A Model of a Combined Heat and Power System for SAGD Operations

Abstract

Abstract This paper presents a model of a combined heat and power system, also known as cogeneration, for the generation of electricity and steam for in situ bitumen extraction operations. The model simulates the operation of a gas turbine-electric generation system with the hot exhaust gases from the turbine being directed to a boiler for steam production. Alternately, a topping steam turbine-electric generation system with exhaust steam directed to the in situ operations is simulated. The model provides estimates of the fuel requirements and electricity generated by the combined process. It enables the investigation of the relationships between electricity generation, steam production and fuel requirements for these operations. Included is an illustrative example of the application of the model for the assessment of the economics of the steam assisted gravity drainage (SAGD) process using alternate fuels, natural gas and petroleum coke. Introduction Bitumen mining and in situ bitumen extraction operations in northern Alberta are large users of natural gas and are projected to grow substantially over the next decade. However, since the turn of the century, the North American natural gas supply-demand balance has tightened and, consequently, gas market prices have risen substantially. Moreover, western Canadian supplies of natural gas are projected to decline in the near-term. Environmental concerns regarding the use of fossil fuels, such as natural gas, have recently become more acute. Consequently, there is a need to identify alternate fuels and heat generation systems that could, if implemented, reduce the cost and environmental impact of bitumen extraction operations. The Combined Heat and Power Model (CHPM) described in this paper was developed specifically for the economic assessment of combined heat and power systems for bitumen extraction using the steam assisted gravity drainage (SAGD) process. It calculates the fuel requirement to provide heat for the SAGD operations as well as the associated electricity generated, which are required for the economic assessment. The economic analysis is performed in a separate model-the SAGD Supply Cost Model (SSCM)-which calculates the supply cost of the extracted bitumen. Description of the CHPM The CHPM simulates the operation of the steam plant of a SAGD operation, both with and without electricity generation. Gas and steam turbine options for the generation of electricity are included (See Figures 1 and 2, respectively). As shown in the figures, mass and heat energy flows of the system are outputs of the model. The principles of conservation of energy and mass are used for the calculation of the flows. Figure 1 shows that, in the case of the gas turbine option, natural gas fuel is burned in a gas turbine to produce electricity. Waste heat in the exhaust gases from the turbine is recovered in a boiler, which produces steam for the bitumen extraction process. There is a steam-water separator, which produces 100% quality steam for this process. Water from the separator is returned to the boiler.