Carbon dioxide boilers

Utility systems as sources of waste. The principal sources of utility waste are associated with hot utilities (including cogeneration systems) and cold utilities. Furnaces, steam boilers, gas turbines, and diesel engines all produce waste from products of combustion. The principal problem here is the emission of carbon dioxide, oxides of sulfur and nitrogen, and particulates (metal oxides, unbumt... 

Fuel switch. The choice of fuel used in furnaces and steam boilers has a major effect on the gaseous utility waste from products of combustion. For example, a switch from coal to natural gas in a steam boiler can lead to a reduction in carbon dioxide emissions of typically 40 percent for the same heat released. This results from the lower carbon content of natural gas. In addition, it is likely that a switch from coal to natural gas also will lead to a considerable reduction in both SO, and NO, emissions, as we shall discuss later. 

C its solubihtyis only 14-15 mg/L. As the temperature increases, so does the solubihty until at 100°C solubihty reaches 30-40 mg/L. This faint solubility at elevated temperatures accounts for the accretion of a primarily CaCO scale in steam boilers. Carbon dioxide exerts a mild solvent action on... 

The selection of boiler-water treatment is also dependent on the type of cooling water. When cooling water reaches the boiler, various compounds precipitate before others. For instance, seawater contains considerable magnesium chloride. When the magnesium precipitates as the hydroxide, hydrochloric acid remains. In some lake waters, calcium carbonate is a significant impurity. When it reaches the boiler, carbon dioxide is driven off in the... 

The steam balance in the plant shown in Figure 2 enables all pumps and blowers to be turbine-driven by high pressure steam from the boiler. The low pressure exhaust system is used in the reboiler of the recovery system and the condensate returns to the boiler. Although there is generally some excess power capacity in the high pressure steam for driving other equipment, eg, compressors in the carbon dioxide Hquefaction plant, all the steam produced by the boiler is condensed in the recovery system. This provides a weU-balanced plant ia which few external utiUties are required and combustion conditions can be controlled to maintain efficient operation. 

Overall comparison between amine and carbonate at elevated pressures shows that the amine usually removes carbon dioxide to a lower concentration at a lower capital cost but requires more maintenance and heat. The impact of the higher heat requirement depends on the individual situation. In many appHcations, heat used for regeneration is from low temperature process gas, suitable only for boiler feed water heating or low pressure steam generation, and it may not be usefiil in the overall plant heat balance. 

PURIFICATION BUILDING 2. PROCESS LABORATORY 3FILLINGUNIT L WAREHOUSE 5. BOILER HOUSE 5. BOILER HOUSE CHIMNEY 7. PURIFICATION LINE. .CD" 8.CARBON DIOXIDE STORAGE TANK YARD ... 

The bicarbonate content of feed should be moderate to avoid excessive liberation of carbon dioxide in the boiler. 

For higher-pressure boilers demineralization is necessary to minimize total dissolved solids in the boiler. This water is normally carried in steel pipework, but if condensate is returned and the condensate has become contaminated (for example, with carbon dioxide or copper ions) more corrosion-resistant materials such as copper are required. Downstream of the boiler, steam pipework is usually steel with steel or stainless steel expansion bellows. 

Carbon dioxide, from the decomposition in the boiler of temporary hardness salts present in some waters, causes corrosion of steel steam pipework and cast iron valves and traps. Corrosion inhibitors may be used, but the choice of inhibitor must take into account the other materials in the system. Neutralizing amines such as morpholine or cyclohexylamine are commonly used. 

Condensate returns lines are often copper. Copper has good corrosion resistance to oxygen and carbon dioxide individually. When both gases are present in the condensate, copper is susceptible to corrosion. Copper picked up in the condensate system and returned to the boiler causes serious corrosion problems in the boiler and any steel feedwater and steam pipework. Boiler tubes should last for 25 years but can fail within one year in a mismanaged or ill-designed boiler system suffering from these faults. 

Carbon dioxide produces a solution of carbonic acid (as in boiler condensate, see Section 53.3.2). Carbon steel is often employed but corrosion rates of up to 1 mm/yr can be encountered. Coatings and non-metallic materials may be employed up to their temperature limits (Section 53.5.6). Basic austenitic stainless steels (type 534) are suitable up to their scaling temperatures. 

Joints in copper components may be a source of trouble. Copper/zinc brazing alloys may dezincify and consequently give rise to leaks . In some waters, soft solders are preferentially attacked unless in a proper capillary joint. Copper/phosphorus, copper/silver/phosphorus, and silver brazing alloys are normally satisfactory jointing materials. Excessive corrosion of copper is sometimes produced by condensates containing dissolved oxygen and carbon dioxide. Rather severe corrosion sometimes occurs on the fire side of fire-back boilers and on electric heater element sheaths under scales deposited from hard waters . 

