Article pubs.acs.org/EF
Some Combustion Characteristics of Biomass and Coal Cofiring under Oxy-Fuel Conditions in a Pilot-Scale Circulating Fluidized Combustor Yewen Tan,* Lufei Jia, and Yinghai Wu Natural Resources Canada, CanmetENERGY Technology Centre, 1 Haanel Drive, Ottawa, Ontario, Canada K1A 1M1 ABSTRACT: A series of tests were conducted using CanmetENERGY’s pilot-scale, oxy-fuel firing capable circulating fluidized bed (CFB) combustor. These tests were done with a variety of coals, ranging from lignite to bituminous, that were cofired with biomass, specifically wood pellets. The amount of cofired wood pellets varied from 20 to 50% by weight. The objectives of these tests were to measure the combustion characteristics of oxy-fuel cofiring with coal and biomass, including flue gas composition, emissions of volatile organic compounds, and emissions of trace metals. Test results showed that stable combustion conditions could be obtained with a CO2 concentration in the flue gas of >90% and that the addition of wood pellets did not appreciably affect combustion conditions. These results provided support for the claim that cofiring solid fossil fuels such as coal and coke with carbon-neutral biomass under oxy-fuel conditions with CO2 capture is a sound approach for achieving negative CO2 emissions.
1. INTRODUCTION Oxy-fuel combustion technology is widely considered as a viable option for the control of CO2 emissions from fossil fuels, especially coals. While most of the research activities have been focused on applying oxy-fuel technology to pulverized coal (PC) combustion, it has recently been recognized that circulating fluidized bed combustion (CFBC) is an alternate technology that could be easily adapted for oxy-fuel combustion.1,2 In fact, CFBC has some advantages compared to PC for oxy-fuel combustion that may make it a better choice for CO2 capture in some cases. For example, the large heat capacity of return leg solids and the degree of cooling of the return solids from the hot cyclone can result in a great deal of bed temperature control, allowing fluidization with oxygen concentrations higher than those that can be used safely in PC oxy-fuel combustion. This high allowable oxygen concentration could allow oxy-fuel CFBC units to operate with recycle flue gas ratios lower than those of PC oxy-fuel combustors, which leads to noticeable lower operating costs. To conduct research on oxy-CFBC technology, CanmetENERGY has converted and operated a pilot-scale 0.8 MWth oxy-fuel CFB for several years, and some of the work conducted with this unit contributed to Foster Wheeler’s Ciuden, Spain oxy-CFB demonstration unit.3−5 Compared to PC combustion, CFBCs also have the significant benefit of being able to combust a wide variety of fuels. They are able to efficiently and easily combust biomass and waste materials such as wood and municipal solid waste. Because of their CO2 neutral characteristics, combustion and cocombustion with biomass have received a considerable amount of interest. In some cases, cocombustion of coal with biomass has been shown to be economically viable.6,7 For PC coal combustion, there is usually a limit to the amount and type of biomass that can be burned along with coal, while CFBCs have much greater flexibility, both in terms of the amount of biomass that be combusted and in terms of the type of biomass Published 2013 by the American Chemical Society
that can be used. When burning these materials and sequestering the CO2 that is produced, oxy-fuel CFB combustors can effectively remove CO2 from the atmosphere. Because of the favorable emission profiles of certain biomass materials, CFBCs can also cofire coal and biomass to achieve improved combustion performance as well as economics. As a result of these advantages, there have been a few studies that have investigated this approach.8,9 It is under this background that a series of combustion tests were performed using CanmetENERGY’s 0.8 MWth oxy-fuel circulating fluidized bed combustor. Although emissions of metals during biomass combustion have been investigated previously,10 to the best of our knowledge, the work presented here is the first that investigated in detail the emissions of major pollutants, volatile organic compounds (VOCs), and metals using a pilot-scale oxy-fuel CFB combustor. The test fuels consisted of a variety of coals (lignite, sub-bituminous, and bituminous) and biomass in the form of wood pellets. The amount of wood pellets in the fuel mixtures varied from 20 to 50% by weight.
2. TEST FACILITY AND PROCEDURE A 0.8 MWth pilot CFBC was used in this work.5 The CFBC has been designed to allow fluidization with either air or O2 mixed with recycled flue gas. It contains a 0.406 m inside diameter refractory lined riser and refractory lined cyclone and return leg. The CFBC is equipped with a natural gas burner for preheating. Independent feed augers can supply multiple fuel types and a sorbent. Oxygen, CO2, and recycle flue gas flow rates are controlled by mass flow controllers. The recycle flue gas system includes a flue gas cooler, a bag filter, a condenser, a condensate knockout, and a recycle gas blower. Gas analysis includes O2, CO, CO2, SO2, and NOx. Four water-cooled tubes can be inserted into the riser for temperature control. The CFBC also has the ability to Received: June 13, 2013 Revised: October 23, 2013 Published: October 25, 2013 7000
dx.doi.org/10.1021/ef4011109 | Energy Fuels 2013, 27, 7000−7007
Energy & Fuels
Article
Figure 1. Schematics of CanmetENERGY’s 0.8 MWth pilot-scale oxy-fuel CFB test facility.
Table 1. Fuel Properties (as analyzed, weight percent) wood pellet
Boundary Dam bituminous
Proximate Analysis, As Received moisture 9.52 25.56 ash 0.38 9.72 volatile 76.46 29.05 fixed carbon 13.64 35.67 Ultimate Analysis, Dry Basis carbon 44.7 46.8 hydrogen 5.36 3.00 nitrogen 0.12 0.69 sulfur
