A comparative study on co-combustion performance of municipal solid waste and Indonesian coal with high ash Indian coal: A thermogravimetric analysis

Abstract

In recent years there has been an increasing utilization of coal blends in the Indian power industry, with Indonesian coal, due to high ash content and shortages in domestic coal production. On the other hand, rapid economic growth is aggravating the municipal solid waste (MSW) related environmental problems. In this study, an attempt has been made to compare the co-combustion characteristics of hydrothermally treated MSW and Indonesian coal with high ash Indian coal, so as to replace the Indonesian coal with MSW. The effect of blending Indonesian coal and hydrothermally treated MSW with Indian coal on ignition behavior was studied. MSW blends of 10%, 20%, 30% and 50% (in wt.%), and an Indonesian blend of 10% with Indian coal were tested in a thermogravimetric analyzer (TGA) in the temperature from ambient to 700 °C with a temperature increase of 10 °C/min. From the results, at 10% of blend, ignition and carbon burnout were similar for Indonesian and MSW blend, analogous to coal combustion and even better than the Indonesian coal blend, which indicated the feasibility for replacing Indonesian coal with hydrothermally treated MSW. Further, the results show a scope to increase the MSW blend in Indian coal up to 20%, as the constituents behave as a single fuel.

Introduction

The management of municipal solid waste (MSW) is one of the major problems in India. The present method of disposal like open dumps and landfills are creating environmental pollution and public health hazards to the inhabitants. In addition, available land for land fill sites has been decreasing at an alarming rate, thus the cost of disposing MSW is increasing. On the other hand, coal is the main source of energy and continues to dominate in the generation capacity mix in Indian electricity generation. A comparatively high level of ash in Indian coal (up to 45%) causes high environmental impacts. Blending of imported coal, especially from Indonesia has become an immediate choice on account of various factors such as inability of the national coal industry to supply appropriate quality of coal and high delivered cost of domestic coal due to high freight [1].

But there are problems associated with the use of Indonesian coal. For co-combustion of Indian coal with Indonesian coal, the boiler needs to be redesigned for a predetermined blending ratio. Even if a boiler is designed for firing blended coal, boiler manufacturers would not be able to guarantee the performance as the burning characteristics of blended coal would depend on homogeneity of blending [2], [3]. Further, the availability of Indonesian coal is not certain [4]. In addition to that, wide variation in operation parameters of boiler due to improper mixing of imported Indonesian coal with indigenous coal is another problem related to imported coal even at low blending ratio, at around 10% as there is a large difference in their calorific values. Considering the above stated two problems, MSW co-combustion with indigenous coal is the most economical short term option to lower the green house gas emissions, associated problems of coal supply and ash handling of the existing power plants [5]. Compared with other kinds of biofuels, MSW co-combustion is normally significantly cheaper and has the advantage that it can be implemented relatively quickly using existing infrastructures. Hence, the co-utilization of coal with MSW is proving to be the cheapest method for generating green power in power utilities [6], [7], [8], [9].

The nature of coal and MSW are quite different, due to their different origins. So co-combustion can even be more challenging, but fuel supply may be cheap and plentiful [6]. Greater availability and lower cost compared to other biofuels could justify their use in co-combustion. But there are some technical barriers that need to be overcome in order to accelerate the expansion of co-combustion technology, to promote clean, efficient use of MSW. The main problems associated with usage of MSW are their high moisture content and its composition, unlike other regular biomass. Since MSW is heterogeneous in nature, it poses a main challenge of devising an effective disposal method for MSW. This may be due to the high organic material (40–60%), high moisture content (40–60%), high inert content (30–50%) and low calorific value content in MSW [10]. The requirement of supportive fuel to sustain combustion even in refuse-derived fuel (RDF) plants in India leads to unsuccessful operation. So a suitable pretreatment technology is required to convert MSW to a required form so that it can be blended with coal easily.

Considerable amount of work has been done by many researchers in the field of pyrolysis of RDF and co-combustion of coal with biomass, sewage sludge, waste tyre etc. However co-combustion of hydrothermally treated MSW with coal has not been studied yet. Further, very few authors carried out tests on co-combustion of MSW and coal, but they have reported a very low efficiency predominately caused by MSW moisture [11].

The aim of this study is to utilize hydrothermally treated MSW for blending with coal so as to reduce pollution and coal imports and to promote effective use of MSW. The objective of this paper is to present the effect of blending on burning characteristics and to establish a suitable level of MSW blending ratio without much deviation compared with coal only burning to replace Indonesian coal presently being used in power generation. The main purpose is to present and discuss the most significant results achieved in the thermogravimetric analysis.