A detailed review examines projected costs for mandated pulp mill air quality improvements to achieve Cluster Rule compliance

MACT I & MACT II Air Compliance: Concepts, Costs, and Impacts

Although any mills with state-of-the-art process equipment may be able to demonstrate MACT I or MACT II compliance with little or no capital investment, mills using older process equipment and high equipment capacity factors will require substantial capital investments to meet the requirements.

This technical article explores the practical engineering concepts, capital costs, and environmental impacts of MACT I and MACT II air emission compliance. The capital costs, operating costs, and emission data used here are derived from numerous mill engineering studies. To represent select MACT I and II compliance alternatives in a mill requiring substantial capital investment, this article develops a hypothetical 2,000 tpd kraft pulp mill for discussion purposes. Although MACT I requirements for waste water discharge and foul condensate compliance have been widely addressed, they are specifically excluded from this discussion.

MACT I AIR EMISSION COMPLIANCE REQUIREMENTS.

The goal of the MACT I air regulation is to substantially reduce hazardous air pollutants (HAP) emissions from pulp mills. The Environmental Protection Agency has developed an exhaustive chemical list of HAPs, and many listed chemicals are present in the pulp mill process vents described here. Methanol is used as the designated surrogate for MACT I HAP compliance. An abundance of mill-specific data is available to facilitate design engineering and cost estimation for MACT I compliance.

To reduces HAP emissions, Cluster Rule MACT I air emission compliance regulates both low volume high concentration (LVHC) gas sources and high volume low concentration (HVLC) gas sources. Mills must achieve MACT I air emission compliance by April 2001 for LVHC gas sources and by April 2006 for HVLC gas sources.

LVHC streams. MACT I regulation specifies that vent gases classified as LVHC streams from the following pulping processes must be collected and treated to 98% efficiency prior to discharge:
• Digesters
• Evaporators
• Turpentine recovery
• Stripper vents

HVLC streams. MACT I also requires that vent gases classified as HVLC streams be collected and treated at a minimum temperature of 1,600°F for a period of 0.75 seconds for HAP destruction prior to discharge. The following pulping system process vents must be collected and treated:
• Oxygen delignification
• Knotters
• Screens
• Deckers
• Brown stock washers
• Weak black liquor tanks (new sources only)

Alternatively, the MACT I clean condensate alternative (CCA) may be exercised instead of brown stock washer vent collection and treatment. The CCA option provides that clean condensates (<400 ppm methanol) or fresh water may be used for brown stock washing, and the resulting washer vents need not be collected or treated. However, collection and treatment of washer vents in HVLC systems has been more commonly selected for MACT I compliance than the CCA option.

Bleach plants using chlorinated bleach agents must collect process vents from all stages (mixers, towers, washers, and seal tanks) and treat the streams to 99% efficiency prior to discharge. Alternatively, the bleach plant vent may be treated to a concentration of 10 ppm chlorine or 0.001 kg chlorine/mg of oven dry pulp.

MACT II AIR EMISSION COMPLIANCE REQUIREMENTS.

MACT II air emission standards were proposed in April 1998, and a second MACT II air emission proposal is expected in early 2000. Promulgation is expected no sooner than November 2000, and a three-year compliance schedule is anticipated.

The proposed MACT II regulation would require reduced emissions from select combustion sources, specifically recovery boilers, smelt dissolving tanks, and lime kilns. The focus of the regulation is HAP particulate matter, specifically heavy metals such as antimony, arsenic, beryllium, chromium, cadmium, cobalt, lead, manganese, mercury, nickel, and selenium and volatile organic carbon (VOC) compounds. The proposed new emission standards will be a fraction (25% to 40%) of current emission rates from many existing particulate collection devices serving combustion sources.

There is not an abundant repository of mill specific test data to support engineering design for proposed MACT II compliance. The National Council of Air and Stream Improvement (NCASI)¹ and several independent mills² have recently completed field tests to assess MACT II compliance requirements. These field tests were designed to quantify current emission rates as compared to proposed emission rates for MACT II compliance. Many of the available data sets exhibit a high degree of variability. It is apparent that careful mill specific testing is necessary prior to completion of detail engineering and cost estimation for capital appropriation.

CONCEPTS FOR MACT I COMPLIANCE.

