Biomass co-firing options on the emission reduction and electricity generation costs in coal-fired power plants
Abstract
Co-firing offers a near-term solution for reducing CO2 emissions from conventional fossil fuel power plants. Viable alternatives to long-term CO2 reduction technologies such as CO2 sequestration, oxy-firing and carbon loop combustion are being discussed, but all of them remain in the early to mid stages of development. Co-firing, on the other hand, is a well-proven technology and is in regular use though does not eliminate CO2 emissions entirely. An incremental gain in CO2 reduction can be achieved by immediate implementation of biomass co-firing in nearly all coal-fired power plants with minimum modifications and moderate investment, making co-firing a near-term solution for the greenhouse gas emission problem. If a majority of coal-fired boilers operating around the world adopt co-firing systems, the total reduction in CO2 emissions would be substantial. It is the most efficient means of power generation from biomass, and it thus offers CO2 avoidance cost lower than that for CO2 sequestration from existing power plants. The present analysis examines several co-firing options including a novel option external (indirect) firing using combustion or gasification in an existing coal or oil fired plant. Capital and operating costs of such external units are calculated to determine the return on investment. Two of these indirect co-firing options are analyzed along with the option of direct co-firing of biomass in pulverizing mills to compare their operational merits and cost advantages with the gasification option.
1. Introduction
The evidence of the effects of anthropogenic emission on global climate is overwhelming [1]. The threat of increasing global temperatures has subjected the use of fossil fuels to increasing scrutiny in terms of greenhouse gas (GHG) and pollutant emissions. The issue of global warming needs to be addressed on an urgent basis to avoid catastrophic consequences for humanity as a whole.
Socolow and Pacala [2] introduced the wedge concept of reducing CO2 emissions through several initiatives involving existing technologies, instead of a single future technology or action that may take longer to develop and stronger willpower to implement. A wedge represents a carbon-cutting strategy that has the potential to grow from zero today to avoiding 1 billion tons of carbon emissions per year by 2055. It has been estimated [3] that at least 15 strategies are currently available that, with scaling up, could represent a wedge of emissions reduction.
Although a number of emission reduction options are available to the industry, many of them still face financial penalties for immediate implementation. Some measures are very site/location specific while others are still in an early stage of development. Carbon dioxide sequestration or zero emission power plants represent the future of a CO2 emissions-free power sector, but they will take years to come to the mainstream market. The cost of CO2 capture and sequestration is in the range of 40e60 US$/ton of CO2, depending on the type of plant and where the CO2 is stored [4,5]. This is a significant economic burden on the industry, and could potentially escalate the cost of electricity produced by as much as 60%.
Canada has vast amounts of biomass in its millions of hectares of managed forests, most of which remain untapped for energy purposes. Currently, large quantities of the residues from the wood products industry are sent to landfill or are incinerated [6]. In the agricultural sector, grain crops produce an estimated 32 million tons of straw residue per year. Allowing for a straw residue of 85% remaining in the fields to maintain soil fertility, 5 million tons would still be available for energy use. Due to an increase in land productivity, significant areas of land in Canada, which were earlier farmed, are no longer farmed. These lands could be planted withfast-growing energy crops, like switch-grass offering potentially large quantities of biomass for energy production [6].
Living biomass plants absorb CO2 from the atmosphere. So, its combustion/gasification for energy production is considered carbon neutral. Thus if a certain amount of biomass is fired in an existing fossil (coal, coke or oil) fuel fired plant generating some energy, the plant could reduce firing the corresponding amount of fossil fuel in it. Thus, a power plant with integrated biomass co-firing has a lower net CO2 contribution over conventional coal-fired plants.
Biomass co-firing is one technology that can be implemented immediately in nearly all coal-fired power plants in a relatively short period of time and without the need for huge investments. It has thus evolved to be a near-term alternative to reducing the environmental impact of electricity generation from coal. Biomass co-firing offers the least cost among the several technologies/ options available for greenhouse gas reduction [7]. Principally, co-firing operations are not implemented to save energy but to reduce cost, and greenhouse gas emissions (in some cases). In a typical co-firing plant, the boiler energy usage will be the same as it is operated at the same steam load conditions (for heating or power generation), with the same heat input as that in the existing coal-fired plant. The primary savings from co-firing result from reduced fuel costs when the cost of biomass fuel is lower than that of fossil fuel, and avoiding landfill tipping fees or other costs that would otherwise be required to dispose of unwanted biomass. Biomass fuel at prices 20% or more below the coal prices would usually provide the cost savings needed [8].
Apart from direct savings in fuel cost, other financial benefits that can be expected from co-firing include the following:
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