Cement, Energy and Environment

HCI The proposed em1ss1ons standard for HCI was developed based on flawed data from a total of four kilns, three of which are area sources. The EPA states that wet scrubbers are the control technology of choice for this pollutant, but wet scrubbers are generally designed to remove sulphur dioxide, not HCI. There is no data that suggests that a wet scrubber using lime will provide the required collection efficiency needed to meet the proposed HCI limit. Another significant issue for cement plants is that wet scrubbers are extremely water-intensive. Plants located in arid locations or plants with limited water supplies may find that wet scrubbers are just not feasible. The PCA estimated that a "typical" cement plant would use 315 gallons of water for each pound of HCI removed. In some cases, that amounts to 30 million gallons of water annually. In the arid southwest 23 plants alone would requir~ more than 600 million gallons of water annually; hardly a benign environmental impact. Wet scrubbers may generate problems caused by their solids and liquids handling systems. If lime is used as the scrubbing reagent, the primary end products will be CaS0 4, CaCI2, and water. Depending on the raw materials used and the nature of the plant configuration, there may also be NaCI and KCI in solution. The CaS04 is insoluble and will make up the bulk of the solid portion of the sludge generated, while the other compounds will remain in solution. Water used during the blowdown of the scrubber will certainly require treatment to control chloride levels, pH levels, total suspended solids (TSS), and biological oxygen demand (BOD) before the water can be released. THC THC can be attributed to a combination of the level of organics in the limestone and the form that those organics take. It can also be attributed to a kiln configuration that has one or more sources or organic releases downstream of the kiln. This is especially the case with in-line raw mills and in-line coal mills that vent to the main stack. The EPA has indicated that ACI could be a viable technology for control of THC as well as mercury, yet it bases that belief on data collected from one plant. Unfortunately, ACI is simply not effective in controlling light hydrocarbons present in the kiln exhaust gas. Additionally, the limited speciation data that is available for THC indicates that these light hydrocarbons (which are not regulated hazardous air pollutants) may comprise a high percentage of THC in kiln em1ss1ons. There is also concern that using ACI to control THC may increase emissions of dioxins and furans. Cement plants burning hazardous waste and municipal incinerators have observed dioxin/furan concentrations several higher than input levels. times Like any control technology, activated carbon injection must be available 24/7. ACI is not a perpetual operation. The activated carbon becomes less effective in capturing pollutants as it captures more and more. At some point, the carbon must be replaced. That means disposing of the saturated carbon and replacing it with new or re-activated carbon. If activated carbon injection is added to the primary particulate matter control device for THC control, then the cement kiln dust (CKD) that plants normally recycle back with the raw materials must now be discarded with the spent carbon. The use of ACI for mercury capture is problematic because the carbon particles would capture mercury and then contaminate the CKD, potentially preventing its reuse in the manufacturing process. The capture mechanism of THC using ACI is quite different from mercury. THC capture using activated carbon injection is a physisosorption process, while mercury capture using activated carbon injection is a chemisorption process. That means that temperature and the speciation of the THC is critical. The very temperatures that help capture THC, hinder capture of mercury. That limitation means that regenerative thermal oxidizers (RTOs) may be necessary to capture THC at many plants. RTOs effectively reduce THC through oxidation caused by a combustion process. There is only one US plant now using a regenerative thermal oxidizer (though not year round) and it has a natural gas usage of about 70 MMBtu/hour. Clearly RTOs will increase cost, energy usage, and greenhouse gas emissions, while also introducing performance limitations. Problems related to the plugging of heat exchangers along with the significant maintenance required, make RTOs a dubious choice at best. If that were not enough RTOs used in cement plants will require wet scrubber pretreatment of the exhaust gases. This is because of the presence of acid gases in the kiln exhaust. RTOs require natural gas for the combustion process. For plants that do not have an existing natural gas supply, this will require a significant infrastructure investment. Propane might be an alternative, but here again : there is an added cost for the storage and handling of this fuel. 35