Main sources of SO_x

Main sources of SO_x (sulphur oxides) emissions are linked to the use of sulphur-containing fossil fuels (coal, lignite, petroleum coke, heavy fuel oil, domestic fuel oil, diesel fuel, petrol, etc.) as well as biomass fuels (low sulphur content in general). These fuels are used in energy production, industry, residential and tertiary activities including transports (road transport, maritime transport, inland navigation…). Several industrial processes also emit SO_x (sulphuric acid production, paper pulp production, oil refining, etc.). Even natural sources emit sulphur products such as volcanoes. At a global level, solid fossil fuels largely used for electricity production are the largest source of SO₂.

SO_x include SO₂, SO₃. In combustion, sulphur present in fuels reacts with oxygen contained in combustion air to form SO₂. Therefore, SO₂ emissions arising from combustion are directly related to the sulphur content of fuels used. SO₃ is produced by oxidation of SO₂, during combustion.

SO₂ is a major contributor to acidification, via the formation in the atmosphere, of sulphate and sulphuric acid. SO₂ residence time in atmosphere depends on meteorological conditions. The average residence time is about 3 to 5 days, hence SO₂ can be transported over hundred kilometres.

To reduce SO_x emissions from combustion processes and other sources, different types of measures are applied. The main applied measures consist in energy efficiency improvement, fuel switching, fuel cleaning, primary and secondary measures.

Sulphur content of fuels

Sulphur content of solid fossil fuels ranges from 0.5 % to more than 5 %. Sulphur content of natural gas is generally very low as well as sulphur content of natural wood (on contrary, waste wood may have significant sulphur concentrations). Sulphur content of liquid fossil fuels ranges from 0.001 % to more than 5 %. The availability of low sulphur content liquid fossil fuels requires the removal of sulphur at the refinery level and the adoption of specific processes.

During the last decade, national and European legislation have toughened the limits required for the sulphur content of petroleum products. As example, table 1 presents the typical limit values applied for some liquid fuels in the EU [1].

Table 1: typical limit values for S applied for liquid fuels in the EU

The amended Gothenburg Protocol of 2012 [2], introduces a limit value for gas-oil of 0.1 % but no other limit value for other fuels.

General approach to reduce SO_x emissions

Fuel switching

Fuel switching (e.g. from high- to low-sulphur coals and/or liquid fuels, or from coal or liquid fuel to gas) leads to lower sulphur emissions, but there may be certain restrictions, such as the availability of low-sulphur fuels and the adaptability of existing combustion systems to different fuels. In many EU countries, some coal or oil combustion plants are being replaced by gas-fired combustion plants. Dual-fuel plant may facilitate fuel switching. Fuel switching can also have beneficial effects on nitrogen dioxide or particulate matter emission levels.

Fuel cleaning

Cleaning of natural gas is state-of-the-art technology and widely applied for operational reasons. Cleaning of process gas (acid refinery gas, coke oven gas, biogas, etc.) is also state-of-the-art technology. Desulphurization of liquid fuels (light and medium fractions) is state-of-the-art technology. Desulphurization of heavy fractions is technically feasible; nevertheless, the crude oil properties should be kept in mind. Desulphurization of atmospheric distillation residues (bottom products from atmospheric crude distillation units) to produce low-sulphur fuel oils is not, however, commonly practised; processing low-sulphur crude is usually preferable. Hydro-cracking and full conversion technology has matured and combine high sulphur retention with improved yield of light products. The number of full conversion refineries is constantly rising. Such refineries typically recover 80 to 90% of the sulphur intake and convert all residues into light products or other marketable products. This type of refinery consumes more energy and requires higher investments. Sulphur content of refinery products needs to correspond to the restricted values ordered by the EU and provided in table 1 as example.

Current technologies to clean hard coal can remove approximately 50% of the inorganic sulphur (depending on coal properties) but none of the organic sulphur. More effective technologies are being developed. However, they require higher specific investments. Thus, the efficiency of sulphur removal by coal cleaning is limited compared to flue gas desulphurisation. There is may be a country-specific optimisation potential for the best combination of fuel cleaning and flue gas cleaning.

