1. Introduction

Most of the generating capacity in Indonesia is based on fossil fuel technology, in which coal power plants still comprised 67% of the energy mix in 2020. However, as the world's focus on sustainability increases, it becomes increasingly difficult to obtain financing for technologies that are based on the use of fossil fuel. Therefore, a green transformation strategy was developed to increase generating capacity with a focus on renewable energy to support the achievement of a more environmentally friendly supply of electricity. The addition of renewable energy generating capacity can be achieved by adding new renewable energy generating capacity, by replacing non-renewable energy generators with renewable energy plants, or by replacing fossil fuels with renewable energy fuels ⁽¹⁾.

This research provides the empirical result in the renewable energy mix achieved through co-firing at existing coal power plants in Indonesia. Co-firing is considered as one of the Indonesian breakthrough programs because it can be implemented without significant investment cost and can provide a solution that allows the management of waste and the reduction of the emissions of greenhouse gases. Low-ratio co-firing is a technology that undergoing development for a long while, but its global application remains very slight compared to other technology options. The commercial implementation of co-firing in coal power plants is based on co-firing trials that show technically feasible results and do not interfere with the reliability of the operation of the plants; however, continuous risk analysis and mitigation are required to maintain the optimum operation of this co-firing program.

The coal-fired power plants that have the potential for co-firing have a total capacity of up to 18,895 MW. If all of the units in all of the coal power plants operated by co-firing commercially with a percentage of biomass PC (Pulverized Coal) Boilers, 6% Circulating Fluidized Beds (CFBs), and 70% Stocker Boilers, 2.7 GW of renewable energy production capacity can be obtained from co-firing, which would require up to 14 million tons of biomass per year (assuming a Capacity Factor of 70%). This is supported by the combination of integrated policy, establishment of a sustainable biomass supply, business schemes, and well designed biomass supply chain ⁽²⁾.

Fig. 1 Energy Mix Projection in Indonesia ⁽¹⁾

The goal of this research is to identify the potential risks that may hinder the co-firing of biomass in coal-fired power plants and their appropriate management plans. The co-firing technology is expected to help achieving Indonesia's target of replacing 10% of the coal with biomass, both from energy generation and waste by 2025. This target also includes maintaining the continuity of service to stakeholders and providing energy services efficiently so that it can become the basis for formulating strategic plans and avoiding waste.

From the research results and experience of using biomass with various coal specifications at several power plants in Indonesia, an optimal risk analysis with mitigation strategies could be carried out. The degree of achievement in co-firing plan is currently evaluated at 67% ⁽¹⁾.

According to the results of the project feasibility studies (including technical, financial, and environmental feasibilities), we will conduct a risk assessment, beginning with determining the activity goal to be achieved and then identifying and describing what risks may occur, as well as their occurrence and causes, key risk indicators, and their effect on the ability to achieve the predetermined goals. Next, we will identify the cause of the risk to show the factors that lead to the risk. Then, we will measure the potential risk level and the scale of the effects of the risk and determine the existing control measures and their effectiveness.

2. Literature review

Several literature references have described the technology, benefits, policies, and effects of co-firing biomass on coal-fired power plants. Hughes ⁽³⁾ maintained that blending biomass and coal as a co-firing fuel is a low-cost option for renewable energy only if policy considerations and incentives permit co-firing to be used as a renewable resource, even if the electricity is provided by an existing power plant that is still operating largely as a coal-fired power plant. Opportunities can be obtained if implemented policies support the co-firing activities. Co-burning biomass with coal can be accomplished with very little capital investment. As long as the biomass material used as the co-fired fuel is produced renewably, the co-firing directly offsets the greenhouse gas emissions from unburned coal. Several local and regional environmental benefits can be achieved from co-burning biomass. While these global and local environmental benefits can be achieved at relatively low costs, neither government regulation nor market forces have led to the adoption of the significant co-burning of biomass.

Richard et al. ⁽⁴⁾ demonstrated that wood biomass can be mixed with coal in the range of 5 to 8% for low co-combustion rates before crushing. The preparation of the wood and separate delivery systems should be supplemented by operating the boiler based on the characteristics of coal used at moderate levels, e.g., 10 ~ 15% biomass. A multi-fuel system, such as a fluidized bed, should be used at a suitably high rate, i.e., 25~50%. Further, boilers intended specifically for the combustion of biomass should be used at levels above 50%.

Sami et al. ⁽⁵⁾ emphasized that co-firing residual biomass, rather than crops grown for energy, leads to additional greenhouse gas mitigation by preventing the release of CH4 from the stockpiled biomass. The compositions of coal and biomass fuels differ greatly, and combustion of the combination of biomass fuel and coal can reduce the emissions of NOx and SOx from existing power plants that use pulverized coal. In addition, because biomass is a carbon dioxide neutral fuel, it can reduce carbon dioxide emissions overall. Depending upon the chemical composition of the biomass, co-firing also can reduce the cost of fuel, waste, and the pollution of the soil and water.

The following literature explanation technology in coal-fired power plants. These references are helpful in developing biomass co-firing plants. Tillman ⁽⁶⁾ explained three general techniques that comprise the co-firing technology family, i.e., blending the biomass and coal in the fuel handling system and feeding that blend into the boiler; preparing the biomass fuel separately from the coal and injecting it into the boiler without affecting the conventional coal delivery system; and gasifying the biomass with subsequent combustion of the producer gas in either a boiler or a combined cycle combustion turbine generating plant.

