INDUSTRIAL WATER TREATMENT TM January/February 2015 1 COAL Wastewater Reuse at a Methanol Production Plant ISSN:1058-3645. COPYRIGHT (C) Media Analytics Ltd. Reproduction in whole, or in part, including by electronic means, without permission of publisher is prohibited. Those registered with the Copyright Clearance Center (www.copyright.com) may photocopy this article for a flat fee per copy. By Mike Snodgrass, Ryan Vargas, (TriSep Corp.) and Jeff Li (Eco Environmental Investments Ltd.) R apid growth and develop- ment in China has dramati- cally increased its need for energy and raw materi- als, but without sufficient domestic oil or natural gas supplies to meet those needs, the country has sought to develop alternatives to petroleum based technologies. As a country with ample coal supplies, China has devel- oped a significant industry based off of coal-to-chemical processes in order to meet its growing demands. Alternative fuel production in the form of methanol from coal has accelerated rapidly in recent years as has the manu- facture of basic chemicals from coal-to- methanol feed stocks. China has quickly become the world’s largest producer and consumer of methanol (1). The Chinese government methanol-to-gasoline fuel blending standards (M85 and M100), in place since 2009, have significantly con- tributed to the consumption of methanol as a fuel alternative. It is anticipated that methanol will substitute more than half of China’s gasoline supply by 2015 (1). Coal Gasification and Water Consumption Methanol, which is used as both an alternative fuel and feedstock for numer- ous other petrochemicals, is generated through coal gasification processes. Coal gasification consists of reacting coal with steam and oxygen, and then generating a synthetic gas, which is then further processed into methanol. An example of a coal gasification process is shown in Figure 1. China’s thirst for methanol, and other coal-based chemicals, has created a significant water issue in the country’s coal-processing regions, most of which are in the arid Northern Provinces. Coal gasification demands tremen- dous amounts of water, as production of 1 metric ton of methanol consumes approximately 20 cubic meters (m 3 ) (5,283 gallons [gal]) of fresh water (1). Significant volumes of wastewater are also discharged by coal-to-chemical pro- cess plants, further compounding water stresses. Unfortunately, the majority of China’s coal reserves are located in provinces with significant fresh water scarcity issues. Viable fresh water in these regions is so scarce that the Chinese government aims to cap annual methanol production capacity at 50 million metric tons by 2015 in order to protect what fresh water that is available (1). The Chinese Province of Inner Mon- golia, in particular, has developed into one of the nation’s most significant coal-producing regions. Between the years 2000 and 2013, coal production in the province has increased more than tenfold, and now holds roughly 28% of China’s overall market share (2). Water scarcity is by far the biggest threat to sustainable population and industrial growth in the Inner Mongolia region. A booming coal industry and growing population has led to significant and severe groundwater depletion (3, 4). This has led the Chinese central government to increase pressure on industry to move towards water reuse (5, 6). Faced with an uncertain water future and growing methanol demand, the ECO Environmental Investments, Ltd., coal-to-methanol plant in Ordos Inner Mongolia launched a water reuse project in 2010 to reduce its dependency on fresh water supplies. Water Reuse Pilot Project In order to achieve its water reuse goals, the Ordos plant initiated a 396 gallons per minute (gpm) (90 cubic meters per hour [m 3 /hr]) pilot project to determine the feasibility of recycling plant wastewater for use as cooling tower and boiler feed make-up water. With an overall goal of reducing fresh water consumption by 20% to 30%, the plant felt the cost of a large scale pilot plant was easily justified. The plant chose to treat and reuse three different waste streams: 1. Wastewater from the production plant, 2. Deionized (DI) water plant discharge; and 3. Cool- ing tower blowdown. The three streams are blended, along with raw (well) water, prior to treatment. Since reverse osmosis (RO) is a key component of the reuse scheme, minimizing the scaling potential Figure 1. Coal gasification process.
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INDUSTRIAL WATER TREATMENTTM January/February 2015 1
COALWastewater Reuse at a Methanol Production Plant
ISSN:1058-3645. COPYRIGHT (C) Media Analytics Ltd. Reproduction in whole, or in part, including by electronic means, without permission of publisher is prohibited. Those registered with the Copyright Clearance Center (www.copyright.com) may photocopy this article for a flat fee per copy.
By Mike Snodgrass, Ryan Vargas, (TriSep Corp.)and Jeff Li(Eco Environmental Investments Ltd.)
R apid growth and develop-ment in China has dramati-cally increased its need for energy and raw materi-als, but without sufficient
domestic oil or natural gas supplies to meet those needs, the country has sought to develop alternatives to petroleum based technologies. As a country with ample coal supplies, China has devel-oped a significant industry based off of coal-to-chemical processes in order to meet its growing demands.
Alternative fuel production in the form of methanol from coal has accelerated rapidly in recent years as has the manu-facture of basic chemicals from coal-to-methanol feed stocks. China has quickly become the world’s largest producer and consumer of methanol (1). The Chinese government methanol-to-gasoline fuel blending standards (M85 and M100), in place since 2009, have significantly con-tributed to the consumption of methanol as a fuel alternative. It is anticipated that methanol will substitute more than half of China’s gasoline supply by 2015 (1).
