There is concerted international effort to reduce green-house gas (GHG) emissions to the atmosphere. Anthropogenic GHG emissions are due mainly to fossil energy utilization. The most abundant GHG is CO₂, which is the product of combustion of carbonaceous fuels. Of the combustion of the three main fossil-fuels: natural gas, liquid petroleum fuel and coal, coal emits the largest amount of CO₂ per unit of heat produced because the C/H ratio in its chemical composition is the highest.

The most effective way of reducing CO₂ emissions is by energy conservation both at the user end, and also by more efficient production of power and/or heat. This will reduce also the emissions of the pollutants such as SO_x, NO_x and particulates. In large power plants, the efficiency of steam generation is quite high and fuel can be saved mainly by improving the thermodynamic cycle of power generation. In contrast with the large power plants, small industrial boilers which produce steam for the manufacturing of textiles, paper, ceramics, etc., and for the heating of households and offices, have relatively low efficiencies, leaving plenty of opportunity for fuel savings and emission reductions by improved fuel treatments and boiler-house practices.

The study presented in this paper focuses on the industrial boiler sector in China. The large coal consumption of this sector, about 400 million tons (Mt)/year, makes it the target of our investigation. However, the problem of a large number of small boilers dispersed spatially at sites nation-wide constitutes challenges to remedial action. The UN Global Environmental Facility (GEF) published data for 1991 as part of a cooperative project entitled “China: Issues and Options in GHG Emissions Control—Pre-feasibility Study on High Efficiency Industrial Boilers”. The report contains recommendations to transfer foreign, advanced technology to Chinese boiler makers for the manufacture of more efficient and cleaner industrial boilers. It would take, however, a long time to replace a significant number of the existing industrial boilers and it is recommended that action is taken in the mean time to improve their performances. These recommendations, which could be seen to be complementary to those of the GEF report, are based on our field studies and the statistical evaluation of the data.

The statistics for 1991 have shown that the number of industrial boilers in China reached 432,000 (983,000 ton-steam/h) with a total annual consumption then of 350 Mt of coal, i.e. around one third of the total coal production in China. The industrial boiler sector is the main contributor to pollution: 6.2 Mt particulate emissions accounting for 36.6% of the total; 5.2 Mt SO₂ accounting for 38.8%; and more than 500 Mt of CO₂. With the fast growth of the economy, the environmental emissions are getting worse. The average operating efficiency of the industrial boilers is in the range of 60–70%. Accordingly, more than 60 Mt coal is wasted and over 100 Mt of excess CO₂ is emitted to the atmosphere annually. It can be estimated that, by now, the boiler population has risen to more than 500,000 with a total coal consumption of over 400 Mt per annum.

Two hundred and fifty certificates of boiler thermal-balance tests from three Provinces: Shanxi, Henan and Jiangsu, were collected for this study. These tests were conducted by specialized teams, licensed by the provincial governments during the past decade. Among the 250 boilers, the medium size boilers (4, 6 and 10 t/h) accounted for 74% of the total number, while few boilers of 1 or 35 t/h were tested. The average capacity of the 250 boilers is some 6.5 t/h, i.e. much larger than the 2.3 t/h, the average size of the total national industrial boiler population. The coal-burning equipment of the 250 boilers is dominated by the type of traveling grate stoker accounting for 96.4%: chain-stoker 72.8% and reciprocating stoker 23.6%, respectively. The higher grade bituminous coal [Lower Heating Value (LHV) 5016 kcal/kg and volatile matter (VM) in dry ash-free coal >20%] is the major fuel in these boilers, though a lean coal (VM=1 20%) is used in the majority of boilers in the Shanxi province. The authors visited the three teams and six boiler houses to confirm the correctness of measurements and test conditions. Table 1 contains the statistical average data of the boiler tests carried out in the three provinces.

The test data are illustrated by histograms of boiler efficiency, excess-air factor, waste-gas temperature, CO concentration and carbon content in slag and fly ash (combined), in Fig. 1, Fig. 2, Fig. 3, Fig. 4 and Fig. 5 respectively. Fig. 6 and Fig. 7 show the distributions of carbon content in slag and fly ash, respectively.

In order to show the relationship between input and dependent variables, it was necessary to better specify the coal type, boiler size, and firing system in the samples. For this purpose, 69 bituminous coal-fir#ed chain-stoker boilers of 4 t/h were chosen from the 250 tests. Fig. 8, Fig. 9, Fig. 10, Fig. 11 and Fig. 12 show the boiler efficiency, the waste-gas temperature, and carbon contents in the slag and fly ash as functions of the excess air, and the efficiency vs the thermal load for this smaller sample of 69 boilers.

The following observations are made:

1. The average boiler efficiency is 65% for the average capacity of 6.5 t/h and for the use of high-quality bituminous coal. This efficiency is lower than that given in the Technical Guidebook for Industrial Boilers which require an efficiency of 76% for the same boiler type, capacity and for using the same type of coal. The average efficiency in Shanxi province is lower than in the others by 5%. One reason for this lower efficiency is the coal type, a lean coal used mainly in Shanxi, which is relatively difficult to burn completely.

2. The highest boiler efficiency is 82% while the lowest is 40%. Only 7.6% of the boilers reach the recommended level of 76%. According to the efficiency range, the boilers can be divided into three groups: (1) less than 65% accounting for 47.1%; (2) 6 70% making up 28%; and (3) higher than 70% taking up 24.8%.

3. One of the main reasons for the low boiler-efficiency and high CO₂ emission is the very high excess-air factor (the excess air factor is the multiple of the stoichiometrically required combustion air): the average is 2.8 compared with 1.5 to 1.8 specified in the standard. The highest value reached is 6.9. Only 20% of boilers are operated in the recommended range of the excess air.

4. The average waste-gas temperature is 188°C, i.e. slightly higher than the recommended 180°C. This may be due to economizers installed in most boilers to recover the heat from the exiting flue-gas by preheating the feed water. However, the high excess-air would be another reason for the low exit-gas temperature.

5. In Shanxi, the average CO is two times higher than normal, so causing low efficiency and more CO₂ emission. The highest CO is 2.8%, resulting in a boiler efficiencies loss of 10.2%, i.e. 10 times higher than normal. The average CO concentration in Henan and Jiangsu seems to be in the reasonable range of 0.05–0.07%. It is not an optimum, however, because a high value of excess-air is used to reduce the CO concentration.