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1、 外文文献：Biomass co-firing options on the emission reduction and electricity generation costs in coal-fired power plantsAbstractCo-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 a
2、s 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 C
3、O2 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
4、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
5、 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 analyze
6、d 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
7、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 wed
8、ge 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
9、 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, ma
10、ny 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 the
11、y 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 ele
12、ctricity 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 agr
13、icultural 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
14、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/gasificati
15、on 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 biom
16、ass 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
17、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 redu
18、ce 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
19、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 pro
20、vide 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:Various pollution-reduction incentives: As co-firing, through synergetic effects, reduces the net SOx, NOx and heavy metal emissions, the plant c
21、ould claim the applicable pollutionreduction incentives offered by government agencies. Financial incentives for plant greenhouse gas (GHG) emission reduction: A co-firing plant that uses biomass to replace an amount of coal in an existing boiler will reduce almost an equal amount of net CO2 emissio
22、n from the plant. On-demand power production: Unlike other renewable energy technologies (e.g.: solar, wind), biomass-based power generation can be made available whenever it is needed. This helps to accelerate the capital investment payoff rate by utilizing a higher capacity factor. An option towar
23、ds meeting a renewable energy portfolio: Cofiring offers a fast track, low-cost opportunity to add renewable energy capacity economically as it can be added to any coalfired plant immediately, with minimum investment. Earning of renewable energy tax credits: The use of biomass as an energy source to
24、 displace fossil fuel can be eligible for special tax credits from many governments. Fuel flexibility: Biomass as a fuel provides a hedge against price increases and supply shortages of coal ore. In co-firing, biomass can be viewed as an opportunity fuel, used only when the price is favorable.biomas
25、s fuels are generally sourced from the areas in the immediate vicinity of the plant (to save on transportation costs), the local communities benefit economically from the production of biomass fuels. All these potential benefits are, however, complex functions of local factors such as the price of c
26、oal and biomass, government policies, capital investment, and the carbon market in the evaluation of the cost effectiveness of electricity production using biomass co-firing. The present paper discusses the effect of these factors on the viability of different technical co-firing options in coal-fir
27、ed power plants. To illustrate these effects, an analysis of the economic aspects of different co-firing options is performed by considering the case of a 150 MW pulverized coal (PC) fired power plant in Canada.2. Co-firing options Biomass co-firing has been successfully demonstrated in over 150 ins
28、tallations worldwide for a combination of fuels and boiler types 9. The co-firing technologies employed in these units may be broadly classified under three types: i. Direct co-firing, ii. indirect co-firing, and iii. gasification co-firing. In all three options, the use of biomass displaces an equi
29、valent amount of coal (on an energy basis), and hence results in the direct reduction of CO2 and NOx emissions to the atmosphere. The selection of the appropriate co-firing option depends on a number of fuel and site specific factors. The objective of this analysis is to determine and compare the ec
30、onomics of the different co-firing options. Brief descriptions of the three co-firing options are presented here.2.1. Direct co-firing Direct co-firing involves feeding biomass into coal going into the mills, that pulverize the biomass along with coal in the same mill. Sometime separate mills may be
31、 used or biomass is injected directly into the boiler furnace through the coal burners, or in a separate system. The level of integration into the existing plant depends principally on the biomass fuel characteristics. Four different options are available to incorporate biomass cofiring in pulverize
32、d coal power plants 10. In the first option, the pre-processed biomass is mixed with coal upstream of the existing coal feeders. The fuel mixture is fed into the existing coal mills that pulverize coal and biomass together, and distribute it across the existing coal burners, based on the required co
