With the continuous efforts to reduce the air pollution by raising emission standards in many countries, catalytic ceramic filters (CCF) are increasingly employed to meet more stringent regulatory emission requirements due to their technical and commercial advantages, such as very high pollution removal efficiency, simple and compact system design and low operational expenditure. Zhengyang biomass power plant, owned by China Energy Group, chose CCF to remove particulate matter (PM), acid gases, sulfur oxides (SOx) and nitrogen oxides (NOx). The power plant was commissioned in December 2020.
Zhengyang biomass power plant has one 130-ton-per-hour natural circulation boiler using a vibrating grate. The electricity generation capacity is 35MWe. The main fuels are corn stover, wheat straw and peanut shells. Owing to the characteristic of the biomass fuels, particularly the high moisture content in the flue gas of 21% volume, could cause serious acid corrosion of the facilities working at lower operational temperatures. The dust load in the flue gas is 15,000 milligrams per normal meter cubed (mg/Nm3 ), representing the design value for the PM removal system, with a high content of alkali metals such as potassium and sodium.
The flue gas treatment system was designed to meet an extra low emission requirement, which mandated the pollution limits (6% O2) as PM <10 mg/Nm3, sulfur dioxide (SO2) <35mg/Nm3, NOx <50mg/Nm3.
Conventional Flue Gas Treatment Processes
Conventional techniques for removing the three regulated pollutants (PM, SOx and NOx) to meet emission requirements have their limitations and disadvantages.
For PM removal, fabric filters will have the issue of ammonium bisulfate (ABS) deposition at their working temperature range, which can cause high pressure drop and even blind the filters, since the SCR system and ammonia injection are upstream of the baghouse. The slipped ammonia (NH3) reacts with sulfur trioxide (SO3) present in the flue gas to form ammonium bisulfate. Due to failures caused by physical and chemical attacks, fabric filters generally need to be replaced every two to three years, depending on the operating conditions. Electrostatic precipitator (ESP) technology is another widely used dust removal technique, which cannot reliably achieve PM removal to less than 10 mg/Nm3.
For NOx reduction (deNOx), selective noncatalytic reduction (SNCR) has limited deNOx efficiency and cannot attain the required NOx emission value. To avoid reheating flue gas, selective catalytic reduction (SCR) is mostly performed in a temperature window between 572 to 752 degrees Fahrenheit (300 and 400 degrees Celsius), with the dust in the flue gas having direct contact with the catalyst. Alkali metals and heavy metals may poison the catalyst and can result in premature catalyst failure. In general, the catalyst needs to be regenerated regularly and replaced every three to five years.
For desulphurization, techniques such as wet flue gas desulphurization, semi-dry scrubbing and dry desulphurization are available for the flue gas treatment. The wet and semi-dry processes will cause excessive heat loss from the flue gas, which lowers the system heat efficiency. The wet process consumes copious amounts of water and produces correspondingly large amounts of wastewater, which requires additional treatment facilities. For the dry processes, the deSOx efficiency increases with temperature in the range of 482-752 degrees F for the lime-based reagent. Where fabric filters are used for PM removal, the deSOx efficiency cannot reach its maximum range because of the lower working temperatures of fabric filters.
Advantages of Catalytic Ceramic Filters
Catalytic ceramic filters, first developed by Clear Edge, combine the advantages of ceramic filters with an incorporated active SCR catalyst for the removal of NOx, dioxin, SOx and VOCs. Catalytic ceramic filters are used in filter plants in much the same way as bag filters, albeit at higher temperatures. The low-density ceramic filters can typically be used in the 356 to 842 degrees F range, depending on the catalyst type.
Following almost 20 years of successful deployment in industrial air pollution control systems, the advantages of a flue gas treatment system using catalytic ceramic filters have been found to include the following.
- Simultaneous removal of PM, NOx, SOx and dioxin from flue gas.
- Effective handling of submicron particles in industrial gas processes with the capability to filter gas to limits of less than 2 mg/m3.
- Superior deNOx efficiency of greater than 95% and long lifetime of catalyst longer than five years, since the catalyst is embedded within the filter walls, hence it is protected from dust deposition and poisoning by heavy metals.
- No ammonium bisulfate deposition on the catalytic ceramic filters, as the operational temperature is higher than its dew point of around 446 degrees F.
- Low system pressure drop-in comparison to a conventional SCR and baghouse.
- Resistant to corrosion and erosion; nonflammable, so no risk of ignition from sparks of biomass particles.
- Modular design—one filter module at a time can be isolated for maintenance without interrupting operation.
- Long service life: five to 10 years operating life of the catalytic filter elements.
- CAPEX savings: simplified equipment train with less equipment for PM, NOx and SOx removal.
- OPEX savings: reduced operation and maintenance costs, less equipment to maintain, increased uptime and long service life of the filter element and catalyst.
System Concept of Flue Gas Treatment
To avoid the disadvantages of the usual flue gas treatment processes and to take advantage of catalytic ceramic filters, the inlet temperature of flue gas treatment system was set to 608 to 662 degrees F. The boiler heating surface economizer was therefore split into two stages, such that the outlet gas temperature of the first stage lies exactly in this temperature range. The input data for the flue gas treatment system design are listed in Table 1.
The flue gas treatment system for Zhengyang biomass power plant consists mainly of a dry desulphurization scrubber, settling chamber and catalytic ceramic filter housing. The settling chamber was designed to remove most sparks and large-size particulates.
The catalytic ceramic filter housing consists of 12 filter modules with a total of 5,040-piece, 3,000-millimeter filter elements supplied by Clear Edge. The modular design allows for cut-off of one module for maintenance without interrupting operation. In the catalytic ceramic filter housing, PM including heavy and alkali metals, desulphurization reagent powder and NOx are simultaneously removed with extremely high efficiency.
The flue gas discharged from the catalytic ceramic filters is cooled down to 230 degrees F after running successively through economizer stage two, air preheater and heat recovery.
For the heat exchangers downstream of the catalytic ceramic filters, the operation conditions are optimal because the flue gas is cleaned, which prevents acid corrosion, dust erosion and dust accumulation and blockage. This considerably reduces not only the operation cost due to a lower pressure drop, but also maintenance costs for these heat exchangers. Moreover, the design flue gas velocity flow through these heat exchangers could be increased, as the flue gas is virtually dust free, the heat transfer becomes more efficient and thus, the required heating surfaces are reduced, which means that the cost for the heat exchangers decreases.
The flue gas treatment system with catalytic ceramic filters demonstrates high performance and reliability since the plant commissioning in December 2020. The emission values measured three months after commissioning are shown in Table 2.
The measured emission values show that the target emission values for the system design were completely achieved, demonstrating high efficiency for the pollutants removal.
Zhengyang biomass power plant is the largest biomass plant in China that utilizes catalytic ceramic filters for flue gas treatment. The catalytic ceramic filter solution stood out from alternative competing concepts. The owner of the plant was convinced by a series of economic, operational and technical advantages offered by CCF technology. The success of the catalytic ceramic filter installation proves that they are an excellent solution even for large biomass boilers, as well as many other industrial applications.
Clear Edge and the Zhengyang biomass power plant contributed to this article.
Author: Ian Chisem
Technical Director, Ceramic Filter Alliance
Read This Article on: Biomass Magazine