Dornburg et al. ⁽⁷⁾ stated that biomass can become more expensive than the coal if it must be transported long distances. Thus, co-firing at some electric facilities is not feasible. Comparing the cost of the fuel and the quantities of biomass, another major market can be identified, i.e., the market that has smaller scale boilers that pay more for their fuel than a large scale facility. Some unit may fit for using biomass because of their location within a reasonable transportation distance and the high cost of the current fuel, thereby allowing more to be spent to obtain and transport the biomass. A wide variety of biomass conversion options exist that have different performance characteristics. Also, the economic and energetic performance depends on many variables, such as the costs of logistics, scaling effects, and the degree of heat used, etc. Therefore, an analysis of the system is needed to identify its optimal operating characteristics.

Hughes ⁽³⁾ reported that co-firing biomass with coal can help reduce the total emissions per unit of energy produced compared to the case coal is used alone. Co-burning biomass with coal has the ability to reduce NOx and SOx emissions from existing power plants that burn pulverized coal. Also, depending on the chemical composition of the biomass, co-burning can reduce the costs of the fuel, minimize waste, and reduce the pollution of the soil and water.

Ayhan ⁽⁸⁾ explained that direct combustion, gasification, and pyrolysis are the technologies for the primary conversion of biomass for the production of electricity. Direct combustion is the process of oxidizing biomass with excess air to produce hot flue gases, which then are used to generate steam in the boiler heat exchange sections. The author indicated that a number of areas require further research and that the technology or computational tools must be developed. Pulverized coal-fired equipment, cyclones, and fluidized beds were found to meet these requirements. The cleaned gasification product gases are delivered directly to a boiler or the combustion section of an industrial or aero-derivative turbine to generate electricity. In indirect gasification cycles, the energy for high temperature steam gasification of the organic fraction of biomass to vapors and gases is provided by an external heat source rather than oxygen.

A group of studies have been conducted on risk assessment, risk matrix, and mitigation that may be applied to co-firing technique. Giannakis et al. ⁽⁹⁾ indicated that risk management is a structured process to manage the risks that occur in achieving certain objectives in the form of a systematic and continuous process. The intention of this process is to identify and measure the level of the risks and determine the best course of action to reduce the likelihood that these risks will occur, minimize their effect, or reduce both, as well as other actions to ensure/create confidence that the desired goals will be achieved.

Markowski et al. ⁽¹⁰⁾ emphasized that a risk matrix is a tool that describes and ranks process hazards found during one or more comprehensive evaluations (e.g., process hazard analysis, audits, or the investigation of incidents). The risk matrix is a valuable tool for assessing semi-quantitative risk assessment and selecting risk control measures.

Huihui et al. ⁽¹¹⁾ stated that managing risks and reducing losses are accomplished by using a set of procedures that include risk identification, estimation, assessment, and transfer in which risk assessment is a key component of the overall risk management framework. In fact, risk is an essential component of all risk management processes because they are methods to categorize and rank risks based upon their relative importance.

When identifying risks, the first step is to refer to a predetermined target/context. Then, based on the best information available and considering the results of the context identification, this activity is conducted using various techniques/methodologies, such as brainstorming, interviews, and the analysis of historical data, missing events, observations, surveys, audit results, risk taxonomy, and benchmarking ⁽¹²⁾.

3. Co-firing

3.2 Co-firing technology

Biomass is an attractive renewable fuel to supplement the combustion of coal in utility boilers because when a biofuel replaces a fossil fuel, there is a net reduction in CO2 emissions. Coal co-firing has been shown to be successful with up to 20% biomass mix. The results of extensive applications have shown that co-firing biomass with coal accomplishes the following: (1) increases the efficiency of the boiler; (2) reduces the costs of fuels, and (3) reduces the emissions of NOx and CO2. Every ton of co-fired biomass directly reduces CO2 emissions by more than a ton. Woody biomass contains virtually no sulfur, so SO2 emissions are reduced in direct proportion to the amount of coal that is replaced. However, biomass can contain considerable alkali and alkaline earth elements and chlorine, and when these components are mixed with other components in the gas derived from coal, such as sulfur compounds, a different array of vapor and fine particulate deposition occurs in coal-fired boilers ⁽⁸⁾.

The proportion of coal substituted with biomass must be carried out in stages with a mixture of waste and wood forest waste/products by 1% to 5% of the total coal demand depending on the type of coal power plant system that is being used. There are three types of coal power plant systems, i.e., the Pulverized Coal (PC) type, the Circulating Fluidized Bed (CFB) type, and the Stoker type. The former two types require 1% to 5% biomass, while the latter one uses 100% biomass. There are 52 coal-fired power plants that will be used to implement the co-firing program of Indonesia, assuming a co-firing energy mix of 10%, CF = 70%, and HTE calorific value = 4,200 kCal/kg, waste = 3,200 kCal/kg) ⁽¹⁾.

Fig. 2 Co-Firing Technology ^(13,14,30)

The co-firing methods that are used to burn a mixture of biomass and coal include direct, indirect, and parallel co-firing ^(14,15). The direct co-firing method is the simplest, cheapest, and the most commonly used. The biomass is mixed with coal and processed through the same or separate milling equipment and feeders and then mixed with coal into the same boiler for combustion. Generally, there is no investment in the cost of specialized equipment with this method because it uses already available equipment. Because the mixture of biomass and coal is burned together in the boiler, the existing boiler monitoring parameters must be considered because of the difference in the fuel that enters the boiler.

Biomass is a renewable fuel that is derived largely from living things in which energy is stored. Generally, biomass has a relatively high volatile content, approximately 60% - 80%, with a low fixed carbon content and lower ash content than coal, so that more reactive than coal ⁽¹⁴⁾.

