the form of CO₂. Through pyrolysis, however, it is converted into stable biochar and is thus permanently removed from the atmosphere. Biochar can remain in the soil for centuries and thus acts as a long-term carbon sink [2].

Properties of the pyrolysis gas

In addition to biochar, pyrolysis produces a gas mixture whose composition depends heavily on the process conditions. Typically, it contains a high proportion of carbon monoxide, hydrogen and methane, supplemented by carbon dioxide and nitrogen. Small quantities of higher hydrocarbons, as well as tar and condensate compounds, may also be present. The temperature, heating rate, residence time and moisture content of the feedstock biomass are key determinants of the gas’s composition. Higher temperatures and short residence times favour the formation of carbon monoxide and hydrogen, whilst lower temperatures promote the formation of tars.

Two-stage combustion as a solution

A key technical problem is the emission of nitrogen oxides during the combustion of the gas. These arise in particular from the nitrogen bound in the fuel. To reduce them, the two-stage combustion process has proven effective [3]. In the first stage, the gas is combusted under oxygen-deficient conditions, whereby nitrogen in the fuel is preferentially converted to molecular nitrogen. In the second stage, additional air is supplied to ensure complete combustion. Homogeneous mixing and precise temperature control are crucial to minimise the formation of thermal nitric oxide. The spatial separation of the two combustion zones also facilitates the control of temperature profiles.

The schematic layout of a two-stage reactor is shown in Figure 1. In the concept shown, the two combustion zones are spatially separated from one another, thereby reducing heat transfer by radiation between the stages (Figure 2). This facilitates precise control of temperature profiles and promotes the selective conversion of nitrogen compounds.

Advantages and challenges

The two-stage process offers several advantages. It acts directly within the combustion process and significantly reduces the formation of nitrogen oxides. As a result, in many cases, secondary measures such as selective catalytic or non-catalytic reduction can be dispensed with. This reduces dependence on chemical additives and lowers operating costs. Furthermore, the systems are characterised by high plant availability and offer stable ignition even under difficult conditions. Added to this is a high degree of flexibility in the use of different fuels. Although the technology requires higher capital expenditure and additional space, the costs are usually recouped within a short time through the savings in operating costs.

Case study: Denmark

In Denmark, the company Stiesdal operates a pyrolysis plant with a capacity of up to 20 mw [4]. Around half of the carbon from the biomass is converted into biochar, whilst the remainder is produced as synthesis gas. Due to the high nitrogen content in the gas and the associated NO_X potential (up to 6700 mg/Nm³@3%O₂ at a 100 % conversion rate), the plant utilises a two-stage combustion process. Initial results show that NO_X emissions can be kept below 150 mg per standard cubic metre, depending on the optimal configuration of the plant. Control is managed via a system developed by SAACKE (se@vis) and is continuously being optimised (Figure 3).

Case study: Norway

Another example can be found in Norway, where the company VOW Green Metals is constructing a plant for the production of biochar for the metallurgical industry (Figure 4). In addition to biochar, pyrolysis gas and condensates are also produced, which are utilised for energy in a specially developed combustion chamber. Here too, the two-stage combustion process ensures that nitrogen oxide emissions remain low and that the fuels are used efficiently. The complete combustion unit, featuring a low-NO_X combustion chamber, is supplied by SAACKE. The plant’s engineering was carried out by SAACKE’s Combustion department. The plant is currently under construction.

Conclusion

In summary, it is clear that the pyrolysis of agricultural residues represents an effective and forward-looking process for reducing CO₂ emissions. By converting biogenic residues into biochar, carbon is sequestered in a stable form and permanently removed from the atmosphere. At the same time, the use of the resulting gas enables additional energy generation. Thanks to modern two-stage combustion processes, this can even be achieved with low emissions and without costly additional measures.

Of particular interest is the possibility of deploying compact and modular plants on a decentralised basis, directly at the sites where the residues are generated. This reduces transport distances, increases regional value creation and facilitates integration into existing structures. It is thus clear that pyrolysis on an industrial scale can be a significant building block for a climate-neutral energy and raw materials economy.