Combustion Technologies

Advanced combustion technologies may improve thermal efficiency and reduce sulphur emissions. These technologies include fluidized-bed combustion (FBC), integrated gasification combined-cycle (IGCC); and combined-cycle gas turbines (CCGT). Stationary combustion turbines can be integrated into combustion systems in existing conventional power plant. This can increase overall efficiency by 5 to 7%, leading, for example, to a significant reduction in SO₂ emissions. However, major alterations to the existing furnace system become necessary. Reciprocating engines can also increase the electrical efficiency by taking advantage of the sensible heat of the exhaust gases generated by e.g. use of a feed-water combined cycle.

In FBC, the combustion takes place through a particulate bed, which can be fixed (FFBC), pressurized (PFBC), circulating (CFBC) or bubbling (BFBC). Fluidized-bed combustion is a combustion technology for burning hard coal and brown coal, but it can also burn other solid fuels, such as petroleum coke, and low-grade fuels, such as waste, peat and wood. Emissions can be further reduced by integrated combustion control in the system due to the addition of lime/limestone to the bed material. The use and/or disposal of by-products from this process may cause problems and further development is required.

The IGCC process includes coal gasification and combined-cycle power generation in a gas and steam turbine. The gasified coal is burnt in the combustion chamber of the gas turbine. Sulphur emission control is achieved by using state-of-the-art technology for raw gas cleaning facilities upstream of the gas turbine. The technology also exists for heavy oil residues and bitumen emulsions.

Combustion modifications comparable to the measures used for NOx emission control do not exist, as during combustion the organically and/or inorganically bound sulphur is almost completely oxidized. A certain percentage, depending on the fuel properties and combustion technology, is retained in the ash. The amount of sulphur retained in ash, can be influenced by added sorbents (e.g. lime/limestone) and combustion conditions (e.g. temperature). Injection of an alkaline reagents into the combustion unit can be considered as process modifications (direct injection of a dry sorbent in the gas stream of the boiler furnace). Pulverised limestone is used (CaCO3). Calcium sulphites and sulphates formed need to be captured by FF or ESP. However, experience has shown that, when applying these processes, thermal capacity is lowered, and the Ca/S ratio is relatively high and sulphur removal low. In recent years, the performance of these processes has nevertheless been optimized to the point that SO₂ removal from 30 to 50% up to 70 to 80 % may be obtained according to the arrangement used and recycling of reaction products [3]. Problems with the further use of the by-product have to be considered.

Secondary measures - Flue gas desulphurisation processes

These processes aim at removing already formed sulphur oxides and are considered as secondary measures. Sulphur removal using wet, dry or semi-dry processes are used to treat the flue gases. Sulphur can also be removed using the recovery of sulphur dioxide from the flue gases. It is then either extracted (regenerative process) or converted into sulphuric acid (sulphuric acid plant). Flue gas scrubbing using water or seawater is another available technology to reduce SO₂ emissions.

In wet scrubbing technologies, the flue gas is first dedusted then cleaned by an atomized solution of alkaline compounds. SO₂ reacts with these alkaline compounds to form by products, whose chemical nature depends on the alkaline compound used. In the case of use CaCO3 or CaO, by-products may be upgraded as gypsum if some technical conditions are achieved (forced oxidation). By-products can also be upgraded using other scrubbing agents. With wet FGD, excellent efficiencies can be achieved ranging from 92 to more than 99% with a near-stoichiometry Ca/S ratio (1.02 to 1.1) [3]. This process is mainly used for reducing SO₂ emissions from large coal power plants.

In the dry process (duct sorbent adsorption), a calcium or sodium-based sorbent is injected in solid form into the flue gases before a fabric filter or an ESP. Hydrated lime (Ca(OH)2) or sodium bicarbonate (NaHCO3) are the most frequently used sorbents. Sorbents need to be very reactive and are activated for this purpose. They react with SO₂ to form calcium or sodium sulphites and sulphates, which need then to be filtered to prevent dust emissions. The dry duct injection process efficiency is lower than with wet FGD (about 50 to 80%) [3] and depends on several parameters such as temperature, SO₂ content of the flue gas, Ca/S ratio and residence time. Duct sorbent injection has great potential for relatively old and small boilers [3]. Large quantities of by-products are produced.

The semi-dry process or spray dry scrubbing is similar to the dry process and also produces a solid residue. It uses moisturised lime or limestone containing about 10% of water to enhance the contact and the reactions. The removal SO₂ efficiency ranges from 85 to 92% with a ratio Ca/S from 1.3 to 1.4 [3]. By-products are mixtures of the original sorbent, calcium sulphite, calcium sulphate and fly ash which can be reused in construction industry, land reclamation purposes as example.