The use of biomass also is adjusted based on its availability near the power plant to save the accommodation costs and to maintain an available supply of fuel. The following are illustrated types of biomass that have been used to test co-firing in Indonesia, i.e., Sawdust (SD); Refuse Derived Fuel (RDF); Recovered Solid Fuel (SRF); Rice Husk; Wood Pallets and Chips; and Palm K Shell ^(16,17,18).

Fig. 3 Types of biomass

The results of the evaluation of co-firing tests on several coal power plants are summarized in Table 1.

Table 1 Typical Characteristics of Biomass Feedstock Compared to Coal ⁽¹⁹⁾

Feed stock	Moisture content (%)	Bulk density (kg/m3)	Low heating value (GJNCV/tone)	Energy density (GJNCV/m3)
Fresh wood	35-58	200 -250	9-12	2-3
Baled straw	15 (air-dried)	140	15	2
Wood chips	20 to 25 (air-dried)	200	15	3
Sawdust	20 to 25 (air-dried)	160	15	2.4
Solid wood	20 (air-dried)	550	15	8
Briquettes	8	650	16	10
Charcoal	2-3	300	27	10
Wood pellets	8	650	17	11
Torrefied wood pellets	2	700	20-21	15
Coal	12	825	20-30	21

The Table 1 shows the types of biomass, from fresh wood to coal, that have different characteristics in water content, specific gravity, calorific value, and energy density, respectively, among which the characteristic closest to coal is Torrefied wood pellets. It can be used as a compact, black solid biofuel for substances such as coal with the same energy content, grindability, and moisture content. It is produced through the carbonization (or slow pyrolysis) of biomass, in which water and volatile organic components evaporate, leaving most of the black carbon behind.

Table 2 Fuel Specification Analysis ⁽¹⁷⁾

Analysis	Parameter	Coal (100C)	Wood Pellets (100WP)	Fuel Mix (5WP95C)
Proximate Analysis (% wt)	Moisture	16.68	8.96	15.02
	Volatile Matter	39.68	72.68	41.06
	Fixed Carbon	38.7	15.94	38.62
	Ash	4.94	2.42	5.3
Ultimate Analysis (% wt)	Carbon	56.6	46.78	56.92
	Hydrogen	4.08	5.28	4.16
	Oxygen	27.2	36.54	26.81
	Sulfur	0.32	0.02	0.27
Hardgrove Grindability Index Ash fusibility temperature	-	41	17	45
Deformation temperature (℃)	Reducing	1,090	1,150	1,090
	Oxidizing	1,150	1,170	1,150
Spherical temperature (℃)	Reducing	1,120	1,170	1,120
	Oxidizing	1,170	1,200	1,180
Higher heating value (kcal/kg)	-	4,536	4,223	4,361

Table 2 shows the results of the analysis of the fuel during the co-firing test. Two different fuels were used during the test, i.e., coal fuel (100C) was used in the first test, while a fuel mixture with a composition of 5% wood pellets and 95% coal (5WP95C) was used in the co-combustion test ⁽¹⁷⁾.

3.3 Effect of co-firing

Most of the technical challenges in co-firing biomass and coal are associated with the quality of the fuels. Biomass differs from coal in a variety of aspects, including its physical qualities and its organic, inorganic, and energy composition. Biomass has less carbon, more oxygen, more silica and potassium, less aluminum and iron, a lower heating value, a higher moisture content, and poorer density and friability compared to coal ⁽⁸⁾. It also has a higher aspect ratio than coal and is substantially less dense. Also, it is more difficult to shrink it to small sizes. Co-fired biomass can be as large as 1/4 inch in diameter or even larger in some cases. These physical characteristics result in several intriguing combustion difficulties ⁽⁸⁾.

The heating value of biomass is far lower than that of most coals. This is attributable in part to increased moisture level and in part to higher oxygen content. Lower heating values may lead lower flame temperatures, and this is true when the high moisture content causes low heating values. However, the low heating values that are attributable to high concentrations of oxygen are not linked to the low flame temperatures. Despite the fact that the burned heating values of dry biomass and dry coal differ by more than 33%, they have identical adiabatic flame temperatures ⁽⁸⁾.

Fig. 4 Technical Effect of Co-firing on PLTU CFB

Fig. 5 Technical Effect of Co-firing on PLTU PC

Co-firing technology has several technological problems. First, some attention must be directed to the problem that the alkaline nature of the biomass can cause the combustion chamber to become fouled and corroded. Deposits of ash reduce the heat transfer and also can cause severe corrosion at the high temperatures. Compared to the deposits produced during the combustion of coal, deposits from the combustion of biomass material are denser and more difficult to remove. Second, the maximum particle size of a given biomass that can be fed into and burned in a particular PC boiler via a specific feeding mechanism constitutes a combination of economy and combustion characteristics and requires additional study. Third, the practical performance of the pulverizer should be assessed. Biomass fuels may require a separate pulverizer to achieve high mix ratios and good combustion performance. Because biomass fuel has a lower calorific value than coal, the flow rate of the mixture must be increased to achieve the same heat output as using only coal. This increased fuel flow rate can cause the flame to move away from the mouth of the burner and create flame stability problems. Enlarged fires also are known to cause higher NOx levels ⁽⁴⁾.

When coal is used as fuel in the co-firing test, the temperature of the gas from the furnace is lower than the existing conditions. The rate of the reduction of the temperature of the gas exiting from the furnace depends on the calorific value of the biomass fuel, the fuel's content of volatiles and ash, and the low calorific value of the wood pellets. The volatile content of the wood pellets is higher than that of coal, so the wood pellets burn faster in the furnace, and the high ash content produces more reasonable heat and causes the solid waste to leave the furnace ⁽¹⁷⁾.

