Flue Gas Treatment for RDF- and Biomass-fuelled power plants

Dipl.-Ing. Martin Sindram, Dr.-Ing. Diethelm Walter

1. Introduction

The use of RDF (Refuse derived fuel) and Biomass in power plants often requires highly efficient FGT (flue gas treatment) processes; most of the RDF- and Biomass-fuelled power plants use dry, lime-based FGT processes. In the UK, a total of about 100 different dry lime-based FGT-units are in operation; out of these, roughly 90% use dry Hydrated Lime or tailor-made blended products based on Hydrated Lime.

2. Dry FGT Processes

Due to their obvious advantages, dry FGT-processes became the predominant solution in modern flue gas cleaning /1/; today, they are used in a great number of different applications /2 – 6/.

The basic dry FGT-process consists of a fabric filter combined with a dry sorbent injection system; the actual flue gas cleaning takes places in the duct and the reagent layer on the surface of the filter bags. Even such a basic FGT-process already allows effective flue gas cleaning; different other components can be added to further enhance the removal efficiency of the dry FGT-process.

Pre-removal of dust, i.e. via a cyclone, offers a simple and cost-effective possibility to select and reuse parts of the fly ash and to reduce the dust level for the actual the FGT-process. Injection of water (quenching) reduces temperature and increases humidity of the flue gas; both effects are beneficial for the removal of acidic gas components. A reactor tower both increases retention time and gas turbulence which results in a better mixing of the flue gas and solids thus increasing the removal efficiency. Recirculation (3 to 5 times for high surface Hydrated Limes) is a possibility to maximize the utilization-rate of the reagent. Some FGT-processes (like the NID- or CFB-process) use significantly higher recirculation rates (up to several 100 times) and can provide high removal rates even with standard Hydrated Limes.

In dry FGT-processes, the sorbent is injected as a powder into the flue gas stream; the addition can be introduced in a number of points within the process; High Temperature- (at 850 – 1000°C; pre-removal of SO₂), Medium Temperature- (300 – 450°C; removal of HF, HCl and SO₂) and Low Temperature- (130 – 180°C; removal of HF, HCl and SO₂) temperature range. The low temperature range has been shown to be the optimum performance in FGT-process for Hydrated Limes (like High surface Hydrated Limes).

3. Sorbents used in dry FGT-processes

Almost all of the dry FGT-processes use Calcium Hydroxide Ca(OH)₂ as reagent for the neutralisation of acidic flue gas components (i.e. HF, HCl, SO_x, ..). Carbon-based materials (i.e. Lignite/HOKNm^®, Activated Carbon) as well as new, mineral based products are used for the removal of micro pollutants such as Heavy metals (Hg) and/or Dioxins/Furans. These materials can be added separately or premixed with Hydrated lime Ca(OH)₂ for a simultaneous removal of acidic components and micro pollutants. Recent developments showed the possibility to use specialized mineral-based materials for this application as well.

4. Practical Examples

4.1 FGT-Process of a heating plant using RDF

This Waste-to-Energy-plant used standard hydrated Lime as the reagent for flue gas cleaning; due to new and tighter emission limits (European directive 2000/76/EU), it was necessary to increase the removal capacity of the existing FGT-process /5/. The heating plant is equipped with a SNCR for NOX-removal; the FGT-process consists of two parallel cyclones for pre-removal of dust and a fabric filter. The reagent is injected directly into the duct; retention time of the reagent in the duct is about 2 seconds. Recycling of reaction products is not done. During the measurement campaign, the flue gas humidity was at 19 Vol.-%; average raw gas concentrations were at 700 mg/Nm³ for HCl and 1.200 mg/Nm³ for SO₂. Normally, power plants using RDF show higher HCl-concentrations this simple FGT-process allows to meet the tight requirements of the European directive 2000/76/EU.

4.2 Biomass-fired Power Plant using Wood-scrap

These power plants usually have to respect the tight emission limits (HCl: 10 mg/Nm³; SO₂: 50 mg/Nm³). Typical for wood-scrap fired power plants are more or less constant raw gas levels with sudden HCl- and/or SO₂-peaks; these peaks are caused by single wood-scrap charges ‘polluted’ by sulphur and/or chlorides.

During the occurrence of such peaks, the FGT-process has to deal with high HCl- and SO₂-concentrations typical for municipal waste incinerators.

In FGT-processes of these power plants, both standard and high surface area hydrated Limes are used. High surface area hydrates are ideal reagents for wood-scrap power plant; if necessary, removal of HCl- and/or SO₂-peaks can be achieved by simply adjusting the required reagent-quantity /2/. Adherence to the HCl- and SO₂-emission limits is necessary for the desired high availability of the power plant. The use of lime leads to an optimization of the total FGT-costs as well as to a high flexibility in the quality of the wood-scrap used in the process.

4.3 Combination of semi-dry & dry FGT-processes

In a number of municipal waste incinerators and RDF-power plants in Europe, the FGT-process consists of a combination of semi-dry and dry sorption. This so-called MKT-process (MKT: modified conditioned dry sorption) combines the effectiveness of the spray absorption (with milk of lime) with the flexibility and efficiency of the dry sorption. The MKT-process is intended for some of the RDF-power plants currently under construction.

In particular with strongly varying or changing HCl- and SO₂-concentrations in the raw gas, the MKT-process offers substantial advantages in the operation of the FGT-process. Practice shows that assessing HCl- and SO₂-raw gas concentrations becomes more and more difficult. In addition, the frequent use to maximum capacity demands high flexibility and efficiency of the FGT-installation. Basic idea of the MKT-process is to achieve a high removal capacity with simultaneous savings in reagent consumption. The spray absorption provides the ‘basic’ flue gas cleaning and operates in as constant a capacity range as possible. The dry sorption stage helps to exactly maintain the emission limits during continuous operation and ensures the necessary absorption capacity when required. For municipal waste incineration, reagent-consumption expressed as sum of quicklime and hydrated lime, varies between 14,1 and 24,0 kg per ton of waste; predominantly it is between 15 to 18.5 kg per ton of waste.