Oxygen is not the only noncondensable gas found in boiler circuits, Problems occur due to the presence of carbon dioxide (C02). Carbon dioxide is steam-volatile and reacts with condensing steam to produce carbonic acid, which attacks steel condensate return lines. 

These factors severely enhance the risks of condensate system corrosion by carbonic acid (resulting from a breakdown of the alkalinity in the boiler water and carbon dioxide [C02] carryover into the steam) and BW carryover. In addition, boiler operation is more difficult because the possible COC is severely limited, there-... 

Consequently, a loss of free carbon dioxide in the water, because of either a rise in temperature (as occurs in a FW heater or boiler) or an increase in pH (all boilers operate at an alkaline pH) leads to a change of equilibrium and the resultant rapid and troublesome precipitation of insoluble calcium carbonate scale on heat transfer surfaces. The reaction is as shown here ... 

In practice, the potential causes of boiler section corrosion are many and often commonplace. Initiators include oxygen, carbon dioxide, acid, caustic, copper plating, chelant, and even the water itself. In addition, mechanical problems may be an initiator of corrosion, which in turn may lead to boiler mechanical failure. 

Carbon dioxide is present in steam as a result of the thermal decomposition of bicarbonates in the boiler. 

Where FW contains bicarbonate or carbonate alkalinity (as calcium, magnesium, or sodium salts), these salts undergo thermal decomposition in the boiler, and the steam-volatile contaminant gas carbon dioxide is introduced into the steam distribution system, as shown ... 

Carbon dioxide carryover also occurs following the deliberate addition of soda ash (sodium carbonate) directly to the boiler. Where boiler designs provide for a significant internal drum or shell, the use of soda ash and caustic soda to prevent calcium and magnesium scales by precipitation reactions (internal softening) may be employed. 

Corrosion of condensate lines is a serious problem. It is compounded where both oxygen and carbon dioxide are present because it causes considerable quantities of hematite (Fe203) to develop. Corrosion of other boiler plant components, such as FW heaters, adds more metals to the mix, and corrosion debris typically includes iron, copper, nickel, zinc, and chromium oxides. 

Both hot and cold processes are employed, although the hot process, which takes place at or above 212 °F (100 °C), is usually preferred for boiler FW applications, because it produces water of lower hardness levels and usually a lower silica content as well. Also, less lime is needed because the carbon dioxide with which it would normally react is driven off at the higher temperatures. Sometimes caustic soda (sodium hydroxide) is used in place of soda, depending on the alkalinity of the water and the chemical costs however, irrespective of the process or chemicals used, the major precipitants are always calcium carbonate and magnesium hydroxide. 

Celgard LLC markets Hoechst Celanese modular membrane technology (Liqui-Cel ) to remove both oxygen and carbon dioxide from boiler MU water and FW. 

DEHA breaks down at high pressure. Its survival pressure is probably not in excess of 1,250 psig, but because of its high volatility and rapid reaction rate, it generally provides complete boiler cycle oxygen-control coverage. Some limited ammonia is also generated, and this may be useful for carbon dioxide neutralization. 

Erythorbates are safe products and there are no harmful breakdown products, although when early formulations utilized ammonia as a PH buffer (and neutralizer for part of the carbon dioxide), copper corrosion problems resulted. However, erythorbates are not steam-volatile,and consequently there is no post-boiler oxygen scavenging potential available. Thus, in the event of complete breakdown of the product at high pressure, oxygen-induced, ammonia corrosion of copper may continue unchecked. 

Carbohydrazide itself is of very low volatility, but it decomposes at relatively low temperatures to produce volatile carbon dioxide and ammonia. In theory, the combined corrosive effects of these two materials should be negated in the condensate system, but in practice, this is not always so and both steel and copper corrosion transport problems may develop, primarily as the result of corrosion-enhancement reactions resulting from oxygen in-leakage. It is presumed, therefore, that (similar to hydrazine) some deliberate after-desuperheating line addition of CHZ is necessary if post-boiler section corrosion is to be avoided. 

The FW oxygen scavengers mentioned earlier are volatile and can therefore provide additional protection against post-boiler section corrosion induced or enhanced by oxygen in-leakage. These particular scavengers also break down under pressure to produce some level of volatile ammonia, which can neutralize any carbon dioxide found in the steam-condensate system. 

Corrosion of steel by carbonic acid is probably the most common problem in the post-boiler section, producing pipe grooving and general metal wastage, especially in threaded joints. This form of corrosion is not self-regulating and the reaction products can produce more carbon dioxide, thus perpetuating the corrosion problem. Typically, the condensate pH level is depressed to around 5.0 to 5.5. 

The formation of carbon dioxide from FW alkalinity varies, depending on boiler pressure, as shown in Table 11.3. 

Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

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