Compared with the existing conditions, the SO2 content in the exhaust gas increased during the co-firing test. Additional research is needed to determine the cause of these conditions and to determine whether the combustion characteristics in the furnace have changed. Generally, because most biomass fuels have low concentrations of sulfur, a reduction in SO2 emissions has been observed in many co-firing applications ⁽²⁰⁾.

The co-firing test has no significant effect on the NOx quantity of the exhaust gas. These results indicate that the co-firing test using 5% wood pellets resulted in a 4.40% increase in the specific fuel consumption. To maintain the same energy output, it is necessary to increase the volume of the fuel due to the lower calorific value, such as wood pellets. For example, PLTU can increase volume by average of 30% to generate the same amount of energy ⁽²¹⁾.

From the results of the 5% biomass co-firing tests at several coal-fired power plants, the power plants had reductions in SO2, NOx, and other co-firing particles. However, there are conditions that actually increase these because the sulfur content in coal affects SO2 emissions significantly. The co-firing process can reduce SO2 emissions only by an insignificant amount because the mixing ratio is still small, the coal calorific specification at the location is low, and the biomass supply condition may be contaminated with outside particles.

Table 3 Emission Quality Standards for Fossil Fuel ⁽²⁵⁾

Parameters	Maximum Rate
	Coal	High Speed Diesel (HSD)	Gas
	(mg/Nm3)	(mg/Nm3)	(mg/Nm3)
Sulfur Dioxide (SO2)	550	650	50
Nitrogen Dioxide (NO2)	550	450	320
Particulate Matter (PM)	100	75	30
Mercury (Hg)	0.03	0	0

According to the standards of emission quality for fossil fuel from the Ministry of Environment and Forestry above, co-firing 5% biomass in coal power plants tends to reduce SO2, NOx, and particulate emissions ⁽²⁵⁾.

Fig. 6 Comparison of Emission Products in Coal Power

Many factors affect the application of biomass co-firing, including the availability of the fuel, the quality of fuel, whether the fuel complies with the specifications of the existing coal-fired power plant, and purchase price (since the price of raw materials must be lower than the price of the existing fuel).

4. Risk Assessment

The co-firing biomass business risk analysis is divided into 3 stages, i.e., risk identification, mitigation, and measurement plans. Risk identification is divided into several risk aspects, including business/commercial, operational, social, environmental, and legal risks. Each risk has written sources of the risk and the effects of these sources.

The target of the project is to ensure that the implementation of the Co-Firing Coal Power Plant with Biomass program as one of the green booster programs will achieve the green objective strategy's target, which is to generate approximately 1 GW from coal power plants with co-firing capacity by 2025.

Project activities are classified according to the taxonomy of risk (risk breakdown structure) and project risk that arises from the development of the company's assets, procurement, and other project activities.

4.2 Risk Matrix

A risk matrix is useful in determining the level of risk against risk likelihood (determine an acceptable level of risk) and to assess/determine whether a risk requires further handling (mitigation) and its priorities ⁽²³⁾.

The risk assessment matrix table is used to describe the level of controlled risk after the probability of the occurrence of risk and its potential effect are assessed ⁽²⁴⁾.

The risk assessment matrix is a well-known method of conducting semi-quantitative risk analysis. The original risk matrix (ORM) and its variants are used extensively in a variety of contexts. The risk matrix approach is introduced briefly in this section ⁽¹¹⁾.

Fig. 7 Risk Matrix for Co-firing

There are 9 risks that have been identified based upon the results of risk analysis and their handling related to co-firing activities; two extreme risks, six high risks, and one moderate risk. We select the extreme and high risks for further analysis.

4.2.1 Discontinuous volume of biomass supply (Extreme Risk)

The causes of this risk, both controlled and uncontrolled, are 1) that the identification data are not sufficiently accurate to describe the existing biomass potential ^(16,17), 2) unavailability of potential biomass near the coal power plant where the co-firing program is planned to be conducted, 3) limited production volume from biomass producers, 4) few biomass sources or biomass producers near the plant, 5) limited biomass with the quality that meets machine specifications ⁽¹⁷⁾, 6) biomass suppliers are still on a small scale/home industry, and 7) the influence of climate change.

Risk mitigation measures, both from prevention and recovery, may be used to reduce the risk level as follows. 1) map the potential to maintain a secure supply of the biomass by providing accurate biomass feedstock, in which there are three types of biomass potential, i.e., the potential of energy plantations (HTE), oil palm plantations (PKS), and waste (SRF or RDF) ⁽²⁵⁾; 2) conduct user trials of several alternative types of biomass (e.g., Wood Chips, Palm Kernel Shell, Wood Pellets, Palm Oil, Garbage); 3) assess the potential distribution and transportation of biomass in Central Java, Indonesia, in which this type of production forest has a potential of 370,129 Ha; 4) the potential for the pellet industry with a capacity of 98,555 tons/year, production of 29,503 tons/year, and potential waste of 782 tons/day using data from Final Disposal Sites found in Central Java Province, i.e., based on the data on the average total biomass potential (AVB); 5) the average number of production forests' potential, PKS potential, and waste potential of 9,224,529.87 tons/day ⁽²⁵⁾, and 6) prioritizing co-firing coal power plants in accordance with the clustering of the potential availability of biomass raw materials near existing coal power plants ⁽⁷⁾. There are several approaches that can be used to estimate the biomass requirements, each of which has advantages and disadvantages. However, it should be noted that the indirect approach is based upon factors developed at the stand level of a forest with a closed canopy, and it cannot be used to make estimates of trees in general. One method that can be used to determine the supply of biomass raw materials is to use a sustainable cycle by calculating the annual need for biomass material and then obtaining the desired number of trees ^(26,27,28).

