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Image shows burning domestic biomass pellets

Large-scale Biomass Combustion

Overview of Large-Scale Biomass Combustion

There is a growing trend to burn biomass on an industrial scale as a source of dependable, low-carbon energy. However, there is a price to pay in terms of toxic emissions such as carbon monoxide, dioxins, particulate matter and oxides of nitrogen, unless these are carefully controlled. In addition methane, whilst not toxic, is a powerful greenhouse gas and therefore emissions of this must be minimised too.

Once we move beyond the domestic scale, both the importance of reducing emissions and the means available to do so increase. Industrial-scale biomass combustion plants have more sophisticated controls than the domestic-scale variety, meaning that the composition of the flue gas can be kept within a tighter range. This allows the use of more advanced abatement technology, including selective catalytic reduction (SCR), electrostatic precipitators (ESP) and oxidation catalytic converters.

Permits are usually required to establish new plants, and compliance with stringent emissions standards is often a condition of obtaining these. Financial incentives may include grant finance or the ability to sell carbon credits, however obtaining either of these is likely to require very careful attention to the impact both on air quality as well as greenhouse gas emissions.

The chemistry of fumes from industrial scale biomass combustion

Discover more about the chemistry of pollution from large-scale biomass combustion and how it can be abated.

Biomass is often described as ‘carbon’, and in broad terms this is true, but unfortunately there are quite a lot of other molecules in there as well, which make ‘clean’ combustion more difficult to achieve.

Even if we did burn pure carbon, we would still get some pollution due to incomplete combustion, in the form of carbon monoxide and soot particles. In addition, the heat of the combustion process would cause a reaction between the oxygen and nitrogen in the air, resulting in the formation of oxides of nitrogen.

The non-carbon molecules in the fuel each have their own effect on emissions too. The most important of these are tar compounds, which if not fully combusted can be emitted as volatile organic compounds, some of which may condense to form particulate matter.

Minerals present in the fuel will generally turn into ash, some of which could be released as dust. Biomass made from stem-products such as straw also contains chlorine which will have an impact on the emissions.

Oxidation of fumes from industrial scale biomass combustion

Discover how catalytic converters can oxidise the fumes from large-scale biomass combustion.

Many of the pollutants created when burning biomass are the result of incomplete combustion, for example carbon monoxide, VOCs, tar compounds, etc. Catalytic oxidation can be regarded as a secondary combustion process in which any organic compounds left over from the primary combustion are oxidised into carbon dioxide and water.

Catalytic oxidation is temperature-dependent, and generally requires temperatures in the range 250-450°C to be effective. However, this is significantly lower than is required for non-catalytic oxidation techniques, such as the use of afterburners, and therefore it is often a more economical and environmentally-friendly solution.

When designing a catalytic oxidation system for a large-scale biomass application, the temperature of the flue gases is always the first consideration. The next stage is to think about chaff and dust, to ensure that the design of the catalytic converter prevents it becoming blocked up. Thirdly, we need to consider corrosive substances in the flue gas such as sulphur and chlorine which can shorten the life of the catalyst unless specialist coatings are used.

Using SCR to reduce pollution from biomass combustion

Selective Catalytic Reduction (SCR) technology can help industrial scale biomass combustion plants meet their permitted emissions limits.

SCR stands for Selective Catalytic Reduction, and is a technology used for breaking down oxides of nitrogen (NOx).

In chemistry, ‘reduction’ refers to a reaction in which oxygen atoms are separated from another element, i.e. it is the opposite of oxidation.

In hot processes, oxides of nitrogen are often formed and these are harmful pollutants which are of increasing concern.

Selective catalytic reduction works by introducing another compound, termed a ‘reductant’, which encourages the oxygen atoms to ‘jump ship’ from the NOx molecules to react with the reductant instead.

In most cases, a catalytic converter is used to promote this reaction, in which case it occurs in the temperature range of 150-600°C.

Without a catalytic converter, a higher temperature is required. The most common reductant used in SCR processes is ammonia.

This can be bought in the form of ammonia solution, or alternatively a solution of urea such as Adblue can be used instead.

Urea solutions are easier to store and handle than ammonia solution and also more widely available, however if they are injected directly into the flue gas they require a minimum temperature of 300°C.

It is very important that the quantity of reductant injected matches the quantity of NOx present in the flue gas. If the quantity is too low, NOx reduction will be compromised, whilst if it is too great, ammonia will be emitted.

Therefore a sophisticated injection system for the reductant is required. A well-designed SCR system is capable of reducing NOx emissions by over 90%.

Measuring the emissions from large-scale biomass combustion

We can interpret the data from test reports related to pollution from industrial-scale biomass combustion.

Both the Industrial Emissions Directive and the Medium Combustion Plant Directive require periodic monitoring of emissions, and these must be carried out by companies which have the appropriate accreditation (e.g. MCERTS).

We are often called in when plants are found to be exceeding their permitted emissions and in this situation we normally use test reports as a starting point for diagnosing the problem.

Monitoring emissions is also an important stage of the calibration of SCR systems and the sign-off of any new installation. SCR systems may incorporate emissions sensors and a data-logging facility which can be accessed remotely if required.

 

Permitted emissions from industrial scale biomass combustion plants

We can help our customers to comply with the regulations covering pollution from large-scale biomass combustion.

In the EU and UK, the biggest development in the regulations covering emissions from industrial-scale burning of biomass is the Medium Combustion Plant Directive (MCPD). This applies to plants with a thermal input in the range 1 to 50 mW and has a range of implementation dates from December 2018 to January 2030 depending on the size of the plants and whether they are new or existing. The main focus of the MCPD is on reducing NOx emissions, however for some fuel types, SO2 and dust are also regulated.

Plants larger than 50 mW are normally regulated under the existing Industrial Emissions Directive.

Plants with a thermal input of less than 1 mW are not covered by the MCPD but are often required to comply with emissions limits in order to be eligible for grant finance.

Catalytic converters for biomass energy schemes

We can design and supply both oxidation and reduction catalytic converters to abate pollution from industrial-scale biomass combustion.

When dealing with fumes from large-scale biomass combustion the performance criteria are often demanding, and each application may have to be treated as a separate project. Pressure drop is usually an important design criteria and therefore we have the expertise and experience to accurately predict this.

The annual usage is likely to be intensive and therefore systems are normally designed so that they can be serviced and overhauled at regular intervals. Particular attention needs to be paid to the presence of contaminants such as sulphur in the flue gas, which may require special catalytic coatings to avoid premature failure.

We can design and supply catalytic converters based on either ceramic or metal substrates. In the case of ceramic substrates, for large applications there are usually multiple substrates in a grid and we will design the housing so that they can easily be replaced when necessary. We are also happy to supply replacement ceramic elements to fit in existing housings.

We also design housings for metal substrates which facilitate their removal for cleaning.

Whitebeam is happy to work with OEMs, end users, consultants or flue specialists to deliver effective catalytic converters in this sector.