Steam Generators for Oil Sands Application

Steam-assisted gravity drainage (SAGD) is considered to be the most viable and environmentally safe recovery technology for extracting heavy oil and bitumen. In the SAGD process, one well is drilled above the bitumen deposit and a second one below the deposit. The upper well is supplied with high-pressure steam, and the lower well collects the heated oil or bitumen that flows out along with any water from the condensation of injected steam. The bitumen and water are pumped out, and they travel to a tank where the two elements are separated. The produced water is then cleaned, and it returns to the boiler where it is converted to steam and reinjected into the well. In the past, warm lime softening process was used to clean the oil field produced water. The produced water cannot be cleaned enough for conventional drum-type boilers so steam is generated in OTSGs making 75–80% quality steam, and hence, 20% of the water is wasted. Environmental regulations are becoming stricter in Alberta causing oil companies to focus on water conservation. As a result, a more viable and environmentally friendly technology called evaporator technology is replacing the warm line softening treatment process.

Evaporator technology produces higher-quality water compared to warm lime softening. The process takes the clarified water and evaporates it out and then condenses it back to produce feed water. It also conserves water and opens up the steam production options to drum-type boilers generating saturated steam. Evaporator water can be run through an OTSG or drum-type boiler (for medium pressure) or through Cleaver Brooks’s newly developed forced circulation oil sands steam generator (FC-OSSG). The special design fea- ture of the FC-OSSG is that it operates like an OTSG but can be mechanically cleaned or pigged by conventional means.

The FC-OSSG addresses the key requirements of oil industry professionals: • Ability to handle the inevitable water quality upsets

• Capability of operating with evaporator-produced water while delivering best-

in-class efficiency • Ability to handle capacities up to 227 t/h of steam (500,000 lb/h) • Meeting stringent regulatory requirements for water usage and emissions • Features reliable steam generators that can work with available produced water • Easy to maintain—all circuits are piggable (a metal piece is sent through each circuit

to clean the inside surface, which is called a pig and the process is called pigging) Figure 3.4 shows the steam generator for oil sands applications. Figure 3.22 shows the

scheme of the forced circulation boiler. There are two parallel circuits, one for the furnace section and another for the convective evaporator section. The circulation system is designed for an average CR of about 5. By varying the flow through the radiant or convective section, one can adjust the CR in either path. The tube wall temperatures in the furnace and evaporator section are measured, which gives an indication of any tube-side buildup of solids. The CR can be increased in that circuit if need arises.

116 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Steam drum

20% quality

Flow > 5 × steam ﬂow

Flow rate decided by

ﬁring rate water quality

12 oﬀ tube metal temperature

FE Convection module 28 parallel passes

2 × 50% circulation pumps

FE 12 parallel passes Furnace module

12 oﬀ tube metal temperature

FIGURE 3.22

Scheme of forced circulation boiler (FC-OSSG) for oil field application. (Courtesy of Cleaver Brooks Inc., Engineered Boiler Systems, Thomasville, GA.)

The issue of water quality upsets is one that is important to oil industry professionals. Existing technologies repeatedly fall short in this area. Even though a system may be operating with evaporated water, which is supposed to be pure, it may still contain oil content above the acceptable limit for drum boilers. This, along with the inevitable water quality upsets, can cause fouling of the heat transfer tubes. Cleaning drum boilers requires access to the inside drums, which are not piggable, so tube cleaning can be costly and time-con- suming. Alternatively, the OTSG can handle water quality upsets, but the equipment is limited in size. An OTSG can handle only up to 136 t/h (300,000 lb/h) of steam (Figure 3.23).

Turbine exhaust in

Economizer

80% steam out

Heat exchanger

70 F water in Evaporator

FIGURE 3.23

OTSG generating 80% quality steam in oil fields.

Steam Generators 117

Subcooled water

Two-phase, steam water

Superheated steam

High heat ﬂux

erature mp

heat ﬂux

Tube wall temperatures

Fluid temperature

Tube wall temperatures

Water enthalpy Steam enthalpy

0 100 x = Quality, % steam by weight

FIGURE 3.24

Typical steam quality versus tube wall temperature in boiling process.

The FC-OSSG is a forced circulation boiler (Figure 3.4). It uses circulation pumps to force the steam water mixture through the furnace and evaporator circuits. A CR of 5–8 is achieved depending on the actual resistance to flow in the circuits. Hence, the steam quality at the exit of the furnace or evaporator is about 12%–20% only compared to 80% for an OTSG. Hence, allowable heat fluxes for DNB are much higher as shown in Figure 3.24 and as also discussed in Chapter 2. Higher fouling compared to OTSG can be handled by OSSG. The tube-side velocities are designed to be very high (on the order of 2.5 m/s). Hence, deposition of solids is ruled out, and frequency of cleaning and potential for failure will be less compared to an OTSG.

In case of upsets or sudden increase in feed water conductivity and turbidity over limits set, the control system reduces the firing rate on the boiler and increases the water flow through the tube to protect it. Increasing the amount of water flowing through the tube reduces the steam quality in the heat transfer circuits, resulting in lesser concentration of impurities and reduced metal temperatures. Heat flux also is reduced due to lowering of heat input. The boiler keeps running, but at a reduced firing rate. Then, if it exceeds another preset value, the operator trips the boiler to protect it.

Unlike in an OTSG, whereby mostly steam travels through the tube, the FC-OSSG is designed so that mostly water flows through the tube. The metal temperature of a water- filled tube closely matches the water temperature rather than the hot gas temperature. For

a constant heat flux (rate at which heat is transferred), the tube wall temperature is a func- tion of the steam quality. At lower steam qualities, the tube predominantly is filled with water, and the metal temperature is closer to the water temperature and is stable. As the steam quality increases, the metal temperature also increases, and beyond a certain point, it starts dropping down, due to the very high velocities in the tube caused by the differ- ence in densities of steam and water. Figure 3.24 shows the relationship between steam quality and metal temperature.

118 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

An OTSG operates at 75%–80% steam quality, to the right of the curve, in the high steam quality region. Trying to predict the metal temperature in this region is very difficult. Conversely, the FC-OSSG operates at 20% steam quality, represented by the flat part of the curve, in the very robust, reliable operating region. Thus, it has much higher tolerance for any heat flux and water quality variations during operation. Several such steam generators are in operation today in Canada, supplied by Cleaver Brooks Corporation.

Steam Generators for Oil Sands Application