(1)

${for}est =\dfrac{Cyc\le\times Energy per hectare}{CF\times 8760 hours}\times G en C ap$

Cycle: length of time required to harvest the species of trees used in the forest (years)

Energy per hectare: total electrical energy from tree species used for every 1-hectare plant planting (kWh/ha)

CF: Generating capacity factor (%)

Gen Cap: Generating capacity

Table 5 Forest Land Area Requirements to Provide Power Generation ⁽³³⁾

Type of Tree	Crop Cycle	Total Energy	1 MW /year Equivalent MWh	Field
	years	kWh /hectare	CF=75% (MWh)	hectare
E. Pelita	5	62,790	6,570.0	523.2
Kaliandra	3	36,104	6,570.0	545.9
Acacia Auri	5	58,604	6,570.0	560.5
Gamal	4	45,209	6,570.0	581.3
Mangium	5	48,837	6,570.0	672.6

A community electricity business scheme should be developed to ensure the supply of biomass by involving the community in feed stock management based upon self-reliance and mutual cooperation, i.e., production forest feed stock, palm kernel shell feed stock, and waste pellet feed stock. A business ecosystem for the management of the biomass supply chain should be developed, long-term biomass sales and purchase agreements should be made to ensure the availability of supply volume, target biomass use adjusted to supply availability, and coal reused to maintain system reliability.

4.2.2 The quality of the biomass supplied does not meet the specifications (Extreme Risk)

The causes of the above risk, both controlled and uncontrolled, are as follows: The calorific value of the biomass supplied is lower than the calorific value specified; officially, to date there are no mechanisms to test the quality of biomass or to determine whether the properties of the biomass meet specifications ^(27,28). In addition, biomass suppliers are mixing materials that are outside the specifications and that are not detected by sampling, and the biomass supplied has been mixed with B3 waste.

Certain mitigation measures that can be used to reduce the risk level in both prevention and recovery are related to the fact that each type of plant has a different energy value. When choosing species, as well as considering technical matters in the field, one of the considerations is the wood's potential calorific value. In the conversion calculation below, there are several basic variables that must be met, the wood's specific gravity, calorific value, and volume, and the specifications for the generator's heat rate ^(27,28).

(2)

$E_{e\le ctric}=\dfrac{V_{wood}\times\rho_{wood}\times wood ca\lor ies\times n_{plants}}{G ene rator heat rate}$

Eelectric: The amount of electrical energy generated as a result of planting a forest area of 1 hectare (kWh/ha)

Vwood: Timber harvest volume in cubic meters by planting 1 hectare of trees (m³/ha)

ρwood: Density of wood (kg/m³)

Wood calories: Calorific value of wood per kilogram (kcal/kg)

Generator heat rate : The value of the heat rate of the generator used (kcal/kWh)

Table 6 Conversion of Wood Energy into Electrical Energy Per Hectare ⁽³³⁾

Type of tree	Wood Density	Wood Calories	Wood Volume	Total Mass of Tree	Total Energy	Generator Heat Rate	Total Energy of Electricity
	kg /m3	kcal /kg	m3 /ha	ton /ha	Gcal /ha	kcal /kWh	kWh /ha
E. Pelita	450	4,000	100	54	180	2,868	62,790
Acacia Auri	400	4,200	100	40	168	2,868	58,604
Mangium	350	4,000	100	35	140	2,868	48,837
Gamal	360	4,000	90	32.4	129	2,868	45,209
Kaliandra	450	4,600	50	22.5	103	2,868	36,104

During the co-firing test of 1,528.58 Tons of 5% sawdust, the average gross electrical energy (Gross) produced was 2,430,410 kWh. For comparison, when 2,420,293 kWh are produced by burning coal, the total fuel consumption is only 1,540.65 Tons of coal. By dividing the total fuel consumption by the total energy produced, the specific fuel consumption (SFC) for co-firing 5% sawdust is 0.629 kg/kWh, while it is 0.637 kg/kWh for 100% coal fuel ⁽¹⁶⁾.

The results of the co-firing test using 5% wood pellets actually contributed to an increase of 4.40% in specific fuel consumption (SFC). However, to produce 1,189,800 kWh, the total specific fuel consumption (SFC) with 5% co-firing wood pellets is 0.580 kg/kWh compared to using 100% coal. The SFC produced is 0.556 kg/kWh, so when using lower heating value fuels, such as wood pellets, it is necessary to increase the volume of the fuel to maintain the desired energy output. This increase in SFC averages up to 30% in co-firing in coal-fired power plants to produce the same amount of energy ⁽¹⁸⁾.

It is necessary to determine the quality standards for types of biomass for each type of coal power plant. This is because the different types of boilers in coal power plants influence the type and characteristics of the feedstock that can be used in the co-firing program. CFB and Stoker boilers can use feedstock with wood chips and Palm Shell types mixed with coal, but it is better to use wood pellets and Solid Recovered Fuel (SRF) feedstock for coal power plants with the PC type of boiler ⁽¹⁶⁾. For the PC and CFB types of boilers, the substitution of biomass for coal is 5% and for the Stoker type of boiler, the use of biomass is targeted to reach 30% based on the research results pertaining to co-firing coal power plants ^(16,17,18).

Fig. 8 Comparison of Specific Fuel Consumption (SFC) in coal power plants

Co-firing 5% biomass can decrease/increase Specific Fuel Consumption (SFC) and Net Plant Heat Rate (NPHR) depending upon the quality of the biomass.