Coal Gasification and Water Consumption Methanol, which is used as both an alternative fuel and feedstock for numer-ous other petrochemicals, is generated through coal gasification processes. Coal gasification consists of reacting coal with steam and oxygen, and then generating a synthetic gas, which is then further
processed into methanol. An example of a coal gasification process is shown in Figure 1.
China’s thirst for methanol, and other coal-based chemicals, has created a significant water issue in the country’s coal-processing regions, most of which are in the arid Northern Provinces.
Coal gasification demands tremen-dous amounts of water, as production of 1 metric ton of methanol consumes approximately 20 cubic meters (m3) (5,283 gallons [gal]) of fresh water (1). Significant volumes of wastewater are also discharged by coal-to-chemical pro-cess plants, further compounding water stresses. Unfortunately, the majority of China’s coal reserves are located in provinces with significant fresh water scarcity issues. Viable fresh water in these regions is so scarce that the Chinese government aims to cap annual methanol production capacity at 50 million metric tons by 2015 in order to protect what fresh water that is available (1).
The Chinese Province of Inner Mon-golia, in particular, has developed into one of the nation’s most significant coal-producing regions. Between the years 2000 and 2013, coal production in the province has increased more than tenfold, and now holds roughly 28% of China’s overall market share (2). Water scarcity is by far the biggest threat to sustainable population and industrial
growth in the Inner Mongolia region. A booming coal industry and growing population has led to significant and severe groundwater depletion (3, 4). This has led the Chinese central government to increase pressure on industry to move towards water reuse (5, 6).
Faced with an uncertain water future and growing methanol demand, the ECO Environmental Investments, Ltd., coal-to-methanol plant in Ordos Inner Mongolia launched a water reuse project in 2010 to reduce its dependency on fresh water supplies.
Water Reuse Pilot ProjectIn order to achieve its water reuse goals, the Ordos plant initiated a 396 gallons per minute (gpm) (90 cubic meters per hour [m3/hr]) pilot project to determine the feasibility of recycling plant wastewater for use as cooling tower and boiler feed make-up water. With an overall goal of reducing fresh water consumption by 20% to 30%, the plant felt the cost of a large scale pilot plant was easily justified.
The plant chose to treat and reuse three different waste streams: 1. Wastewater from the production plant, 2. Deionized (DI) water plant discharge; and 3. Cool-ing tower blowdown. The three streams are blended, along with raw (well) water, prior to treatment. Since reverse osmosis (RO) is a key component of the reuse scheme, minimizing the scaling potential
Figure 1. Coal gasification process.
INDUSTRIAL WATER TREATMENTTM January/February 20152
prior to the membranes was critical, so lime softening was incorporated into the reuse process. Polyaluminum chloride (PAC) is also added to the lime-softening process to enhance any organic removal. The treatment process selected had the following steps:
A portion of the first-pass RO system would be used for cooling tower make-up, while the entire permeate stream generated by the second-pass RO would feed the plant’s mixed-bed ion exchange
(MBIX). A complete process schematic is shown in Figure 3.
Pilot Plant PerformanceThe main problem experienced during the pilot project was rapid RO fouling, which necessitated extensive cleaning and system downtime. RO performance rapidly declined as chemical cleanings proved ineffective at restoring flow. It was discovered that the RO membranes were fouling because of total suspended solids (TSS), which can be easily seen on the front ends of the elements shown in Figure 4. Whenever RO membranes are used for wastewater reuse, pretreatment
is absolutely critical, otherwise fouling will be rapid and severe, increasing operating costs for plant owners.
Upon further investigation, it was determined that there was significant ultrafiltration (UF) hollow fiber break-age, allowing solids to pass through to the RO. Mechanical integrity of the UF membranes is key in achieving and maintaining a high effluent quality for the RO system. In order to protect the RO system, the UF modules were regularly repaired to seal up any broken fibers. While RO performance (fouling) was marginally improved with frequent UF fiber repairs, system efficiency further decreased because of additional system downtime.
Despite the challenges with suspended solids fouling the RO because of UF mechanical integrity issues, the plant was convinced that water reuse was a viable option to help ease the strain on their fresh water supplies. It was decided that the water reuse plant would be expanded to 1,188 gpm (270 m3/hr). However, it was decided that a more robust and durable UF membrane design was needed in order to properly protect the RO membranes from excessive solid fouling. After reviewing multiple mem-brane design formats, the plant selected a submerged spiral-wound UF membrane. The plant was confident enough in the expansion design that no pilot testing was performed with the spiral wound UF.
Expansion Plant DesignThe pilot project proved successful in terms of achieving the desired effluent water quality. Therefore, the same pro-cess design concept was used for the new plant expansion. In addition to increasing reuse capacity, the plant targeted a signifi-cant reduction in wastewater discharge as another key project objective. The new treatment system was designed to reduce overall liquid waste from 300 to 130 tons per day, a 56% reduction.