Most pulp mills have existing LVHC gas collection and incineration systems that were probably installed in the 1970s following promulgation of the original Clean Air Act. Most of the previously named process vent sources may already be collected and treated. However, the foul condensate stripper vents are a relatively new source of foul gas generation. Many mills currently combust the collected LVHC gas streams and stripper gas streams in an existing lime kiln, dedicated incinerator, power boiler, or recovery boiler

LVHC system upgrades. Many mills require a capital upgrade to the existing LVHC system due to historical production increases, maintenance issues, and the addition of new sources for MACT I compliance. The scope of work for LVHC system upgrades includes:
• Steam eductor replacement
• Piping for new gas sources
• LVHC gas coolers
• Mist eliminators
• Combustion nozzles
• Condensate removal system
• Piping, electrical, controls, and structural components

Typically, the existing LVHC systems will not meet the 99% availability required by the Cluster Rule. Historically, existing LVHC system availability may range from 90% to 95%. Capital upgrades must address existing system deficiencies that contribute to this lost time. Finally, the existing LVHC gas system primary combustion device will require a back up to ensure 99% system availability. The LVHC stream is commonly incinerated in a lime kiln, power boiler, or dedicated incinerator. One unit is typically designated as the primary incineration device, and the second unit is designated the secondary device.

FIGURE 1: LVHC system modifications for MACT I compliance.

HVLC system upgrades. Regarding HVLC gas systems, most pulp mills do not currently have gas collection and treatment systems. The few existing HVLC systems typically collect only the brown stock washer hoods, and the system capacity will not accommodate the additional gas streams required for MACT I compliance. The scope of work for HVLC system installation includes:
• Brown stock washer hood replacement
• Decker hood replacement
• Hoods for open knotters and screens
• Motive fan
• Incineration device (or nozzles)
• Gas coolers/condensers
• Condensate removal system
• Piping, electrical, controls, and structural components

Washer hood replacements with “closed” design are typically required to minimize the volume of HVLC gas that is collected. HVLC gas condensers and coolers are required to lower the temperature, specific volume, relative humidity, and absolute gas volume to be collected and treated.

The pretreated HVLC gas stream is typically combusted for HAP destruction in a power boiler, recovery boiler, or dedicated incinerator, and the HVLC system is mandated to demonstrate 96% system availability. A reliable mill steam generating boiler will generally satisfy the availability requirement, so no secondary combustion device may be required for the HVLC gas stream.

Bleach plant scrubber systems. Bleach plants generally have existing scrubber systems for treatment of chlorinated gas emissions from mixers, towers, washers, and seal tanks. The existing bleach plant scrubber systems typically operate in the range of 90% to 95% treatment efficiency.

The MACT I regulation mandates that the scrubber system operate at 99% efficiency, 10 ppm outlet chlorine concentration, or 0.001 kg chlorine/mg of oven dry pulp. Existing scrubber systems may not collect all the bleach towers, mix chests, or filtrate tanks as required by the MACT I regulation and may not achieve the scrubber outlet emission rate. Consequently, replacement or upgrade of the existing scrubber system is commonly required. The scope of work for replacement of the bleach plant scrubber system includes:
• Demolition of the existing scrubber
• Washer hood refurbishments
• Installation of new scrubber and auxiliaries
• Fan and drive unit replacement
•Piping, electrical, controls, and structural components

CAPITAL COSTS FOR MACT I COMPLIANCE.

Table 1 summarizes the capital costs for MACT I compliance in a typical 2,000 tpd kraft pulp mill and addresses upgrades for the LVHC system, HVLC system, and bleach plant scrubber system.

As Table 1 shows, capital costs for LVHC system upgrades in such a pulp mill may total $752,000, while the costs for the HVLC system may total $4,500,000. In addition, capital costs for the bleach plant scrubber replacement and system upgrades in the same pulp mill might total an additional $692,500.

IMPACTS OF MACT I.

Incremental HAP emission reductions will result from installation of upgrades for MACT I compliance in the LVHC system, HVLC gas system, and bleach plant scrubber system. Table 2 summarizes the estimated, incremental HAP emission reductions following capital upgrades as defined for a 2,000 tpd kraft pulp mill.

Appreciable operating costs are incurred to support the MACT I air emission systems in the kraft pulp mill. Operating costs include steam, electricity, chemicals, and operating and maintenance expenses. Typical annual operating costs are approximately $1.50/o.d. ton of pulp production, as Table 3 shows.

Table 3 also shows the cost effectiveness range—$/ton incremental HAP removed—for the MACT I environmental projects. Cost effectiveness is defined as the total annualized capital and operating cost divided by the tons of pollutant removed using a 20 year equipment life cycle.

CONCEPTS FOR MACT II COMPLIANCE.

The goal of the proposed MACT II regulation is to substantially reduce air emissions of HAP particulate matter (heavy metals) and volatile organic compounds. Table 4 summarizes the proposed MACT II emission requirements for existing process units. For new source process units, the proposed emission allowances are reduced by a range of 40% to 85% from the values listed in Table 4.

MACT II compliance potentially impacts recovery boiler, electrostatic precipitator, smelt tank, and lime kiln equipment. Capital costs in each of these equipment areas would include labor, equipment, materials, subcontract, and engineering expenses.