Based upon tests conducted by the PT PLN (Persero) Research and Development Center, the changes in SFC and NPHR values depend largely upon the calorific values of the coal and the biomass. The graph above indicates that the SFC value of the average power plant decreased; this value actually increased in only two plants. This is because the calorific value of the biomass is lower than that of coal alone, and, after mixing, the calorific value is lower than when only coal is used.

Fig. 9 Comparison of Net Plants Heat Rate (NPHR) in Coal Power Plants

If feedstock from an energy plantation forest is used, 8 million tons of biomass/year are required depends on the quality of the biomass supply for 52 co-fired coal power plants in the first stage. From solid waste fuel, 800 tons/year are required assuming that CF is 70%, that the calorific value of wood is 4200 kcal/kg, and that the calorific value of SRF is 3200 kcal/kg ⁽¹⁾. Thus, a strict punishment clause must be included for biomass suppliers whose supply does not meet specifications, and a biomass quality testing laboratory must be set up as a transaction point ^(16,17). The government must issue a Business Permit for Use of Natural Forest Timber Forest Products and a Business Permit for Use of Industrial Forest Timber Products for co-firing and create sufficient biomass storage to maintain the availability of biomass for power generation ⁽⁸⁾.

4.2.3 The efficiency of co-firing power plants decreases (High Risk)

The causes of this risk are the biomass's calorific value is lower than that of coal.

Table 7 Typical Characteristics of Biomass Feed stock Compared to Coal ⁽¹⁹⁾

Plants	Capa-city	Co- Firing composition	Type of Biomass	Caloric Value (HHV-Ar Basis)
				Coal	Biomass	Fuel Mix
	MW	%		kCal /kg	kCal /kg	kCal /kg
A	300	5	Sawdust (SD)	4,212	2,694	4,136
B	300	5		3,865	3,909	3,258
C	300	5		4,199	1,867	4,288
D	330	5		4,296	3,951	4,358
E	360	5		3,905	3,101	4,423
F	400	5		4,047	2,694	4,330
G	500	5		5,395	3,535	5,041
H	660	5		4,237	4,294	4,240
I	300	5	Wood Pellet	4,644	4,486	4,636
J	400	5		4,047	4,487	4,356
K	660	5		4,128	4,280	4,137
L	300	5	Solid Recovered Fuel	4,788	2,901	4,652
M	400	5	Rice Husk	4,459	3,241	4,213

Table 7 shows the results of the tests that the PT PLN (Persero) Research and Development Center conducted at 13 coal-fired power plants in Indonesia that use a fuel mixture of Sawdust (SD), Wood Pellet (WP), Solid Recovered Fuel (SRF), and Rice Husk with a generating capacities of 300 - 660 MW with various average calories of the fuel mix, some of which were lower and some of which were even higher than the calories associated with coal fuel. Of course, with the same type of boiler and nearly the same generating capacity, this result must be analyzed and evaluated to achieve a much higher calorific value after co-firing with biomass.

Further, the biomass storage at the coal power plant site is not appropriate. There are non-combustible materials in the biomass that was supplied ⁽⁸⁾, and the biomass has a high content of alkali and chlorine, which causes the calorific value to decrease rapidly ⁽⁶⁾.

The following mitigations, both prevention and recovery, can be used to reduce the risk level mentioned above: 1) develop a community electricity business scheme to ensure the supply of biomass by involving the community in feedstock management based on self-reliance and mutual cooperation, i.e., production forest feedstock, Palm Kernel Shell (PKS) feedstock, and Waste Pellet feedstock; 2) build a business ecosystem for biomass supply chain management, contract long-term biomass sales and purchase agreements to ensure the availability of the supply volume; 3) cooperate with biomass suppliers near the power plant to prepare adequate biomass storage facilities; 4) encourage the government to establish Indonesian national standards (SNI) for various types of biomass; 5) sort the biomass strictly to obtain the best quality that fits the plant's specifications; 6) measure power plant equipment affected by co-firing (tube boiler and others) before and after the trial period ⁽¹⁷⁾; 7) conduct tests to identify the most optimal proportion of biomass to use; 8) change the type of biomass that is used, reject and impose penalties on suppliers who do not conform to specifications, and 9) create sufficient biomass storage to maintain the availability of biomass for the generation of power.

4.2.4 Reliability of Co-Firing Power Plants that Implement Co-Firing Decreases (High Risk)

Some of the causes of the risk above, both controlled and uncontrolled, include residue from burning biomass that damage generating equipment, changed operation patterns, biomass quality that does not meet the required specifications, and biomass suppliers who commit fraud.

The following are mitigation measures, both prevention and recovery, that can reduce the risk level. First, strictly monitor the quality of the biomass. Second, adjust the Standard Operation Procedure to the co-firing operation pattern; direct combustion is the most mature technology in the process of converting biomass into electrical energy, and it is used nearly all over the world. Even so, it has two significant drawbacks, i.e., its high level of emissions and its low efficiency ⁽²⁹⁾. Third, refuse and penalize suppliers for not complying with biomass supply specifications, and revert to using coal to maintain system reliability. Related with the second risk mitigation measure, the fuel feed of the co-firing coal power plant must be adjusted to the boiler specifications at the existing coal power plants.

a. Fixed Bed Combustion

Underfeed Stoker is a relatively inexpensive technology for small and medium-sized boilers up to 6 MW thermal, suitable for low ash biomass, such as woodchips, pellets, and sawdust, as well as small particles ( < 50 mm). It is simple to operate and control feeding because of the continuous supply into the furnace.