33、-firing rate. This is the simplest option, involving the lowest least capital costs, but has a highest risk of interference with the coal firing capability of the boiler unit. Alkali or other agglomeration/corrosion-causing agents in the biomass can build-up on heating surfaces of the boiler reducin
34、g output and operational time 11. Furthermore, different combustion characteristics of coal and biomass may affect the stability and heat transfer characteristics of the flame 12. Thus, this direct co-firing option is applicable to a limited range of biomass types and at very low biomass-to-coal co-
35、firing ratios. The second option involves separate handling, metering, and pulverization of the biomass, but injection of the pulverized biomass into the existing pulverized fuel pipe-work upstream of the burners or at the burners. This option requires only modifications external to the boiler. One
36、disadvantage would be the requirement of additional equipment around the boiler, which may already be congested. It may also be difficult to control and to maintain the burner operating characteristics over the normal boiler load curve. The third option involves the separate handling and pulverizati
37、onof the biomass fuel with combustion through a number of burners located in the lower furnace, dedicated to the burning of the biomass alone. This demands a highest capital cost, but involves the least risk to normal boiler operation as the burners are specifically designed for biomass burning and
38、would not interfere with the coal burners. The final option involves the use of biomass as a reburn fuel for NOx emission control. This option involves separate biomass handling and pulverization, with installation of separate biomass fired burners at the exit of the furnace. As with the previous op
39、tion, the capital cost is high, but risk to boiler operation is minimal.2.2. Indirect or external co-firingIndirect co-firing involves the installation of a completely separate biomass boiler to produce low-grade steam for utilization in the coal-fired power plant prior to being upgraded, resulting
40、in higher conversion efficiencies. An example of this option is the Avedore Unit 2 project in Copenhagen, Denmark. In Canada, Greenfield Research Inc. has developed a similar CFB boiler design that utilizes a number of units of the existing power plant systems like ID fan etc. to reduce the capital
41、cost. In this system, a subcompact circulating fluidized bed boiler is designed specifically to have a piggy-back ride on an existing power plant boiler. Since it is not a stand-alone boiler it does not need many of the equipment or component of a separate boiler. This unit releases flue gas at rela
42、tively high temperature and joins the existing flow stream of the parent coal-fired boiler after air heater. Thus, the flue gas from the co-firing unit does not come in contact with any heating elements of the existing boiler, thus avoiding the biomass related fouling or corrosion problem, which is
43、the largest concern of biomass cofiring. This boiler is totally independent of the parent unit, and as such, any outage in the co-firing unit does not affect the generation of the parent plant. Thus this indirect combustion-based option offers high reliability. The piggy-back boiler produces low pre
44、ssure steam feeding into the process steam header of the power plant. Fig. 1 shows the photograph of one such unit built by Greenfield Research Inc., for a 220MWe Pulverized coal-fired boiler in India. In this specific case, the piggy-back boiler fired waste fuel from the parent boiler as that was t
45、he need of the plant. Fig.12.3. Gasification co-firing Co-firing through gasification involves the gasification of solid biomass and combustion of the product fuel gas in the furnace of the coal-fired boiler. This approach offers a high degree of fuel flexibility. Since the gas can be injected direc
46、tly into the furnace for burning, the plant can avoid expensive flue gas cleaning as one would need for syngas or fuel gas for diesel engines. As the enthalpy of the product gas is retained, this results in a very high energy conversion efficiency. If the biomass contains highly corrosive elements l
47、ike chlorine, alkali etc., a certain amount of gas cleaning may be needed prior to its combustion in the furnace.Another important benefit of injection of gas in the furnace is that it serves as a gas-over firing designed to minimize NOx. Although less popular, indirect or external and gasification
48、cofiring options have certain advantages, such as the possibility to use a wide range of fuels and easy removal of ash. Despite the significantly higher capital investment requirement, these advantages make these two options more attractive to utility companies in some cases.3. Current status of bio
49、mass co-firingThere are a number of co-firing installations worldwide, with approximately a hundred in Europe, 40 in the US and the remainder in Australia and Asia (Fig. 2) 9,13. Most of these installations employ direct co-firing, mainly because it is the simplest and least cost option. Examples include the 635 MWe EPON Project of Gelderland Power Station in Holland which uses direct co-f