There are other processes such as regenerative processes. They have not been described in the last EU BREF for Large combustion Plants [3] as there are only few references in the world. In regenerative processes, a regenerating agent is used to recover SO₂. The sodium sulphite bisulphite process is one of these regenerative processes. Sodium sulphite (Na₂SO₃) reacts with SO₂ to form sodium bisulphite (NaHSO₃), which is then evaporated to crystallise sodium sulphite and recover SO₂. A recovery rate up to 95% can be achieved. These types of processes are only used for specific applications.

Costs of reduction techniques of SO₂

Costs are an important issue when selecting SO₂ emission reduction techniques. The following expenses may be relevant:

• imputed depreciation allowance and imputed interest,
• labour costs,
• expenses for auxiliary and operating materials,
• energy costs,
• maintenance and repair costs, expenditure on monitoring, expenses for external services,
• taxes, environmental levies (e.g. charges for waste water), fees, public charges.

Costs increase in general less than the capacity of the reduction technique so that larger units are often more cost-effective. Dry additive processes are less cost effective for high sulphur content fuels compared to wet scrubbing processes.

A methodology for cost estimation of abatement options of SO₂, NOx and TSP (Total Suspended Particulates) for Large Combustion Plants (LCP) with a thermal capacity of more than 50 MWth, developed by TFTEI. ERICCa_LCP (Emission Reduction Investment and Cost Calculation) is available at: http://tftei.citepa.org/en/work-in-progress/costs-of-reduction-techniques-for-lcp. Other information from TFTEI is available for Cement production, Glass production, and some other processes.

http://tftei.citepa.org/en/documents/revision-of-the-gothenburg-protocol#documents-provided-to-help-decision-making-costs-data-for-some-activities

By-products and side effects

Side effects of emission abatement options/techniques can be positive or negative and should be accounted for.

Options that lead to usable by-products should be selected, as should options that lead to increased thermal efficiency and reduced waste whenever possible. Although most by-products such as gypsum, ammonia salts, sulphuric acid or sulphur, are usable or recyclable products, factors such as market conditions and quality standards need to be taken into account.

Side effects can generally be limited by properly designing and operating the facilities. Side effects include

• impacts on energy consumption and hence greenhouse gas emissions,
• impacts on other air pollutants,
• impacts on the use of natural resources such as limestone,
• cross-media effects, e.g. on waste or water.

More particularly, the following table presents positive and negative side effects for selected flue gas desulphurisation processes.

Table 2: positive and negative side effects for selected flue gas desulphurisation processes

References used

[1] Amended Council Directive 1999/32/EC of 26 April 1999 relating to a reduction in the sulphur content of certain liquid fuels and amending Directive 93/12/EEC https://publications.europa.eu/en/publication-detail/-/publication/a093e44c-bf78-434b-92c7-d726dc5ad012

[2] 1999 Protocol to Abate Acidification, Eutrophication and Ground-level Ozone to the Convention on Long-range Transboundary Air Pollution, as amended on May 2012. https://www.unece.org/fileadmin/DAM/env/documents/2013/air/eb/ECE.EB.AIR.114_ENG.pdf4

[3] European Commission. Thierry Lecomte and all’s. « Best Available Techniques (BAT) Reference Document for Large Combustion Plants » Industrial Emissions Directive 2010/75/EU (Integrated Pollution Prevention and Control) – 2017
http://eippcb.jrc.ec.europa.eu/reference/lcp.html

Information provided by stake holders and accepted by the clearing house evaluation committee

This section presents technological developments in reduction techniques provided by stakeholders. The documents presented in this section have been firstly submitted to the Clearing House Evaluation Committee (CHEC). They are available publicly after agreement of the CHEC.

SO₂ abatement primary measures

SO₂ abatement secondary measures

A first draft of document was submitted by Solvair in 2018 to the 4th TFTEI meeting. The CHEC agreed to publish the document on 10 October 2019, after complements of the first version provided by Solvair, following comments of the CHEC.

This document presents the sodium bicarbonate based process developed to reduce SO2 and HCl emissions from activities like waste to energy (WtE), industrial boilers, large combustion plants, sintering plants, mineral wools, cement and glass production. Performances are presented for several plants in operation. CAPEX and OPEX are also provided for some examples./p>

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