Recovery boiler upgrades. New or reconstructed kraft and soda chemical recovery units must comply with a new total gaseous organic (TGO) emission limit of 0.025 lb/ton black liquor solids. New noncontact recovery boilers with dry bottom electrostatic precipitators are considered to be in compliance with the MACT II HAP particulate matter regulation. Also, preliminary HAP emission test data suggests that properly designed and operated non-contact (NDCE) design recovery boilers may comply with the MACT II HAP particulate matter regulation of 2,010 lb/mmt black liquor solids.

However, recovery boilers with direct contact evaporators (DCE) may not comply, or may marginally comply, as represented by the emission data in Table 5. In addition, some chemical recovery boilers operating with DCE at high capacity factors may not comply with the TGO emission limit.

Proposed MACT II compliance may require capital conversion of the direct contact recovery boiler to low odor (non-contact) design. Recovery boiler low odor conversion costs for a 2,000 tpd kraft mill may total $14,495,000, as Table 6 summarizes. The recovery boiler conversion project would include:
• Demolition of the cascade evaporator
• Demolition of the economizer section
• Demolition of the wet bottom on the electrostatic precipitator
• Installation of a long flow economizer
• Installation of a black liquor concentrator
• Structural and building supports
• Piping, instrument and electrical controls

PM₁₀ (particulate matter < 10microns) matter emission rates from existing recovery boiler electrostatic precipitators may not comply, or may marginally comply, with the proposed MACT II regulation as Table 5 illustrates. Electrostatic precipitator upgrades for a 2,000 tpd kraft mill may total $2,000,000, as Table 6 describes. The recovery boiler electrostatic precipitator upgrade for increased PM₁₀ collection efficiency includes the following scope of work:
• Demolition of the existing electrostatic precipitator wet bottom
• Demolition of existing site interferences
• Installation of a bottom dry ash system (2 ash hoppers) on the electrostatic precipitator
• Two additional electrical fields
• Ash conveyors
• Structural supports
• Electrical supply and controls

FIGURE 2: Recovery boiler modifications for proposed MACTII compliance.

Smelt tank upgrades. The smelt dissolving vent stack scrubber may require upgrades or replacement for MACT II PM₁₀ and HAP particulate matter emission reduction, as Table 7 shows. Capital costs for replacement of the smelt vent scrubber in a 2,000 tpd kraft mill may total $499,000. Table 6 summarizes these costs.

Lime kiln upgrades. Based on mill data as represented in Table 7, upgrades for MACT II compliance in the lime kiln and recausticizing area may be required. The lime kiln scope of work for MACT II compliance includes upgrading or replacing the combustion controls, fuel gun(s), and induced fan and drive unit. Also, the lime kiln flue gas electrostatic precipitator (ESP) or scrubber unit may require an upgrade to achieve reduced PM₁₀ or particulate matter HAP emissions.

Capital costs for the lime kiln area scope of work in a 2,000 tpd kraft mill may total $1,799,500 (Table 6). Additional electrical fields and a new continuous emission monitoring system unit is included in the capital cost.

IMPACTS OF MACT II COMPLIANCE.

Incremental HAP particulate matter and VOC emission reductions will result from installation of upgrades for MACT II compliance in combustion sources such as recovery boilers, smelt tanks, and lime kilns. Table 8 summarizes environmental reductions of PM₁₀, HAP particulate matter, and project effectiveness of MACT II compliance in a 2,000 tpd kraft pulp mill. Again, project cost effectiveness is defined as the total annualized capital and operating cost divided by the tons of pollutant removed using a 20 year equipment life cycle.

Utility, operating, and maintenance costs are also incurred to support the MACT II air emission compliance systems in such a pulp mill. Summary costs total $3.23/ton pulp (Table 9).

CONCLUSIONS.

As this article describes, mills with state-of-the-art equipment may be able to demonstrate MACT I or MACT II compliance with little or no capital investment. However, substantial capital costs may be required for upgrades of existing process and pollution control equipment in mills with older equipment operating at high capacity factors. Other observations are as follows:

• Substantial utility, operating, and maintenance costs may be required to support the MACT I and proposed MACT II control systems.

• Mill site specific field testing is key to establishing the baseline emission rates, technology selection, and associated capital and operating costs for MACT I and proposed MACT II compliance.

• Process modifications or changes in raw material supplies to effect particulate matter HAP may be considered in the program for proposed MACT II compliance.

References:
1.) NCASI Technical Bulletin No. 701, October, 1995.
2.) Mill Test Reports (Confidential)

Jerry Garner is sr. process specialist for BE &KEngineering, Birmingham, Ala.