Grate Furnaces suits for use with biomass with high water content, non-uniform size, and high ash composition. The homogeneous distribution of biomass in the furnace causes the primary air supply to be distributed evenly, which prevents slagging, greater fly ash production, and a greater need for excess oxygen for combustion that reduces the boiler's efficiency. Additional technology is needed to obtain good performance, particularly with respect to the proportion of NOx and emissions.

b. Fluidized Bed Combustion

This type of combustion has temperatures in the range of 700 - 1000oC, and the furnace can provide a more homogeneous temperature so that the combustion that occurs is more efficient. However, the specification for the size of the biomass particles is quite strict ( < 80 mm).

c. Pulverized Fuel Combustion

Biomass is injected pneumatically into the combustion chamber. This system requires fuel that has relatively constant quality. The maximum particle size should not exceed 20 mm, and the moisture content should not exceed 20% by mass. Fuel feeding must be regulated carefully because of the explosive combustion characteristics of the fine particles of fuel. Generally, a mixture of fuel and air is injected tangentially into the inlet to the furnace to form a rotational air flow (vortex flow). Combustion of biomass and coal occurs simultaneously because of their small particle sizes, so load control can be achieved efficiently and load changes can be completed quickly ⁽³⁰⁾.

4.2.5 Increase in cost of coal power plants that implement co-firing (High Risk)

Some of the causes of this risk, both controlled and uncontrolled, are 1) that the cost of transporting and treating biomass is too high ⁽⁷⁾; 2) maintenance costs have increased; 3) the unbalanced supply and demand of biomass ⁽⁸⁾, and 4) the price per calorie of biomass is higher than that of coal ⁽²⁸⁾.

Fig. 10 Comparison of the Cost of Production in coal power plants

The use of biomass co-firing tends to reduce the cost of production in the range of 0.021 to 0.34 USD Cent/kWh, based upon the Regulation of the Board of Directors of PT PLN (Persero) 001.P/DIR/2020 dated March 5, 2020 in calculating the highest benchmark price of biomass for co-fired power plant co-firing. The highest benchmark price of biomass is one of the references used to determine the Owner Estimate Price in the biomass procurement process ⁽³²⁾.

Co-firing carried out at several coal-fired power plants using contracted coal and mixing 5% wood pellet biomass can reduce fuel costs if the biomass price is lower than the coal price. However, the price of biomass exceeds the purchase price of coal in some locations, so that the cost of production still exceeds the price of coal.

Certain mitigations, both prevention and recovery, that can be performed to reduce the risk level above include 1) negotiating with biomass suppliers to lower the prices of biomass below the prices of coal; 2) limiting the price per calorie of biomass to a maximum of the same as the price per calorie of coal; 3) encouraging the government to issue policies and regulate biomass prices to offer suppliers and users fair economic value; 4) change the type of biomass used, and 5) revert to using coal to maintain the cost of the supply.

4.2.6 Implementing co-firing in coal power plants violates the provisions of environmental regulations (High Risk)

Some of the causes of the risks discussed above, both controlled and uncontrolled, are that the combustion products do not meet Emission Quality Standards ⁽²⁵⁾; the coal power plant's environmental impact analysis document currently does not accommodate the use of biomass; biomass storage causes environmental pollution ⁽⁶⁾, and co-firing biomass in a coal power plant is not yet legalized internally or externally ^(3,13).

Some mitigations, both prevention and recovery, that can be used to reduce the risk level above are 1) coordinate with the Ministry of Environment and Forestry to confirm the existence of an environmental permit for co-firing at coal power plants ⁽³¹⁾; 2) encourage the government to stipulate SNI (Indonesian National Standard) for various types of biomass; 3) submit a revised environmental impact analysis to relevant parties; 4) create a Standard Operation Procedure for storing biomass at the location of the coal power plant; 5) perform exhaust gas treatment to meet Emission Quality Standards ⁽²⁵⁾; 6) stop using biomass until the permit is issued ⁽³⁾; and 7) cooperate with the government to issue regulations immediately on the implementation of co-firing in Indonesia ⁽¹⁾.

4.2.7 Coal power plants that implement co-firing have difficulty managing Fly Ash Bottom Ash (FABA) (High Risk)

Some of the causes of the risk above, both controlled and uncontrolled, include the presence of a mixture of biomass in the coal material that affects the chemical composition and physical properties of the ash that is produced ⁽⁵⁾, such that FABA cannot be used ⁽²¹⁾.

Certain mitigations, both prevention and recovery, that can be done to reduce the risk level above include 1) standardizing the amount/volume of mixed biomass for each type of coal power plant (volume, size, composition) ⁽⁴⁾ and 2) looking for other alternative uses that can accept the quality of co-firing ash ⁽²¹⁾.

4.2.8 Decreasing biomass quality (High Risk)

Some of the causes of the risk above, both controlled and uncontrolled, are improper placement of biomass, not using the biomass according to the first-in-first-out method, and increased water content or moisture ^(16,18).

Some mitigations, both prevention and recovery, that can be used to reduce the risk level above include 1) covering the biomass with a waterproof material; 2) drying the wood by increasing the temperature of the air, reducing the humidity of the air, and increasing the contact between the dried wood and dry air. Drying in the sun at a relatively low temperature (40 - 60 oC) also minimizes the emission of odorous volatile organic compounds from wood; 3) stirring regularly using a machine, stirrer vehicle, or wheel loader so that the quality of the biomass remains homogeneous, and 4) performing a visual inspection of the biomass, either after it has just arrived or before it is unloaded and used. Visual checks include checking for the possibility that it is mixed with dirt or other materials and treating biomass that may have decreased in quality.

5. Co-firing in other economies

International experience has shown that co-firing is viable economically when state-funded programs are implemented. In addition to direct management and control, regulatory tools include 1) carbon taxes; 2) feed-in tariffs; 3) direct subsidies, and 4) renewable energy portfolio standards that require a minimum share of renewable energy in electricity generation [ 31].