To achieve a reduction in wastewater discharge, the following changes/addi-tions were made to the system design: 1. Sludge collection systems were added to increase water recovery; 2. UF reject is recycled back to the front end of the treatment system; and 3. Reject from the second-pass RO is recycled back to the feed of the first-pass RO. Final design
Figure 2. Coal-to-methanol production plant.
Figure 3. Coal-to-methanol plant wastewater plant process flow diagram.
INDUSTRIAL WATER TREATMENTTM January/February 2015 3
of the treatment plant expansion, which used the same treatment approach used in the pilot project, is shown in Figure 5. The feedwater chemistry used as the design basis for the project is shown in Table A.
The existing clarifiers used in the pilot project were deemed sufficient to meet the demands on the plant expansion. However, all other processes required a significant increase in size and scope. The expansion of the water system included the construction of a new lime softening system, multimedia filters, a submerged spiral-wound UF system, a double-pass RO system, new sludge collection sys-tem, filter presses, holding tanks, and a state-of-the-art control room. These were housed within a new building that held the expanded treatment facility. Design summaries of both the UF and RO systems are shown in Tables B and C, respectively.
A low-fouling RO membrane chem-istry was selected for the first-pass RO unit to combat against organic fouling. Since soluble organic compounds will pass through the UF, there is the pos-sibility the RO membranes in the first pass could experience some fouling. Recovery in the first-pass RO system is limited because of calcium carbonate, silica, and fluoride scaling. Without the lime softening process, maximum recovery through the first-pass RO would have been significantly less.
Construction of the plant expansion began in 2013 with commissioning in January 2014. Start-up of the new plant focused on the lime softening process first, followed by the UF/RO systems. Figure 6 shows two parts of the reuse plant—lime softening clarifier (left) and a UF system. Not shown is a two-pass RO system.
Apart from the typical growing pains associated with the start-up of a new wastewater treatment system, plant per-sonnel have been extremely satisfied with the UF effluent quality and resulting RO performance. Through the first 5 months of operation, the RO systems have yet to be cleaned, a significant improvement from the pilot project. Some scaling has been present on the UF system, but that is suspected since several sparingly soluble salts are at or beyond their saturation limits. Fortunately, chemically enhanced
INDUSTRIAL WATER TREATMENTTM January/February 20154
backwashes with citric acid have proven extremely effective at cleaning scale from the UF system. Performance of the UF has been very consistent over the initial months of operation with respect to permeate flux and trans-membrane pressure (TMP). UF operating data shown in Figure 7 is typical of day to
day system performance.Preliminary estimates based on the
first 3 months of operation by ECO engineers indicates that the reduction in fresh water consumption resulting from the expansion is on pace to save the plant ¥7.8 million ($1.3 million USD) on an annual basis.
ConclusionsThe combination of a coal-rich and water scarce region found in Inner Mongolia poses unique challenges for coal to methanol producers. With demand and production expected to grow as part of China’s current 5-year plan (2011-2015) (7), water management, sustainability, and environmental impact issues will be at the forefront of the country’s coal producing areas.
ECO Environmental Investments coal-to-methanol facility in the New Dalu Chemical Industrial Base is one such plant grappling with the need for water reuse and increasingly stringent wastewater treatment regulations. The expansion of the wastewater system at ECO’s methanol plant following the completion of its pilot phase represents what is an expected trend within China as other new and existing plants explore similar investments in more robust water treatment and reuse systems.
In the case of ECO’s newly expanded wastewater plant at its Inner Mongolia coal-to-methanol facility, the use of a submerged spiral-wound UF membrane is a departure from more traditional hol-low-fiber systems. The unique challenge of operating a rugged low maintenance UF system as pretreatment for RO in a relatively remote region have made the use of a spiral-wound membranes an advantage for meeting the needs of the plant.
Early data from the commissioning process indicates that the expanded system is producing excellent water quality for reuse within the plant. With well-conditioned water being supplied by the new UF system to the RO units a key pain point experienced in the opera-tion of the pilot wastewater treatment system has been alleviated and points towards a new treatment option for future similar plants in the region. While optimization of the treatment process continues, the plant anticipates being able to meet its wastewater discharge reduction numbers.
AcknowledgementsA special thank you for support of this article goes to Liu Xin Chun, Alan Zhang, Xiao Hui, Vincent Cheung Kam Fung, and Terence Lam.
Figure 5. Wastewater reuse expansion plant process flow diagram.
Figure 6. Lime softening clarifier (left) and ultrafiltration system (right).
Figure 7.
INDUSTRIAL WATER TREATMENTTM January/February 2015 5
Parameter Units ValueMembrane chemistry - PVDFPore size µm 0.03UF configuration - SubmergedFlow capacity m3/hr 270No. trains - 3Flux lmh 42.5System recovery % 90Backwash frequency min 30
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This paper was originally presented at the 75th International Water Conference, which was con-ducted Nov. 16-19, 2014, in San Antonio, Texas. Conference information is available at www.eswp.com/water.