The current carbon tax has not been enough to solve climate change, but the social value of carbon remains a valid theoretical indicator for measuring greenhouse gas emission reduction policies, plans, and programs. The level of support for co-firing in several European countries starts from 20 to 64 Euro/MWh through the Feed-in Tariff and Green Certificates schemes. An avoidable CO2 value of 30 Euro/t would make co-firing biomass in coal-fired power plants economically viable in Germany. Recently, when evaluating the prospect of co-firing in four European countries, it became apparent that it would be advantageous if a carbon price of 5 Euro/t would make co-firing with biomass prices lower than 2.3 Euro/GJ. A carbon price higher than 50 Euro/t would allow the use of pellets ⁽³⁴⁾.

A Renewable Obligation scheme is being used to support co-firing projects in the UK. Renewable Obligation for Electricity utilities in England, Wales, and Scotland began at 2% in 2002, and they had increased to 48.4% in 2019-2020 with certificate issued are tradeable for amount of electricity generated from renewable resources. Having introduced the Renewable Obligation, the share of co-firing in renewable energy generation in the UK has increased. By 2017, when the scheme closed for new capacity and was replaced by a contract-for-difference mechanism, all major coal-fired power plants in the UK were retrofitted to co-firing ⁽³⁵⁾. Although the system was introduced in a technology-neutral way, this no longer has been the case after 2009. Since post-2012 systems discourage co-firing at low biomass proportions, coal-fired power plants have responded by switching to dedicated biomass units. After the UK, South Korea was the second largest market for industrial wood pellets in 2020 ⁽³⁶⁾.

Denmark and the Netherlands have taken a different approach. They directly subsidize co-incineration power generation. Since January 2009, Denmark has paid a subsidy of 2 Euro cents/kWh for dedicated and co-firing systems ⁽³⁷⁾. The Netherlands imposes a feed-in tariff premium on the wholesale price of co-firing electricity based on the post-2013 sustainability criteria ⁽³⁸⁾. In 2020, Denmark and the Netherlands were the third and fifth largest markets, respectively, for industrial wood pellets.

In 2003, Japan first introduced a standard renewable energy scheme, but it was replaced by feed-in tariffs in 2012. In this program, electricity producers must use renewable energy to generate part of their electricity, and they receive a fixed term contract that specifies their purchase price for electricity. Feed-in tariffs for biomass power plants in Japan range from 13.65 to 33.6 yen/kW, depending upon the type of biomass used. The renewable energy standard helped increase the biomass generation capacity from 1.3 GW in 2004 to 2.3 GW in 2011, and within the framework of feed-in tariffs, biomass power generation reached 3.5 GW (123/166) in 2018 ⁽³⁹⁾. Japan was the fourth largest market for industrial wood pellets in 2020. Strauss ⁽³⁶⁾ pointed out that growth in demand in the next several years will exceed that of any other country.

All of the cases mentioned above are in high-income countries. By 2020, few middle-income countries used co-firing. Nonetheless, there is technical potential to burn biomass with coal in their power plants, and this potential only increases co-firing's relative importance. Many rich countries are replacing old coal-fired power plants with plants that produce renewable energy. However, power plants in middle-income countries are younger and have lower budgets. Thus, the spread of biomass co-firing in ASEAN is still limited, although developed countries have validated the technological and economic feasibility of co-burning biomass in coal-fired power plants.

Indonesia is a country with a fairly large source of biomass fuel, and it has the opportunity to implement biomass co-firing in a number of coal-fired power plants that have been built over the last 15 years. The urgent need to reduce the levels of greenhouse gas emissions in Indonesia has motivated research and testing of biomass co-firing as a practical technique in existing coal-fired power plants. To create and protect the implementation of co-firing activities in Indonesia, the country also must be equipped with risk analysis and mitigation to optimize opportunities and reduce potential state losses in achieving the renewable energy target of 23% by 2025.

5. Conclusion

Based the risk analysis result, there are several mitigation activities that must be implemented to keep the implementation of the biomass co-firing program at coal-fired power plants in Indonesia running smoothly and in accordance with the expected targets. Normal or safe operating parameter limits for coal power plants that will carry out co-firing must be adjusted according to the design parameters/reports on the results of the performance of each coal power plant's system or equipment. Operators should refer to the manufacturer's design manual to determine the safe limits for the operation of the coal power plants' equipment when co-firing trials are performed.

At each coal-fired power plant where the plan is to implement biomass co-firing, tests must be tested performed initially to ensure that the co-firing can be done safely and will not adversely affect the reliability of the generator. When co-firing is implemented, it is necessary to map the potential and adequacy of the availability of biomass, as well as the utilization of biomass sources that are located as close as possible to the power plant. The characteristics of biomass are different from coal, coal material tends to maintain its calorific value under certain weather conditions, while biomass cannot maintain its calorific value under certain weather conditions so it requires special handling.

Obviously, the co-burning of biomass has many advantages. It creates large new domestic businesses, helps develop local economies, creates many jobs, and encourages the development of the forestry sector. The biomass co-firing business has a multiplier effect on local economic growth and environmental recovery, but only a small part of the community is aware of this business opportunity. Finally, government support is needed with respect to policies, knowledge, education, and assistance for the community concerning the need and availability of biomass as co-firing fuel for generators. We still have some limitations in this paper, i.e., we used data from Indonesian coal power cases to identify and assess risk factors. However, the data may not be applicable to a different environment or economic situation. Future research is expected to include a comparison-based study in co-firing to find the variations among different economies.